Wind speed

As the speed of the wind at a site constantly varies, wind speeds are quoted as standardised speeds for a height of 10 m above ground blowing across open country. That standardised wind speed is then adjusted to site conditions, taking account of:

• * The terrain – effective wind speeds will be lower in urban and suburban areas than rural areas
• * Altitude – wind speeds are generally greater at higher altitudes
• * Position relative to orographic¹ features such as hill and escarpments, and proximity to the sea

Wind speed can be expressed in several different ways:

• * Ten-minute mean wind speeds with a 50-year return period (that is, there is a 2% probability of that speed being exceeded in any one year) are used in structural calculations because they express the greatest forces likely to be encountered
• * Mean annual wind speeds are representative of conditions experienced throughout the year and are therefore used in ventilation

For any location, the wind speeds used for structural calculations will be many times greater than those used in ventilation calculations.

Wind direction

The wind speed at a site will vary with the wind direction. The analysis of wind loads and ventilation pressure differences should therefore consider the forces produced when the wind blows from each the eight cardinal wind directions (N, NE, E, SE, S, SW, W, NW). For larger or more sensitive buildings (e.g. a naturally-ventilated commercial building) it may be necessary to calculate forces for every 30º segment.

Most sites have a prevailing wind direction (the direction the wind blows most frequently), which is affected by large-scale weather patterns and local topography. The wind forces produced by the prevailing wind are of particular importance because they represent the commonest conditions the building will experience.

Wind loads and forces

Once the site wind speed has been calculated for eight or twelve directions the air pressures and wind loads acting on the building can be calculated. The methods vary for different types of structure (and for the assessment of ventilation forces), but in outline:

1. The wind speed is used to calculate a peak velocity pressure (or the ‘dynamic pressure’, which is effectively the same thing), taking account of turbulence produced by neighbouring buildings, terrain and exposure.
2. The peak velocity pressure is multiplied by an external pressure coefficient (C_pe) or an internal pressure coefficient (C_pi) to obtain the pressure on the external or internal surface.
3. Finally, the pressure on a surface is multiplied by its area to obtain the wind load on that surface.

The wind forces acting on the building depend on the size and shape of the building and its position relative to the wind direction. Windward surfaces, such as walls facing directly into the wind, will experience positive pressures and therefore positive wind loads, while the sides and leeward faces will experience negative pressures and therefore negative wind loads (that is, suction). This is illustrated in Figure 2-03, which shows the varying intensity of air pressures on the same simple building illustrated in Figure 2-02.

Figure 2-03 Areas of Positive and Negative Pressure Produced by Wind Action on a Building’s Surfaces

Assessing infiltration

The susceptibility a building to air infiltration is expressed by its air permeability, that is, the volume of air which passes through each square metre of its exposed surface in an hour with a pressure difference of 50 Pa between inside and outside. It is expressed as m³/m².h at 50 Pa (cubic metre per square metre per hour at fifty pascals).

The air permeability is measured by pressure testing, which is carried out using portable fans fitted to a doorway or other opening. The openings of the building are closed and all vents sealed, then the fans are used to raise the air pressure within the building. The rate of air flow generated by the fans is measured at several pressure differences, then the air flow at the standard pressure difference of 50 Pa can be calculated.

The overall rate of air exchange for a building or space is usually given in air changes per hour (ach). One air change per hour notionally represents the replacement of all the air in the space by an equal volume of incoming air. So, 2 ach for a space with a volume of 60 m³ represents the movement of 120 m³ of air. There is no simple relationship between the air permeability rate and ach, because one is based on the area of a building’s envelope and the other on its internal volume.

The rate of air infiltration will depend on the air permeability of the fabric and the environmental conditions, including wind speed and direction, and the internal and external temperatures. Rates vary enormously between buildings: a house built to meet the Passivhaus standard⁴ requires an air change rate less than 0.6 ach, while in the UK some existing dwellings have air change rates well over 20 ach.

Figure 2-07 shows the reduction in heating energy which can be achieved by reducing air infiltration for a number of building types.

