Buildings and energy

Brian Edwards, in his comprehensive reference volume The Rough Guide to Sustainability (2007, p67) discusses the link between fossil fuel combustion and CO₂ emissions and there are some further, useful statistics relating to the energy used in buildings in a paper published by the United States Environmental Protection Agency, titled: Buildings and their Impact on the Environment: A Statistical Summary, (2009, p2). It is of interest to note that households spend on average $2,000 per year on energy bills, and that more than half of this sum goes on providing thermal comfort and satisfactory lighting.

The sun and solar geometry

A basic understanding of solar geometry and the ability to read sun-path diagrams are key skills for the designer of low energy architecture and there is a concise explanation of solar geometry and of the relationship between sun and earth by Bougdah and Sharples in Environment, Technology and Sustainability, Volume 2 (2010, pp19–26). Using a sun-path diagram, the sun’s position in terms of both altitude (angle above the horizon) and azimuth (position in relation to the compass points) can be determined for any time of day at any place on the planet. Web-based sun-path calculators are readily available and they give an instant and detailed result, including sunrise and sunset times, but they do not give the whole picture in one view, as a sunpath diagram does. A description of sun-path diagrams, and an explanation of how to use them may be found in Sun, Wind and Light Architectural Design Strategies (2001, p8) and a range of northern hemisphere sunpath diagrams are included in Appendix A, (pp323–327) of that volume. Websites such as www.gaisma.com and www.jaloxa.eu provide a complete range of diagrams either by location or by latitude.

Topography and wind

Bougdah and Sharples, in Environment, Technology and Sustainability, Volume 2 (2010, pp27–34 and pp41–43) provide helpful diagrams indicating the effects of wind flow in relation to buildings and landscape. They, and Randall Thomas in Environmental Design, an Introduction for Architects and Engineers (2006, p63) provide descriptions and diagrams of the effects of windbreaks. The UK Building Research Establishment (BRE), Digest 350, Climate and Site Development Part 3: Improving Microclimate Through Design (1990) further interrogates implications and solutions in relation to buildings’ sensitivity to wind.

The United States Department of Agriculture (Natural Resources Conservation Service) has a website which contains a wealth of general and technical information on windbreak design, including appropriate plant species, and suggests that a well-designed windbreak protecting a rural building can result in energy savings in winter heating costs of 15 to 25% because cooling of the building envelope and, importantly, air infiltration are reduced. The strength of the wind matters, and Olgyay, in Design With Climate (1963, p99) tells us that the heating requirement for a house subject to a 20mph wind would be 2.4 times that of the same house in a 5mph wind. Where trees are used both as windbreaks and for summer shading overall energy savings of up to 40% have been reported in studies on Farmstead Windbreaks by Quam, Wight and Hirning at North Dakota State University (1993). Their work contains further useful information on windbreak composition and very good explanatory diagrams of windbreak designs. The effects of hilly topography on wind are discussed by Dean Hawkes et al in The Selective Environment, An Approach to Environmentally Responsive Architecture (2002, pp124–125 and 128–130) and covered comprehensively, with diagrams, by Brown and DeKay in their essential handbook Sun, Wind and Light (2001, pp17–23).

Water and landscape for cooling

The BRE document Environmental Site Layout Planning: Solar Access, Microclimate and Passive Cooling in Urban Areas (2000, pp37–43) by Littlefair et al provides references to research which confirms our intuition that bodies of water will provide a source of cooling. The larger the body of water the better is the rule. A lake might provide a reduction in air temperature in its vicinity of up to 2°C (3.6°F). However, a small pool in a garden, if it replaces materials which would otherwise readily absorb and re-radiate heat, will have a much larger local cooling effect.

Trees and cooling

Trees, which absorb radiation and cool the surrounding air, can lower the air temperature near buildings and therefore lower the indoor temperature, although there seems to be little hard evidence and few agreed rules. In Environmental Site Layout Planning Littlefair et al (2000, p40) cite evidence suggesting that the temperature will be lowered in this way for a distance of up to five times the height of an orchard, and that the temperature drop within and downwind of a forest might be as much as 6°C (10.8°F).

Green spaces in cities do not merely provide an environment for recreation but bring beneficial cooling for a considerable distance, 150m or more into the urban fabric according to Baruch Givoni in Climate Considerations in Building and Urban Design (1998, pp308–310). The urban issues are dealt with in further detail by Brown and DeKay in Sun, Wind and Light: Architectural Design Strategies (2001, pp121–124).

Trees and their leaves

The UK Building Research Establishment (BRE) Digest 350, Climate and Site Development Part 3: Improving Microclimate Through Design (1990, p6) lists the transparency of different species of tree crowns to solar radiation and the range of their foliation periods. As an example, deciduous trees in the UK will be in full leaf from about May to September and there will be a period of a month or so at each end of this period when they will be in partial leaf, before and after which they will be quite bare. This is the type of information useful to the low energy designer who can manipulate orientation, form and openings to benefit from tree shade or permit access to winter sunlight.

Further strategic design comment, along with foliation data for trees in the United States may be found in Sun, Wind and Light Architectural Design Strategies Brown and DeKay (2001, pp268 and 269), and in other countries and regions similar reference material on the characteristics of local tree species may be consulted and applied to shading and passive solar designs.

The position of a shade-giving tree in relation to a building façade is discussed by Thomas (2006, p59) in Environmental Design, an Introduction for Architects and Engineers. Pelsmakers, in The Environmental Design Pocketbook (2012, p80), proposes a slightly different rule, which places the trunk of the tree at a distance from the building of 1 to 1.5 times its mature height.

Sun and wind

A more complete explanation of global winds may be found in Matthys Levy’s (2007) Why the Wind Blows, which contains much of interest for the budding meteorologist. The Rough Guide to Weather (2007), by Robert Henson provides a good jumping-off point for all things weather-related and gives a concise history of global climatic regions and an explanation of the seasons. The characteristics of each climate region are clearly described by Baruch Givoni in Climate Considerations in Building and Urban Design (1998, pp333–441).

Why should the main façade face the sun?

Well, the closer to south (or north in the southern hemisphere) the better is the rule because, as Simos Yannas shows us in Solar Energy and Housing Design, Volume 1 (1994, p88), energy use in northern hemisphere housing doubles if windows face mainly east, west or north rather than south.

North-facing windows create a significant energy problem in temperate and cold climates and should be minimised. Randall Thomas, in his very readable environmental science primer Environmental Design, an Introduction for Architects and Engineers (2006 p58), confirms the general rule that energy performance will be optimized as long as the main façade – the façade with the greatest solar-oriented window area – is oriented to within 45° of the midday sun. Brown and DeKay, in Sun, Wind and Light: Architectural Design Strategies (2001, p167, quoting Balcomb et al: Passive Solar Design Analysis, 1984) define the limits more closely, stating that if the façade is within 30° of south the fall-off in solar performance will be less than 10%.

Shading

Well designed solar control shading can almost eliminate solar gain. Pelsmakers (2012, pi 41), in The Environmental Design Pocketbook provides examples of shading devices which would block up to 90% of solar gains. Solar control shading design is determined by the orientation of a building’s openings and relates to the time of day and time of the year when a building needs to be shaded. The best way to design solar shading is to model it, either using modelling software or by using physical models and a heliodon, a device which uses a light source to represent the sun’s elevation and azimuth at any place and time on the planet. However, for solar-oriented openings the general climate-related rules, which will do as a starting point for design, are:

• Cold and temperate regions: design a horizontal projection for a sun cut-off angle half way between the sun’s altitude at midday on the summer and winter solstices, but with a maximum projection of 1.5m. For mid-latitude regions the following formula is sometimes used:

Summer projection = h/tan(ss)

Winter projection = h/tan(ws)

where h = height of window, ss = elevation of sun at summer solstice, ws = elevation of sun at winter solstice. Design the projection to be between the two figures.

• Hot regions: use the altitude of the sun at the spring equinox to determine the depth of the overhang, but bearing in mind that some hot summer regions have cool spring and autumn periods when solar gains are beneficial

• Hot equatorial regions: here the high midday sun can be blocked with a relatively narrow horizontal projection. But the sun may also heat the equator-facing elevation in mid-afternoon for some of the year, and at a much lower angle to the horizon. In these regions the spring equinox at mid afternoon, say 3.00pm, is a better starting point for determining the projection depth. Remember that the sun will also reach the non-equator-facing façade in summer and that a combination of vertical and horizontal projections will usually be the most effective solution for shading of all elevations.

Brown and DeKay, in Sun, Wind and Light Architectural Design Strategies (2001, pp262–267), provide a set of tables which act as a short cut to the formulae and allow some sizing of projections for a range of latitudes and for solar-oriented as well as east and west facing windows. They also describe (pp57–60) a method of determining when shading will be needed, and of what type. There is a wealth of insight and practical guidance in CIBSE Day lighting and window design: Lighting Guide LG10 (1999) and much is applicable not only to the UK but to other similarly mild, temperate climates.

Spring and autumn equinox issues – to shade or not to shade?

This particular aspect of solar geometry, that the elevation of the sun is the same on both 21 March and 21 September, is often forgotten and the important differences between heating and shading needs at these times must not be missed. The excellent Passive Solar Architecture Pocket Reference by Haggard et al (2009, p24) has a good section on shading and covers this issue, as does Pelsmakers in The Environmental Design Pocketbook (2012, p141, Fig 6.5.3).

Compact form and ventilation – a dilemma

In some climate regions there is obviously a conflict between the desire for energy-saving compactness on the one hand and a more spread out form which encourages cross ventilation and increases surface area for cooling on the other. Givoni discusses this in Climate Considerations in Building and Urban Design (1998, pp50–53) and suggests that an adaptable configuration might be the answer.

Olgyay, in Design With Climate, Bioclimatic Approach to Architectural Regionalism (1963, pp87–91) argues that a slightly elongated solar-oriented form provides the best balance between heat loss and beneficial solar gain. The Oikos website’s article in Issue 36 has an interesting study of the energy benefits of compact forms, with some helpful numbers. Hall and Nicholls, in The Green Building Bible Volume 2 (2006, pp 70–73) present a good explanation of the impact of building form and surface area in which they also consider the insulation values of the fabric elements, giving clues about where insulation needs to be increased as a result of decisions about building form.

Heavy buildings heat up and cool down more slowly than light buildings

In a high thermal mass building the rate of change of maximum indoor temperature might be about half of that of the outdoor temperature. Givoni, in Passive and Low Energy Cooling of Buildings (1994, p60) has calculated that in a lightweight building, on the other hand, the rate of change of temperature might increase to 0.8 times – in other words, quite closely matching – that of the external temperature. Although each building will vary considerably depending on the construction materials and methods, in high thermal mass buildings there may be a time lag of 10 hours or more between the peak external temperature and the highest internal temperature. Both Olgyay in Design With Climate, Bioclimatic Approach to Architectural Regionalism (1963, pp116 and 117) and Thomas in Environmental Design, an Introduction for Architects and Engineers (1994, p158) discuss this important principle.

The time lag between maximum outdoor and maximum indoor temperature can be designed to occur at a useful time of night (or day). The Australian Government’s Your Home Technical Manual (2010 section 4.5) includes a very interesting table indicating the relative thermal lag times of some building materials, concluding that 250mm of concrete would have a thermal time lag of 6.9hrs, 250mm of rammed earth 10.3hrs and 1,000mm of sandy loam a very slow 30 days. In Ecohouse 2: A Design Guide (2004, p128), Sue Roaf considers that in a domestic situation each wall inside a house would need to be around 100mm thick to provide sufficient thermal mass to dampen temperature fluctuations and store heat generated within the building.

High thermal mass needs night time ventilation

Research by Givoni to be found in Passive and Low Energy Cooling of Buildings (1994, pp60–65) has shown that the maximum internal temperature of a building whose mass is cooled at night might be 3°C (5.4°F) lower than in the same building without the aid of nocturnal cooling. With a maximum outdoor temperature of 36°C (96.8°F) that potentially means a significantly more comfortable 27.5°C (81.5°F) maximum indoor temperature. In arid and desert areas the internal temperature might be 6–8°C (10.8–14.4°F) lower than the outdoor maximum. Openings therefore need to be designed to enable the cool nocturnal air to ‘scour’ the exposed thermal mass, and Givoni shows us examples of how to maximise the effect of nocturnal ventilation with the design of openings. In climatic regions where high summer daytime temperatures would place the interior outside the comfort zone nocturnal ventilation will be more effective than daytime comfort ventilation.

Thermal mass and insulation

The relation between thermal storage and insulation value is well presented by Yannas in Solar Energy and Housing Design, Volume 1 (1994, pp27 and 98). Research carried out in Alaska by Carlson and Siefert, Building in Alaska – Thermal Properties of Walls (reprinted 2002) identifies the thermal storage capacities of different wall constructions, confirming the relatively high thermal storage capacity of timber.

Thermal mass and insulation work together

Studies by Kosny et al, in Thermal Mass – Energy Savings Potential in Residential Buildings (2001) indicate that, for greatest energy savings, the best location for the insulation layer in a multi-layer wall is central. This confirms a significant body of research suggesting that thermal mass should be in direct contact with the building interior if it is to dampen internal temperature fluctuations. Furthermore, it seems walls with exposed thermal mass might reduce energy needs by almost 10% to 20% over lightweight walls of equivalent insulation value, depending upon climatic location.