Figure 2-07 The Effect of Infiltration on Heat Loss and Energy Use

Data derived from SAP 2012 for dwellings and iSBEM for non-dwellings

Limiting infiltration

Minimising infiltration requires careful attention to the design and construction of building elements, junctions between elements and service penetrations:

• * Lightweight aggregate blockwork is permeable and requires a parge coat of plaster to seal it. In contrast, structural insulated panels (SIPs) have low permeability because they have large, unbroken sheet facings bonded to insulation, with a limited number of junctions between panels.
• * Cavities within construction should be avoided wherever possible, because introducing a cavity provides a route for air movement.⁵
• * Junctions between elements should be designed without unnecessary gaps and cavities, and should be sealed, with tapes and sealants matched to the range of differential movement likely to occur. Laps are easier to seal than butt joints.
• * Service penetrations offer significant scope for air leakage, as pipes, ducts and cables often run through elements. The optimum solution is to avoid penetrations wherever possible. Where that is not practicable, penetrations should be properly sealed, particularly at vapour control layers, air barriers and other membranes.
• * Effective sealing of pipework and recessed light fittings is likely to require special components.

Beneficial air movement in air spaces

There are three conditions where air currents within air spaces can be thermally beneficial:

• * Dynamic insulation systems for cavity walls – consisting of rigid insulation boards containing vertical channels. Heat moving through the wall from the building interior warms the air within the channels, creating a stack uplift. The warmed air rises to a collector at the top of the wall to be fed directly into the occupied space or used in conjunction with a positive input ventilation system.
• * Transpired solar collectors – consisting of micro-perforated cladding panels with a cavity behind them. The panels are warmed by solar gain, so air drawn through the perforations is also warmed. The air, which is further warmed by heat transferred from the building interior, rises up the cavity and is collected at the top and drawn into the building’s ventilation system.
• * Trombe walls – consisting of thermally massive walls faced with glazed cavities. Solar radiation passes through the glass and is absorbed by the wall, raising its temperature. Unventilated trombe walls rely on conduction to transfer the heat into the occupied space, while ventilated trombe walls have top and bottom vents which allow warmed air from the glazed cavity to reach the occupied space.

Figure 2-08 The Effects of Air Currents in Cavities on Heat Loss in Cold Flat Roof

Respiration

The primary requirement of air for building occupants is that it is suitable for respiration, having the right levels of oxygen and carbon dioxide. The minimum concentration of oxygen for respiration is about 12% at normal atmospheric pressure, while carbon dioxide concentrations should not exceed 0.5%. Expired air contains approximately 16% oxygen and 4% carbon dioxide, so the main challenge when designing a building’s air supply is to provide sufficient fresh air to keep the carbon dioxide concentration below the maximum exposure level, and preferably below 0.25%.

The air supply required to maintain those levels varies with the metabolic rate of the building occupants, which is, in turn, proportional to the activity level. Table 2.1 gives typical values. The rate of air flow provided by a ventilation system is usually quoted in litres per second per person (l/s per person). The ventilation rate required for a building or space is typically given in ach (air changes per hour). The air change rate can be combined with the volume of the building to define the rate at which air must be delivered by the ventilation system.

Providing suitable air change rates may mean that different ventilation strategies are needed for different parts of the same building.

Table 2.1 Air Supply Rates to Limit Carbon Dioxide Levels

Pollution

While pollutants such as gases, vapours, dust, fibres and biologically viable material (spores and bacteria) are present in the atmosphere to a varying extent, there are many activities within buildings which can produce pollutants in potentially harmful concentrations. Industrial and manufacturing processes can produce a wide range of pollutants, ranging from dust in sawmills to the emission of volatile chemicals in paintshops; office photocopiers and laser printers emit ozone and volatile organic compounds; underground car parks have high rates of carbon monoxide emission. Table 2.2 gives the UK’s exposure limits for the commonest pollutants.

Table 2.2 Maximum Permitted Concentrations of Key Pollutants

Potentially harmful concentrations of pollutants in buildings can be addressed by:

• * Removing or controlling the source of pollution (e.g. changing a production process)
• * Extracting the pollutant at the point of generation (e.g. fitting an extract hood to a cooker or an industrial saw)
• * Filtration to remove the pollutant from the air (e.g. filtration of incoming air where there is heavy traffic outside the building)
• * Reducing the concentration of the pollutant to safe levels by diluting it with incoming air (e.g. providing a large supply of fresh air in a workshop where volatile organic compounds are given off during manufacturing)

Odours

Human beings are sensitive to a wide range of airborne odours; some pleasant, some unpleasant. Within buildings people are subject to many odours, including body odour, toilet odour and cooking odour, with the mix largely dependent on the function of the building. The perception of odours, together with their intensity and quality – and whether or not they are acceptable – rests with individuals and cannot be measured directly.