Do not forget that heat transfer through a wall will occur irrespective of whether the insulation is on the outside or in the middle of a multi-layer construction – insulation does not completely halt heat flow. In the northern European climate it would be expected today that walls and the ground floor would have 200mm of insulation and roofs 300mm, and that these figures would increase for very cold regions. Haggard et al, in the Passive Solar Architecture Pocket Reference (2009, p30) give a useful table of the insulation values needed for roofs and walls in different climatic regions.

When and where to use thermal mass?

The Australian Government’s Your Home Technical Manual (2010, section 4.9) advises that where diurnal changes of 7–10°C (12.6–18°F) exist thermal mass becomes useful and where diurnal changes are more than 10°C (18°F) then the adoption of thermal mass is ‘highly desirable’. The Royal Institute of British Architects in their on-line Sustainability Hub (see Night Ventilation of Thermal Mass) also suggests 5–7 °C (9–12.6°F). Whatever the exact cut-off point, the basic rule is that the greater the diurnal temperature range the higher the quantity of thermal mass needed to dampen temperature changes.

Heavy or light weight – rules for climate

Olgyay, in Design With Climate, Bioclimatic Approach to Architectural Regionalism (1963, pp122–124) reminds us that there needs to be a balance between insulation and thermal mass and identifies the ideal locations of thermal mass in different climatic conditions. It is generally the case that in hot-humid locations lightweight construction is preferable but Givoni, as detailed in Climate Considerations in Building and Urban Design (1998, p400) has experimented with thermal mass in hot-humid conditions and he concludes that temperature stability will be improved if thermal mass can be employed with effective nocturnal ventilation. Dean Hawkes et al, in The Selective Environment, An Approach to Environmentally Responsive Architecture (2002, p39) confirm that homes in cold climates should use thermal mass as a heat store for evening comfort.

Heavy or light weight – rules for occupancy

Pelsmakers, in The Environmental Design Pocketbook (2012, p148), states that lightweight buildings without thermal mass should only be proposed for buildings with intermittent use, and holiday homes are cited as an example. A building in constant use is more compatible with a heavyweight construction as heat is retained for use.

What we wear and what we do influences comfort

Acceptable temperature limits for low humidity regions are defined by Givoni in Climate Considerations in Building and Urban Design (1998, pp38–39) who proposes 18–25°C (64.4–77°F) in winter and 20–27°C (68–80.6°F) in summer. He also provides a very useful summary (on pp5–22) of the human responses to the thermal environment and confirms that the rate of heat exchange between humans and their internal environment will depend greatly upon clothing and physical activity. Givoni’s research confirms that the internal temperature can be significantly lowered without occupant discomfort if people wear more (or more appropriate) clothing. Pelsmakers, in The Environmental Design Pocketbook (2012, p136) reminds us that the temperature at the feet should remain at or above 19°C (66.2°F) and that discomfort will be experienced if the temperature at foot level reaches 3°C (5.4°F) lower than at head level. For hot-humid climates Givoni proposes elevating the acceptable upper temperature by 2°C (3.6°F) because people become acclimatized to such conditions. In a fascinating piece of research by the BRE Productive Workplace Centre, Nigel Oseland, in Adaptive Thermal Comfort Models (1998, pp41 and 42) defines the temperature benefits of various activities and of putting on or removing various items of clothing: putting on or taking off a collar and tie results in a net temperature benefit of 0.8°K, changing office chair type 0.3°K, consuming a cold drink 0.9°K.

Windows and glass

Glass is an important and complex topic in relation to energy use in buildings: too much glass, or the wrong glass, and poorly placed windows will result in buildings being too hot in summer and too cold in winter. Glass is a poor insulator compared with many other building materials: in winter it readily allows heat from within to escape. A triple-glazed window might halve the heat loss of a double-glazed window, but it takes around 50% more energy (embodied energy) to make a triple-glazed window. It is therefore essential to correctly apply the properties of windows and glass and many authors have covered this topic.

In The Environmental Design Pocketbook (2012, pp203 and 204) Pelsmakers discusses thermal performance and compares the performance of double and triple glazing (see p144), in doing so confirming that triple glazing improves thermal comfort in a building because the temperature near to the inner pane is closer to room temperature, limiting surface temperature differences which humans experience as discomfort. In fact, during a very cold winter day, in a room heated to 21°C (69.8°F) the temperature inside close to a high performance double-glazed window might be 16°C (60.8°F) while the near-surface temperature will be 18°C (64.4°F) with a triple-glazed unit. Haggard et al, in the Passive Solar Architecture Pocket Reference (2009, p35) consider the different functions of apertures in buildings and discuss the architectural implications. They also usefully tabulate (p36) the thermal and daylighting properties of different window types.

The key properties of glass which are of interest to the low energy designer are:

• Overall U-value, the measure of resistance to heat transfer of the window, including glass and frame. The lower the value the better.

• g-value, or Solar Heat Gain Coefficient (SHGC), the measure of the ability of the window to permit solar gain. The higher the g-value the greater the solar gain will be. A single glazed pane will be best, but will have a poor U-value.

• Low emissivity (low-E), the coating which in cold regions or seasons is used to prevent heat from leaving a room. It can also be employed in hot climates to prevent heat entering a room. The key is which surface of the glass the coating is applied to. The surfaces in a double glazed unit are numbered from the outside to inside, 1, 2, 3 and 4 etc.

• Shading coefficient (SC), a measure, relative to a single-glazed window, of the amount of heat passing through a window. The lower the figure the greater the ‘shading’. Beware that often glazing with a low SC is tinted and thus transmits significantly less daylight.

• Daylight transmission value (Tvis), the measure of the amount of daylight transmitted by a window assembly. The higher the value the more daylight will enter the building.

The various properties have to be balanced in relation to the specific climate region and the orientation of the building façades.

Fabric first: insulation, infiltration and windows – the golden rules for retrofit

Because the rules of thumb are universal many apply equally to retrofitting of existing buildings and to new-build projects, and this rule is particularly relevant to retrofitting. Pelsmakers, in The Environmental Design Pocketbook (2012 pp229–238) includes a chapter on retrofitting existing housing and considers the payback period for various measures. It is widely acknowledged that the most cost-effective energy-use reduction measure, other than lifestyle change, is to insulate the loft and the cavity wall of a traditionally constructed house in a temperate or cool climate, with a payback period of less than 5 years. Replacing single-glazing with high performance double or triple-glazing will have energy-use benefits (cutting energy bills by up to 40% over single-glazing in cold climates) and the payback period will be 15–30 years.

Heat losses through the fabric of a typical dwelling in a temperate climate – that is a house that has not been specifically designed for minimum energy use – are discussed by Bougdah and Sharples in Environment, Technology and Sustainability (2012, pp61–63) and they, and Brian Edwards in The Rough Guide to Sustainability (2007 p62) provide a version of the generally acknowledged heat loss percentages through different fabric elements. The Australian Government’s Your Home Technical Manual (2010, section 4.7) on insulation gives an indication of the heat gains through the same fabric elements in a temperate climate. Remember, materials with a high thermal mass will slow heat flow, but they will not halt it.

Reducing heat loss – earth sheltering

The temperature of the ground under the surface fluctuates with the seasons, but at about 15m depth the ground temperature stabilizes at around the mean annual air temperature which, according to The British Geological Survey, is 8–11 °C (46.4–51.8°F) in the UK. This is the origin of the very rough rule of thumb, often quoted in discussions about underground temperatures, which says that the temperature below the surface is 10°C (50°F), and that very general rule is applied in many temperate and hot-summer/cool-winter climate zones. The temperature remains reasonably constant because heat moves very slowly through the high thermal mass of the earth. The reality is that at depths of less than 15m the earth temperature will fluctuate. An article on earth-sheltered homes by Rob Roy, director of the Earthwood Building School, on the website Mother Earth News (October/November 2006) describes building an earth-sheltered home 3m under ground in New York State in the USA. The earth temperature is said to be 4°C (39.2°F) in early March and 16°C (60.8°F) in late August, a useful indication of the beneficial heat and coolth storage capabilities of the earth. Earth-sheltered structures, like green roofs, need to be insulated in most climate regions, to hinder migration of heat from the structure into the surrounding earth. A useful ‘climate suitability matrix’ may be found in Sun, Wind and Light: Architectural Design Strategies (2001, p56) by Brown and De Kay and there is further explanation of the design and operation of earth shelters, with case studies, in The Green Studio Handbook by Kwok and Grondzik (2007, pp169–174).

Green roofs, and climate change

From Randall Thomas’ Environmental Design, an Introduction for Architects and Engineers (2006, p158) there is evidence of a time lag of 12 hours between maximum external and internal temperatures in an office building with a green roof with 500mm of soil, 100mm of insulation on a 150mm concrete slab. If the interior slab is exposed internally then the roof acts both to reduce heat loss and balance temperature changes. A thick green roof is known as intensive whereas shallower depths are also possible and these are known as extensive green roofs. An extensive green roof will only support low-height and resilient planting. Kwok and Grondzik in The Green Studio Handbook (2007, p52) note that for small trees 1m of soil depth will be needed, and perhaps 1.8m for large trees. Remember that the additional load of a green roof must be taken into account in structural design.

The UK Environment Agency, in the report, London’s Warming: The Impacts of Climate Change updated in May 2012, predicts that summers in 2050 will be between 1.5 and 3.5°C (2.7 and 6.3°F) hotter than today and that the urban heat island effect (UHI), which currently adds 5 to 6°C (9 to 10.8°F) to summer night time temperatures, will strengthen. Coupled with an increased incidence of sudden heavy rainfall and the lack of oxygenating vegetation in our cities, green roofs are seen as one of the means to develop buildings which aim to mitigate the impacts of climate change. Figures from the UK Environment Agency suggest that 4–6 kWh/m²/yr of energy savings can be obtained by using a green roof and this translates as between 7 and 17% of the space heating energy use of a well insulated house.

Reducing heat loss – internal shutters

Yannas, in Solar Energy and Housing Design, Volume 1 (1994, p84) calculated the performance of insulated internal shutters and concluded that in a 24 hour period if they were closed for 12 hours they would provide an average U-value (insulation value) equivalent to double glazing. In buildings which have 24 hour occupation (say a home/office) the shutters would have to be designed to admit daylight if day time heat losses were also considered, and this presents an interesting problem for innovative designers to tackle. The effectiveness of heavy curtains is disputed by some, prescribed by others. Hall and Nicholls, in The Green Building Bible Volume 2 (2006, p36) propose thick curtains as a substitute for insulated shutters.

Are we an indoor species?

Dr Nick Baker has written a fascinating paper, ‘We are all Outdoor Animals’ (on The Daylight Site) in which he reminds us that until quite recently the human species spent most of its time outdoors. Now, according to statistics from the US Environmental Protection Agency in Buildings and their Impact on the Environment A Statistical Summary (2009, p4) we spend more than 90% of our time in buildings, placing obvious demands on energy consumption. More than half of the carbon-derived energy used in a home in a temperate climate is used for space heating, so the sun’s power should be harnessed to make a very valuable contribution. The BRE in Exploiting Sunshine in House Design (1988, pp53–59) states that for a typical house in a temperate climate increasing the amount of double glazing to a south facing façade will result in only small energy savings, and shading must be used to prevent overheating in summer if the window area exceeds 20% of the wall area. And, as we are reminded by Dean Hawkes et al, in The Selective Environment, An Approach to Environmentally Responsive Architecture (2002, p142), passive solar buildings aim to optimise solar-oriented glazing and minimize openings in the opposite façade.

Cold rooms, which are not solar-oriented (north facing in the northern hemisphere) may benefit from direct gains by virtue of a careful manipulation of the section, and north facing windows should be restricted to 10% of the elevation area. The BRE, in Exploiting Sunshine in House Design (1988, pp53–59) provides evidence to suggest that in a temperate climate a typical, relatively high mass construction (say masonry) will provide enough thermal mass to dampen diurnal temperature swings by way merely of the internally exposed masonry walls.

Direct, indirect and isolated solar gain

The different ways in which the sun’s power may be harnessed in passive solar architecture are described by many authors. Haggard et al, in the Passive Solar Architecture Pocket Reference (2009, pp9 and 10) provide handy explanatory diagrams and a brief appraisal of the application and performance of six systems. Hall and Nicholls, in The Green Building Bible, Volume 2 (2006, pp39–41) delve further into the complexities of passive solar collectors.

Direct solar gain

In direct solar gain solutions the sun is permitted access into the building, falling on interior surfaces which then re-radiate heat. Thermal mass is usually used in conjunction with direct solar gain and the Australian Government’s Your Home Technical Manual (2010, Section 4.5) advises that for maximum effectiveness the exposed area of a high thermal mass floor in a direct solar gain design should be around 6 times the area of the window which exposes it to the sun. The orientation of the window is important and should be within about 30° of the midday sun. There is an energy balance for solar gain and heat loss between solar-oriented and non solar-oriented windows. Hall and Nicholls, in The Green Building Bible, Volume 2 (2006, pp34–37) rightly conclude that effective solar collecting windows need to have a low U-value (high insulation value) and be capable of being insulated at night (see previous section on internal shutters).