Moreover, the perception of an odour – pleasant or unpleasant – changes over time. Sensitivity lessens over the first half an hour of exposure; over weeks and months it may come to be regarded as normal. The ventilation rates required to address unpleasant odours are therefore based on the perception of someone coming into the space from outside.

For spaces where the occupants are mainly sedentary a rate of 10 l/s per person will be sufficient (presuming the space to be a no-smoking zone).

Smoking

Tobacco smoke requires specific consideration, because it is both unpleasant and harmful: it causes irritation of mucous membranes in the respiratory system and eyes, while longer term exposure increases susceptibility to respiratory problems and, of course, lung cancer.

Air supply levels to counteract tobacco smoke have been based on considerations of odour for a non-smoker entering a space where smoking is permitted. A rate of 15 l/s per person is recommended for a space with typical number of smokers, rising to 40 l/s where all occupants are assumed to be smokers. The ventilation rates required to avoid mucous membrane irritation are approximately a quarter to a half of the rates for odour control.

Bans on smoking in public places and work places means that the need to provide ventilation for smoking is not as significant as 20 or 30 years ago.⁶

Moisture levels

High moisture levels in buildings can adversely affect comfort, induce problems that have an adverse impact on health, such as mould growth and house dust mites, and damage the building fabric (see chapter 3: Moisture and the building fabric p. 83). Water vapour forms part of the air within a building, so the main mechanism by which moisture travels is air movement.

Moisture levels may be controlled by drawing moisture-laden air out of the building as close as possible to the point of generation (e.g. in bathrooms, or laundry/cooking areas), while introducing drier air from elsewhere in the building. The necessary rate of air flow will depend on the rate of moisture generation (see chapter 3: Human activity p. 80).

Setting air supply rates

The consideration of each aspect of air supply for the health and comfort of building occupants results in a range of requirements for minimum rate of air flow: the requirements for respiration being the lowest and the requirements for the control of odours being the highest. In practice, providing the air supply necessary for the control of odours will meet most of the other requirements. The exceptions, where a higher air supply rate may be required, are:

• * Spaces where pollutants are generated
• * Spaces where moisture is generated

New buildings – passive protection measures

Passive measures limit the transmission of radon from the ground to the occupied space. They commonly involve the installation of a radon-resistant membrane, typically low-density polyethylene (LDPE), across the entire footprint of the building as far as the outside face of the external walls. (This is sometimes referred to as a radon barrier.) The construction details will vary, but groundbearing concrete floors, such as that shown in Figure 2-10, can incorporate a gas-resistant membrane between the slab or deck and the screed.

New buildings – active protection measures

Where higher concentrations of radon are likely, ventilation is required below the radon-resistant membrane to draw radon from beneath the building and reduce the pressure of radon against the underside of the membrane.

Figure 2-10 Passive Radon Protection Measures on a Groundbearing Concrete Floor

Providing ventilation beneath a suspended floor is straightforward, as shown in Figure 2-11. Groundbearing floors will require collection chambers, referred to as radon sumps, which are vented to the atmosphere: pre-fabricated plastic sumps are commonly used.

Figure 2-11 Active Radon Protection Measures for a Suspended Floor

Active systems should include provision for retro-fitting of an extraction system (mechanical or passive stack) which can be installed if high radon concentrations were to be found inside the building after construction.

Existing buildings

It is rarely possible to fit a radon-resistant membrane to an existing building. Remedial measures are therefore designed to:

• * Reduce radon pressure beneath the floor – by fitting a radon sump under a solid floor or by increasing the amount of ventilation beneath a suspended floor. A radon sump can be formed from the outside by making a hole in the external wall below ground level, digging out a small cavity beneath the floor, then fitting an extract pipe which discharges at roof level. The best results are obtained with fan-powered systems.
• * Prevent radon reaching the building interior – by installing positive pressure ventilation which forces air into the building and eliminates the normal under-pressure which draws radon into the building. Sealing gaps and cracks in floors can also reduce radon transmission, but may result in the decay of timber floors;
• * Remove radon from the interior – by increasing ventilation rates. This will only be of limited benefit.

Cross-ventilation

Cross-ventilation (Figure 2-12) relies on differential pressure generated across the building by the wind: air enters the building on the high-pressure windward side and is drawn out on the low-pressure leeward side. The rate of air exchange depends on the wind speed and direction, and the size and location of vents. As a rule of thumb, cross-ventilation will be effective where the depth of the space does not exceed five times the floor-to-ceiling height.