The sunspace

From Brown and DeKay’s work, Sun, Wind and Light: Architectural Design Strategies (2001, pp171 and 172) we can deduce that in a temperate climate the area of glazing of a south-facing sunspace (north-facing in southern hemisphere) should be between 10% and 20% of the floor area of the building to be heated. With correct orientation (i.e. solar-oriented, or within about 30°) and summer shading a sunspace might, according to various sources including the European research project EASEE: Education of Architects in Solar Energy and Environment, contribute between 15% and 30% to energy savings.

Often a sunspace appears to perform poorly due to lack of understanding, by designers and building users, of how it operates throughout the seasons:

• In summer it needs to be shaded and ventilated to the outside – not open to the inside if hot outdoor air is to be excluded

• On winter days it will act as a buffer zone and, on sunny days, even as a short-term heat source if it is closed to the outside and open to the inside

• On sunny autumn and spring days it will provide much useful heat to adjacent spaces if it is open to indoors and closed to the outside

The presence of thermal mass in the sun space will help in controlling temperature swings and a high performance glass should be used for good insulation values. Performance will be further improved if the sunspace is insulated at night.

Warm air moves towards cool air

This is true whether we are considering outdoor air moving towards indoors or air moving within and between spaces in a building. Haggard et al, in the Passive Solar Architecture Pocket Reference (2009, p7) provide a useful chart of the heat sources and sinks which operate in the exchange of energy between people and their environment.

The Trombe wall

The Trombe wall, named after the French engineer Félix Trombe, a pioneer in solar engineering, takes advantage of the fact that glass permits the passage of short wave infra-red radiation which in this case heats a high mass wall behind the glass. The wall works as follows:

• The sun heats the wall

• Heat transfers from the wall, with a time lag, to occupied spaces behind by conduction through the wall

• At the same time a ventilation stack develops between glass and wall due to heated air rising

• Vents allow controlled convection of warm air, without a time lag, from the solar-heated stack into the occupied spaces behind

A Trombe wall needs to be at least 1m high to create an effective stack, according to Hall and Nicholls in The Green Building Bible, Volume 2 (2006, p39). They discuss this rule because a constraint which results from the use of Trombe walls is the loss of views, but below-window-sill storage walls are a possible solution. A rule of thumb for the required area of a Trombe wall is difficult to define because the subject is location-sensitive. However, for a house, a figure of 10% of the total floor area (as for glazing area for a sunspace) would be a reasonable starting point and for a 1,000m² two storey building perhaps half that figure.

Some useful guidelines for climatic regions in the United States appear in Brown and DeKay, Sun, Wind and Light Architectural Design Strategies (2001, pp171—175), who confirm that the high mass wall element of the Trombe wall should be 300–400mm thick, or 250–350mm if no vents are introduced. There is no clear rule about the dimension between glass and wall, but in most built examples of Trombe walls the dimension is minimal, enough for construction only. Since the Trombe wall is likely to be built in regions with cold nights the glass outer skin is a source of heat loss and should be insulated at night.

The water wall

Prof. David Bainbridge, in articles published at Motherearthnews.com, provides us with an abundance of first-hand detail on water wall design and construction. He has pointed out that the water wall may also be used for free cooling as long as it is positioned such that breezes may carry the stored coolth into the building and it is he who has given a practical indication of the volume of containers to be used. There are examples of water containers being used as heat/coolth stores in a clerestory, avoiding the problem of view blockage, and such a solution is illustrated in Chapter 5 of this book as a tool for cold climate strategies.

The pond roof

The Skytherm House in Atascadero in California, USA, was the test-bed for this heat storage method. Movable ‘bladders’ of insulation were devised, making night-time insulation of the heat store possible. Night-time insulation is crucial to avoid heat radiating outwards into the cold night sky rather than being conducted down through the structure, with a time lag, into occupied spaces. The pond roof may also be used for cooling and in this case the pond must be shaded and insulated during the day. Design information may be found in Brown and DeKay, Sun, Wind and Light: Architectural Design Strategies (2001, pp176–177). There is a useful review of the main design themes and the available literature by Spanaki titled, Comparative studies on different type of roof ponds for cooling purposes: literature review, (2007) and Givoni, in Passive and Low Energy Cooling of Buildings (1994, pp152–163) provides detailed scientific study data on roof pond experiments in various locations.

Earth pipes with fluid

This solar energy system relies on the sun having warmed the earth. Coils of pipe are filled with a refrigerant fluid which extracts heat from deep in the ground where temperatures are relatively stable, at around average surface air temperature. A ground source heat pump (GSHP) upgrades the temperature of the fluid into low grade heat, which is adequate for heating water for an underfloor heating system. The heat source might also be used to heat air. Bougdah and Sharples, in Environment, Technology and Sustainability (2010, pp113–115) provide a thorough overview of different types of heat pumps and their operation. They define the rule of thumb for length of pipe needed as 30m length for each 1 kW of energy to be extracted from the earth. Kwok and Grondzik, in The Green Studio Handbook, Environmental Strategies for Schematic Design (2007, pp131–134) examine the design implications of various configurations and set out a sample sizing of a system. Downsides to coil and GSHP systems include the fact that GSHPs are powered by electricity (which might, though, be sourced from the sun) and that only low temperature hot water (at around 40°C [104°F]) can be produced. Too much overlapping or crowding of pipe coils reduces the efficiency of the system because too much heat will be drawn from the earth and the sun might not be able to replenish it over a long period. The efficiency, measured as Coefficient of Performance (COP), of ground source heat pumps is improving each year.

Heat recovery

The idea is to recover heat from rooms which generate it, such as kitchens and bathrooms and to move that heat to rooms which might need it, such as living rooms and bedrooms. Year-round mechanical ventilation with heat recovery (MVHR) should only be adopted for airtight and super-insulated buildings because otherwise air is being exchanged directly with the outside, and this is a premise of the Passivhaus standard of energy efficiency. The standard, which is now being applied in many countries, sets limits in new buildings of:

• space heating energy maximum 15kWh/m²

• airtightness 0.6 air changes per hour

• fabric U-value maximum 0.15 W/m²/K

• window U-value maximum 0.8 W/m²/K

There are also Passivhaus standards for retrofit, known as the Passivhaus: EnerPHit standards and a useful summary of the standards may be found in the The Environmental Design Pocketbook by Pelsmakers (2012, pp222 and 223).

Inter-seasonal heat/cool storage

Inter-seasonal storage is not a new idea but there is renewed interest in its potential for low energy solutions in both small scale, single building and large scale district heating applications. A number of working examples exist, including the pioneering Freiburg self-sufficient solar house project by the Fraunhofer Institute for Solar Energy Systems. The house, built in 1992, used a hydrogen-based seasonal heat store and proved that inter-seasonal solar storage could work. In Italy, the University of Calabria in the late 1990s created a successful water-based inter-seasonal store to heat an office building, the water reaching a temperature in October of 70 to 80°C (158 to 176°F) and remaining at a functional temperature through the winter. A summary of these, and other, experiments may be found on the Fujita Research website. Water, if contained in a highly insulated vessel, has benefits over many other storage media, including its large capacity to store heat and its inert physical properties. Pelsmakers, in The Environmental Design Pocketbook (2012, p286) considers that 250 litres would be a suitable tank volume for a typical home. Phase-change materials, which store and release energy as they pass from liquid to solid and back, are another medium for inter-seasonal heat storage. A large installation of phase-change materials was adopted by the City of Melbourne for its innovative Council House 2 (CH2) project.

Natural ventilation using wind and bouyancy

It is important to remember that with natural ventilation the internal environment will not be controlled within a narrow comfort band. The CIBSE manual Natural Ventilation in Non-domestic Buildings (2011, p4) confirms that temperatures will exceed 25°C (77°F) on occasion. However, with the measures discussed in other sections of this book, such as solar shading, use of thermal mass and nocturnal ventilation, the amount of time a building will exceed thermal comfort conditions will be limited.

Single sided ventilation

In rooms with window openings to one side only the opening acts as both the inlet and outlet for air. Thomas, in Environmental Design, an Introduction for Architects and Engineers (2006, p125) is just one author who confirms the rule of thumb that single sided ventilation will work up to a maximum room depth of 6m. Bougdah and Sharples, in Environment, Technology and Sustainability (2010, p83) comment on the nature of the window in a single sided ventilation scheme, confirming generations of experience that a window with both high and low openings – such as a traditional vertical sash window – is the most effective for summer ventilation as cool air is drawn in at low level and warm air, rising by stack effect within the room, will be exhausted at high level. A further advantage of high and low level openings is that they will have some effect even during times of still air because the buoyancy of air will create a stack. Givoni, in Passive and Low Energy Cooling of Buildings (1994, p46) tells us that if a room has a single window the size of the window will have little impact on the internal airflow, although the impact is greater if the wind is oblique to the wall. Ghiabaklou, in the paper ‘Natural Ventilation as a Design Strategy for Energy Saving’ (2010, p319), concludes that external vertical fins (wing walls) projecting from one side of the opening will improve the ventilative performance of a window.

Cross ventilation

Cross ventilation will work whether the wind is normal (at 90°) to or obliquely aligned with the façade openings. Some researchers have concluded that cooling by cross ventilation will be most effective if the inlet is smaller than the outlet. Ghiabaklou in Natural Ventilation as a Design Strategy for Energy Saving (quoting Sobin, 2010, p317) proposes that the ideal ratio of inlet to outlet is 1: 1.25, and Givoni, in Passive and Low Energy Cooling of Buildings (1994, p48) states that such an arrangement provides the highest air speeds. Flow rates will be determined by the smaller of the two openings. There is agreement on the fact that the bigger the openings are the greater the flow and speed of air will be. In Passive and Low Energy Cooling of Buildings Givoni (1994, pp26 and 27, and pp 46–49) concludes that because air speeds are higher when the inlet opening is small and the outlet large, such an arrangement might be best suited to rooms such as bedrooms, where the location of desirable cooling is known. Large inlets, on the other hand, lead to better distribution of internal air speeds and so might be best suited to spaces in which people might occupy all parts, such as living spaces. Brown and DeKay, in Sun, Wind and Light: Architectural Design Strategies (2001, p65) provide a table which can be used to determine the area of ventilation opening, as a percentage of floor area, related to wind speed and cooling capacity.

Stack ventilation

Air inside a building will commonly be warmer than outside air and the natural buoyancy of warmed air will induce a stack effect. The effectiveness of a stack will be proportional to the square root of temperature difference between top and bottom of the stack. Thomas, in Environmental Design, an Introduction for Architects and Engineers (2006, pp125 and 126) ponders the problem that there is a lack of detailed knowledge about stack ventilation, but he provides some guidance on the area of inlets and outlets as a percentage of floor area. In the RIBA’s on-line Sustainability Hub, in the section on natural ventilation, Dr Nick Baker provides a useful step-by-step design tool for stack ventilation as well as an in-depth explanation of its operation and application. Solar chimneys, which use the sun’s heat to drive a column of air within an enclosed shaft, need to be one storey higher than the top floor to be ventilated.

Cooled earth

Givoni, in Passive and Low Energy Cooling of Buildings (1994, pp15–17) experimented with cooled earth solutions and concluded that the difference between outdoor maximum air temperature and the temperature of the cooled earth could be 12–14°C (21.6°-25.2°F) in arid climates and 10–12°C (18–21.6°F) in humid regions. The implication is that the cooler earth provides a sink to cool ventilation air. There are different methods of cooling the earth, shading being the key one. Cooling using water is likely to be restricted to regions and locations with a plentiful local water supply, but given increasing global concerns about water supply and demand this method seems limited in application.

Free cooling – pond roof

Experiments in Mexico by Givoni, detailed in Passive and Low Energy Cooling of Buildings (1994, p155) concluded that a pond roof, shaded in daytime with floating insulation, could modify the outdoor maximum temperature by up to 13°C (23.4°F), from an outdoor maximum of 35°C (95°F) to an indoor maximum of 22°C (71.6°F), which compares with an internal temperature of 33°C (91.4°F) without the pond roof. Further information and references may be found earlier in this narrative bibliography, in the section on the pond roof as free a source of heat.

Earth cooling pipes with air

Van Lengen, in The Barefoot Architect (2008, pp240–242) considers the use of underground ventilation pipes in dry tropical climates. He suggests using 100mm diameter clay pipes buried at around 2m depth. Givoni, in Passive and Low Energy Cooling of Buildings (1994 pp214–218) has experimented with earth cooling tubes and provides much detail on his own and others’ work. Van Lengen states that there are no rules for the length of pipes, but although currently information is limited, there are now some emerging rules:

• Bury pipes at least 1.8m deep to take advantage of stable, lower earth temperatures

• Use 100–300mm diameter pipes of a material which is resistant to fungal growth

• There is some evidence that cooling ability peaks at pipe lengths of 50–70m although hundreds of metres may be needed for larger buildings

• The pipes may be used in reverse to heat incoming air if the soil temperature is higher than desired internal temperature

• Earth pipes are not generally advisable in hot-humid climates due to condensation within the pipes contributing to internal humidity as well as concerns about the pipes harbouring fungal spores

• Earth pipes are unlikely to be cost-effective as the only cooling method due to the extent of pipes and excavation needed

Further information and useful references can also be found at www.builditsolar.com

Sue Roaf et al, in Ecohouse 2: A Design Guide (2004, p132) indicate that the air in the earth cooling pipes may be circulated by a fan, which can be powered by a photovoltaic panel.