Typically, cross-ventilation to provide background ventilation will be provided by trickle vents above openings or incorporated into window and door frames, which will give about 0.1–0.2 ach. Open windows on opposing sides of a building will also provide cross-ventilation, and are often used to address overheating in summer. The necessary window area is usually specified as a fraction of the floor area, with 5% of floor area appropriate for sash windows or hinged lights opening more than 30 degrees, and 10% for those opening between 15 and 30 degrees. Fully open windows can provide 6–8 ach.

The direction of the prevailing wind should be taken into account; however, even when the wind is blowing parallel to the ventilation openings pressure differences across the building will still produce some air movement. Where ventilation openings are positioned at different heights (e.g. on different storeys) there will be some drive from the stack effect, which may reinforce or counteract the wind forces. Effective cross-ventilation requires a good flow of air between different sides of the building, so will be less effective in buildings where the interior is divided by many walls or partitions.

Single-sided ventilation

Rooms and spaces with openings to only one side, or with limited opportunity for cross-ventilation, can be naturally ventilated by exploiting the air movement generated by temperature differences between the inside and outside of the building (see above: The stack effect p. 46). Air movement can take place through twin wall vents where one is near floor level and the other is close to the ceiling, or, to a more limited extent, at the top and bottom of a large open window. (A large traditional sash window with top and bottom sashes opened will give a similar effect to twin vents.)

In the summer, when the internal air is cooler, air will be drawn in at high level and drawn out at low level. In the winter, when external air is cooler, air will be drawn in at low level and drawn out at high level (see Figure 2-05). The ventilation rate achieved by single-sided ventilation depends on the temperature difference and the distance between the upper and lower openings. There will also be some air exchange driven by the wind. On some faces of a building the wind force may be the predominant driver, in which case the ventilation rate will be determined by the wind speed.

Single-sided ventilation will be effective where the room depth does not exceed twice the room height. Trickle vents can provide 0.1 ach, while fully openable windows (sized to the rules given above) will provide 4–6 ach.

Stack ventilation

Stack ventilation (also referred to as ‘passive stack ventilation’) uses the stack effect to move air up and out of a building at high level, drawing fresh air into the building at lower level (Figure 2-12c). The temperature differential between air inside the building and outside the building results in the upward drive (see above: The stack effect p. 46), and wind action across the ventilation terminal also draws air out. The stack effect has been exploited in this way for thousands of years (see box: The wind tower p. 68).

Stack ventilation can be used as part of a whole building natural ventilation system, either using shafts to ventilate buildings several floors high, or simply using multistorey spaces such as atria as the stacks. Figure 2-13 shows how a central atrium can be used to ventilate the floors to either side of it, venting air from the top and drawing air into each floor. The performance of such systems depends on stack height, temperature difference and the free area of vents. Typically, the opening at the base of the stack should be 3–4% of the floor area.

Stack ventilation is often used instead of extract fans to remove moist air from bathrooms and other wet rooms, through vertical ducts which terminate above the roof. The rate of extraction is variable, because it depends on the temperature differential, which will be less in summer, and the wind speed.

Controls

Natural ventilation relies on climate-driven forces so the ventilation rates achieved will be variable. Controlling such systems is, in most cases, a matter of opening and closing vents.

For dwellings and buildings of similar complexity, manual operation of vents and windows is generally sufficient; although where moisture control is the main function of the ventilation system, humidity controlled vents may be used. In larger buildings, controls can be integrated into the building management system, but are still limited to opening and closing vents, albeit in a sophisticated way.

Supply-only ventilation

In a supply-only ventilation system (Figure 2-14a) air is drawn into the building to a central air-handling unit and circulated by ducts to different spaces in the building. The building is maintained at a higher air pressure than outside (the systems are often described as ‘positive pressure systems’), so exhaust air is expelled through passive vents. Air can be filtered in the central air handling unit to remove contaminants before it is circulated, and it can also be heated or cooled.

Basic supply-only systems are often used in dwellings, while more complex systems which include filtration are used for operating theatres and clean rooms because the positive pressure prevents contamination from the rest of the building, and allows air to be filtered and conditioned before it is circulated.

Extract-only

In extract-only ventilation systems (Figure 2-14b) air is drawn out of the building or room either by centralised fans and ducts, or by individual fans. As the air pressure in the building is then lower than atmospheric pressure (commonly referred to as ‘negative pressure systems’) fresh air is drawn into the building through vents, louvres and other permanent openings.