Using daylight to reduce energy use

The relationship between the amount of daylight available outside and the amount that will be distributed through window openings into a room is known as Daylight Factor (DF). Kwok and Grondzik, in The Green Studio Handbook, Environmental Strategies for Schematic Design (2007, pp57–61) discuss the importance of Daylight Factor and although each country has its own standards of daylighting requirements the authors make a suggestion, for design purposes, of the factors which might be aimed at for different building and room types. Thomas, in Environmental Design, an Introduction for Architects and Engineers (2006, pp98 and 99) points out that the designer needs to find a balance between the amount of daylight being introduced and the consequences of designing with excessive glazing, such as unwanted heat losses or heat gains. Both Thomas and Kwok and Grondzik re-state the generally understood rule that if a room has a Daylight Factor at the working plane which exceeds 5% the room will appear well daylit but at less than 2% it will seem gloomy. Somewhere in between will result in a daylit room in which the need for artificial light during working hours is minimised.

Window size for sidelighting

Much research points to the fact that the area of a window below the height of the working plane makes little or no contribution to the daylighting of the room. Thomas, in Environmental Design, an Introduction for Architects and Engineers (2006, p100) confirms the rule of thumb equation for the relationship between Daylight Factor, room area and window area: average DF = 20(Ag/Af)%, where Ag = area of horizontal glazing, Af = area of floor. If, in order to minimise the times when artificial lighting will be needed we want to achieve a 2.5% Daylight Factor in a domestic living room of 20m² the window area will have to be 2.5m², which would be around 25% of the wall area, slightly more than the overall figure of 20% recommended for low energy performance. This confirms that it is hard to achieve a uniform distribution of an adequate quantity of daylight in a room and that calculation and modelling (either physical with a heliodon and artificial sky and/or by using modelling software such as IES) are needed. However, the rules of thumb provide designers with a sound basis for decision making.

Height of window and depth of room

The problem with daylight is that it diminishes very rapidly as it travels from the window into the depth of a room. Haggard et al, in the Passive Solar Architecture Pocket Reference (2009, p67) provide handy sectional diagrams which explain the key means of maximizing the reach of daylight into interior space, which are generally agreed to be:

• Increased ceiling and window height

• Adding a light shelf

• Sloping the ceiling downwards into the room

• High reflectance of internal finishes

Thomas, in Environmental Design, an Introduction for Architects and Engineers (2006, p98) also reminds us that tall windows in higher rooms result in a greater throw of light deeper into the room, and greater uniformity in the distribution of daylight. The following equation is commonly used to determine the appropriate relationship between room dimensions and window height: L/w +L/h ≤ 2/(1-Rb) where L and w = respectively length and width of the room, h = the window height above the floor and Rb is the weighted reflectance of the room (typically taken to be 0.5). If the left side of the equation is not less than or equal to the right side then the room will be poorly daylit.

Rooflights and light from above

Because lighting levels of about 5,000 lux exist directly overhead on an overcast day it is sensible to consider toplighting where possible, remembering that solar control and heat loss must be managed. Randall Thomas, in Environmental Design, an Introduction for Architects and Engineers (2006, pi 00) states that the average daylight factor for a rooflight is: DF = 50(Ag/Af)% where Ag = area of horizontal glazing, Af = area of floor. If a daylight factor of 5% is desired in a room of, say, 20m² then the area of the rooflight glazing needs to be 2m². Using the previous rule of thumb equation for sidelighting the same room would need 5m² of glazing and very careful attention to reflectance, colour, room shape and window design, which indicates the significant advantage of rooflights. A wealth of further information relating to many aspects of toplighting, with useful diagrams, can be found in the Daylighting Guide for Canadian Commercial Buildings (2002, pp32–38). Pelsmakers, in The Environmental Design Pocketbook (2012, p144) proposes a maximum of 12% rooflight area to floor area as a means of limiting heat losses. In hot climates, with high sun angles heat gains are a challenge to be overcome by glazing specification, by limiting rooflight area and by shading.

Haggard et al, in the Passive Solar Architecture Pocket Reference (2009, p68) consider the roof monitor, which is a vertical rooflight form commonly adopted as a ‘northlight’ (in the northern hemisphere) for factories, workshops and studios. They propose, though, to turn it around to face the sun (equator-facing), pointing out that in this orientation the area of glazing can be reduced by up to about 4% of the floor area, and the space can benefit from winter solar heating. A key benefit to opposite-of-equator-facing roof monitors is that they do not admit the sun with its attendant problems of colouration and glare (light which comes from the opposite of the equator is neutral in colour and consistent throughout the day), and so the ‘northlight’ is favoured by craftsmen and artists. However, credence should be given to the idea of an equator facing, solar controlled roof monitor with modern glazing technology and inventive control of glare and uniformity, especially if useful winter solar heat gains can be harnessed.

Light-coloured interiors, reflectance and window area

White paint will provide a reflectance of 85%, whereas a light grey paint will give 70% and dark carpet 10%. Silver & Mclean, in their Introduction to Architectural Technology (2008, p102) state that internal surfaces should have reflectance values of at least 50% for walls, 70% for ceilings and 30% for floors. There is a mathematical relationship between the reflectance of interior surfaces and the window-to-wall (WWR) ratio. When compared with a light room, the WWR will need to be almost double for rooms with dark finishes. A good explanation of this (and other daylighting rules), with some useful diagrams is to be found in the Daylighting Guide for Canadian Commercial Buildings (2002, p27). The relationship between room depth and interior reflectance is expressed in the equation L/w +L/h ≤ 2/(1-Rb), where L and w = respectively length and width of the room, h = the window height above the floor and Rb = the weighted reflectance of the room. This usually assumes a Rb of 0.5, and if the left side of the equation is less than or equal to 4 then the full depth of a room with windows to one wall will be uniformly daylit.

Exterior colour influences energy use

Givoni, in Passive and Low Energy Cooling of Buildings (1994, pp30–33) studies the external colour of a building and its influence on energy use. A reflective, light-coloured building envelope is not only a highly effective but also an inexpensive solar control device, since colour generally involves no extra cost to a project. The effectiveness increases in places with greater diurnal temperature swings, such as would typically be found in arid regions. The colour of a northern façade (south-facing in the southern hemisphere) has little impact on energy consumption in temperate mid-latitude climates, but in regions where the powerful early morning and late afternoon sun reaches the opposite-to-equator-facing elevations colour once again becomes a significant issue.

Olgyay in Design With Climate, Bioclimatic Approach to Architectural Regionalism (1963, pp86 and 87) confirms that the roof of a building receives more solar radiation in summer than any other surface, regardless of latitude, making this a significant element in the fabric heat gain equation. The idea of the ‘cool roof’, which reflects solar energy and reduces cooling needs, is a topic of much current research. Energy savings of at least 10% are possible by incorporating cool roofs in hot climate regions or those with hot summers. The EU Cool Roofs Council rightly points out that there may be a winter heating energy penalty in climates which have hot summers but mild winters, because in achieving summer reflectance the free winter heating capacity of the roof would be lost. This presents the low energy designer with a dilemma. There is therefore also research into building products which can change state – and colour – to vary the solar reflecting/absorbing qualities of roofs and façades depending on the season.

Keeping it simple

In The Environmental Design Pocketbook (2012, pp26 and 136) Pelsmakers considers the effect of occupant behaviour on the performance of buildings, and one theme is the intuition which people have towards their environment: it is counter-intuitive in the northern European climate on a hot day to seal a building with closed shutters, windows and doors against heat because we prefer to encourage ventilation. But this would be the correct response. The main theme, though, is that we should ensure our buildings are simple to operate and that users should have control over their environment.

Give clear instructions

A new building will usually have what is known as a maintenance manual, handed to the client upon completion. The manual contains, amongst other things, the instructions and warranties, and repair and maintenance procedures for the equipment and controls built into the project. What is not often provided, however, is a clear operating manual for control of the environment. Without such essential information, written in a form which the users of the building will understand, the ways occupants can manipulate a building to provide comfort with minimal energy inputs will never be learned.

Hot-dry climate

Found at latitudes between 15 and 30° north and south of the equator, the main focus in this climate region will be on summer cooling but some hot-dry climate zones experience comfortable winters and some can expect very cold winters, so in some cases the winter condition also needs to be addressed, and a heating system might be needed. Givoni, in Climate Considerations in Building and Urban Design (1998, pp333–342) provides a detailed commentary on this climate region and on the designer’s response. Maximum daytime temperatures might reach 50°C (122°F). It is assumed that summer night time ventilative cooling can be achieved because most hot-dry regions experience large diurnal temperature swings with night cooler than day.

Adoption of the strategies indicated in the rules of thumb diagrams could result in maximum summertime indoor temperatures of up to 10°C (18°F) lower than the outdoor maximum. Van Lengen, in The Barefoot Architect (2008, pp224–266) provides further information on methods for cooling in hot-dry regions and a particularly informative section on wind towers and wind catchers. Along with shaded courtyards, carefully planned ventilation openings, earth cooling pipes and light colours he identifies further, traditional cooling methods. The windcatcher, an ancient Persian invention, also features in Rudofsky’s Architecture Without Architects (1981, figs. 113, 114 and 115). The windcatcher works by guiding summer breezes into the shaft at night, for nocturnal cooling. A shutter, or baffle, within the tower is closed during the day to prevent the ingress of hot air. In the winter in hot-dry regions with plentiful sun the scoop faces the sun and the shutter is open during the day to allow in air which has been warmed by the thermal mass of the tower. In winter the shutter is closed at night to prevent heat loss. Further examples and descriptions may be found in Brown and DeKay’s Sun, Wind and Light Architectural Design Strategies (2001, pp188–190).

Hot-humid climate

Found at latitudes of 10 to 15 degrees each side of the equator, these climate regions typically experience a fairly constant annual average temperature of around 27°C, with highs of about 30°C (86°F), or more on clear days. High relative humidity (80–90%) is experienced all year and rainfall is also high. Some regions enjoy the trade winds (easterlies) and some are subject to potentially destructive typhoon or hurricane winds.

The preoccupation in this climate is to provide effective ventilation with minimum heating of the building envelope. Haggard et al, in the Passive Solar Architecture Pocket Reference (2009, pp61 and 62), while confirming that high humidity makes these the most difficult regions in which to provide comfort conditions, particularly in summer, provide us with instructive sectional diagrams explaining ventilation strategies, including for high-rise buildings. Van Lengen, in The Barefoot Architect (2008, pp145–147) adds suggestions on the implications on roof form for effective cooling by ventilation. Givoni, in Climate Considerations in Building and Urban Design (1998, pp 379–402) dedicates a chapter to hot-humid climate zones and his excellent explanation of the varying characteristics of these regions includes a discussion of the dilemma that in regions with a beneficial, easterly prevailing wind the powerful early morning sun is also in the east and must be intercepted before it enters east-facing windows if overheating is to be avoided and ventilation optimised.

Cold climate

In this book a cold climate is one with cool, comfortable summers and average autumn/winter/spring temperatures of around 0°C (32°F), and this follows Givoni’s definition in Climate Considerations in Building and Urban Design (1998, p417). In these climatic regions the designer’s focus is on reducing the need for heating energy in the winter, and a significant influence on winter temperatures is wind chill. Bougdah and Sharples, in Environment, Technology and Sustainability (2010, p14) group cold and polar climate regions, in which average winter temperatures might be -15°C (5°F) or below. Givoni, in Climate Considerations in Building and Urban Design (1998, pp417–430) addresses the cold climate at both the building and urban scale and makes reference to super-insulation and to direct solar gain, emphasising the importance of orientation. These are factors which apply in both cold and polar conditions, and the rules of thumb work for both too.

Cold-winter/hot-summer climate

There are places which experience cold or very cold winter conditions and warm or hot and humid summer conditions, and these need to be treated differently from the merely hot or cold regions. The cold-winter/hot-summer climate regions currently occur on, and inland of, the eastern seaboards of China, Japan and the USA and also in many continental locations on earth, where the tempering influence of the sea does not exist. Givoni in Climate Considerations in Building and Urban Design (1998, pp431–441) examines this distinct climate type and, as with the other regions, provides us with building and urban design considerations. Givoni also proposes the notion of a changing building form which could provide an increased envelope for cooling in summer and a reduced, compact form in winter.

Temperate climate

Van Lengen, in The Barefoot Architect (2008, pp270–293) considers the temperate climate region as one in which the goals are to keep heat out in summer and prevent heat loss in winter. In other words, the designer’s focus is to reduce winter heating needs and prevent overheating in summer, and this is confirmed in a useful comparison table by Hyde in Climate Responsive Design (2000, p57). The result is that many of the rules of thumb come together in a strategy which also takes advantage of free heating in winter and buffers living spaces against cold winds and lack of sun. Van Lengen in The Barefoot Architect (2008, p279) introduces a fireplace into his temperate climate strategies, and as long as there is a sustainable source of fuel, and airtightness is not a primary goal, this seems a sensible provision. Haggard et al, in the Passive Solar Architecture Pocket Reference (2009, pp58 and 59) provide information about glazing areas and areas for thermal mass in temperate (and other) climatic regions in their consideration of passive heating.