Extract-only ventilation is well suited to addressing moisture levels, because such systems extract moist air at the site of generation, preventing it from travelling through the rest of the building. The extraction system can include filtration, which enables pollutants to be removed from extracted air before the air passes into the atmosphere, so preventing the further spread of pollutants.

Balanced ventilation

In balanced ventilation systems (Figure 2-14c) air is drawn into one or more central air handling units then distributed throughout the building by ducts to air outlets. Simultaneously, air is extracted from other outlets to the air-handling unit and passed out of the building. The use of separate input and output ductwork means that moist or polluted air can be extracted close to the point of contamination, which helps to prevent distribution through the building, and allows fresh air to be provided to specific locations

Balanced ventilation systems will often include a heat exchanger, to minimise the heat loss resulting from extraction and will often form part of full conditioning systems. The possibility of heat exchange makes balanced ventilation popular in dwellings with low energy use targets.

Mixed-mode ventilation

Mixed-mode ventilation is a hybrid approach which integrates natural ventilation (through openable windows and vents) with mechanical ventilation systems. Mixed-mode ventilation operates in three main ways:

• * Complementary – The mechanical ventilation runs at the same time as the natural ventilation is working. Often, the mechanical system provides the background ventilation, while building occupants can open windows if they want to. Alternatively, the mechanical system may be used to augment natural ventilation when weather conditions result in low flow rates.
• * Changeover – The ventilation provision changes between natural and mechanical according to need. The changeover may be seasonal, with windows openable in mild weather but locked shut in winter; or daily, with natural ventilation during the day and mechanical ventilation at night to provide cooling.
• * Zoned – Some spaces in the building are naturally ventilated while others are mechanically ventilated. For example, open-plan office areas may be naturally ventilated, while meeting rooms – which have higher ventilation requirements – are mechanically ventilated.

Mixed-mode ventilation can reduce energy consumption and allow smaller mechanical systems to be installed. It also improves the building occupants’ satisfaction by giving them more control over their conditions. However, it is more complex to specify and requires careful commissioning to ensure natural and mechanical systems do not work against each other.⁹

Ventilation effectiveness

The effectiveness of a ventilation system measures how well it delivers the supply air to the building’s occupants. A system which extracted the supply air before it reached the occupants would have an effectiveness of 0 (this can happen if a supply and an extract grille are located too close together), while a system which mixed the supply air fully with the room air before it was breathed by the occupants would have an effectiveness of 1. Displacement ventilation, which introduces cold air close to floor level and extracts warmed exhaust air at ceiling level, has an effectiveness greater than 1.

Air currents

Ventilation air introduced into a building will not be at room temperature (although it will not necessarily be at the temperature of outside air). Vents and inlets should therefore be sited so as to prevent building occupants being exposed to draughts of cold air. However, even if the incoming air is well mixed, air currents can, of themselves, affect the perceived temperature: currents of air travelling at 0.1 m/s can be felt by the building occupants and will lower the perceived temperature of the air. At air speeds greater than 0.15 m/s the temperature of the air should be raised to counteract the perceived cooling. Air speeds greater than 0.3 m/s will cause discomfort and are unlikely to be acceptable, except for summer cooling.

Noise

A ventilation system which provides a direct connection for air movement between the interior and exterior of a building will also provide a route for sound transmission. Where external noise levels are high – for example because of traffic – sound transmission through vents and ducts may be disturbing to the building occupants.

For some projects the problem may be avoided by the use of acoustic vents which will attenuate the noise (see chapter 4: Controlling noise from outside p. 113), but in some locations the likely disturbance may be so great that a ventilation system which forms a direct connection between inside and outside will be unacceptable.

Ductwork can also transmit sound between rooms and zones, causing a nuisance and, in some cases, threatening privacy. The noise from the fans that are part of a mechanical ventilation system can also be transmitted through out a building via ventilation ductwork. Attenuating ductwork or silencers may be fitted to reduce noise levels.

Heat loss

The deliberate movement of air into and out of a building for ventilation purposes will result in heat transfer, which may result in a heat gain to the building (typically in the summer), or heat loss (mainly during winter).

To reduce the contribution of ventilation to heat transfer, mechanical ventilation systems can include heat exchangers, which transfer heat from warm exhaust air to the colder fresher air which is being drawn into the building. Heat exchangers are rarely compatible with natural ventilation, because they have too great an effect on the air flow.