Prefabricated building units and modern methods of construction (MMC)
Abstract:
This chapter identifies what constitutes prefabricated buildings and offsite construction, and provides an overview of both old and new systems that are emerging under the banner of modern methods of construction (MMC). It hypothesises that the construction industry has been labelled as being reluctant to change, due in part to a ‘materials led’ approach to building design and delivery; and how government legislation is forcing change engendering a ‘building design led’ culture which seeks to satisfy future building needs and assure sustainability. It suggests that knowledge sharing and partnering are prerequisites for providing swift and effective sustainable design advancement by maximising the use and skills of industry experts across many previously unrelated fields.
18.1 Materials led building design
Buildings and shelters have evolved using readily available, locally sourced materials such as stone, timber, clay, etc. Use of these natural materials as fundamental building elements has developed empirically over time, increasing in skill and complexity, giving rise to ‘materials led’ rather than ‘building design led’ advancement. This approach to building has left a legacy, where the construction industry has been criticised as ‘reluctant to change’ and historically tied by robust technologies. However, with global needs pressing governments to impose ecologically challenging codes and guidelines, the construction industry is being forced by legislation to change; reluctance can no longer be a barrier to that change.
The construction industry has a major role to play in delivering an environmentally sustainable future by providing responsible building design. Driven by legislation, this is forcing the design of new materials to fulfil specific functions. As a result buildings (or parts of buildings) are increasingly becoming ‘building design led’ rather than ‘materials led’.
Sustainable delivery throughout the life cycle of the building is as much about quality, maintenance, reliability and running costs as it is about designing with the right materials. It is little use maximising material choice and detail design if poor construction techniques or interfacing ruins the design integrity at the very beginning of its life cycle.
Offsite construction principles, often referred to as ‘modern methods of construction’ (MMC), enable the designed building, panel or component to be constructed without compromise, providing an environment where repeatable performance, minimal waste and high levels of quality can be assured. This enables buildings to be designed holistically, without needing to ‘dumb down’ the detailing to suit the available skill base, so promoting the design and development of materials and systems which rely upon accuracy and control during construction, to deliver the intended design life and energy map for the building.
Whether referred to as offsite, prefabrication or modern methods of construction, generically this provides an environment in which a ‘building led design’ approach can be exploited fully. The factory environment is also predisposed to look beyond the horizon of the immediate building project to consider standardisation of the processes, methods, materials and design.
18.2 Offsite construction
Mtech Consult has developed the following definition based on the BuildOffsite Glossary of Terms (2006):
Offsite Construction includes all those elements of construction that involve the pre-assembly (joining together or prefabrication) of a number of construction components, which would otherwise be traditionally incorporated in the works as separate components.
There are many good reasons to manufacture a building in a factory rather than making it on site:
• Reduced preliminaries due to shorter time on-site.
• Reduced project management costs – manufacturer will supplement these skills with directly employed project managers.
• Reduced design/professional fees – a large proportion is absorbed into manufacturers’ overheads.
• Reduced programme times leading to earlier return on capital employed.
• Reduced defects and rework due to high first-time quality.
• Provide controlled procurement strategies.
These principal benefits drive the wish to prefabricate buildings and elements of buildings in factories. The extent to which people can benefit from these principles and hence the extent to which prefabrication or Offsite is applicable depends on three basic factors: the physical design of the building, the preferred local methods of working and, most relevant to this book, the materials that are used. It is much easier to create an offsite prefabricated box element of a building in a factory if you make that box from timber or steel than it is if you make it from masonry or stone.
Prefabrication and industrialisation of construction methods have been available for some time. From the Crystal Palace or hospitals in the Crimean War, right back to traditional oak frame buildings, ‘offsite’ has long been a small but successful part of the construction industry. In the past, prefabrication has been used periodically to satisfy short-term needs where solutions were required quickly and in bulk, but after the need passed there was generally a return to onsite techniques.
After the Second World War, prefabricated buildings were used to respond successfully to the UK’s need for rapid replacement of housing stock at a time of acute materials, labour and site-based skills shortages. Later, fully finished classroom units were manufactured in response to the upturn in educational needs in the 1960s. Furthermore, the 1960s witnessed the development of industrialised high-rise buildings using pre-cast concrete structural elements. This specific building technique was associated with some dramatic failures and has stigmatised developments in the offsite sector. Recently, various organisations in the UK have started to evaluate and develop new construction methods that involve an increasing amount of work being undertaken offsite and incorporating a high level of innovation. Modern offsite technology distinguishes itself from the ‘prefabrication’ label by utilising modern materials, employing robust quality control systems, testing and evaluating products through managed R&D programmes and demonstrating durability and regulatory compliance verified by third party endorsements such as the British Board of Agrément (BBA) approvals or BRE certification.
In recent years the measure of ‘benefits’ perceived as being gained from offsite methods has focused primarily on cost savings. Traditional methods of cost comparison are deeply embedded in the existing culture of the construction industry. They are based on initial purchase cost rather than whole life costs or best value procurement and tend to put offsite at a disadvantage. To measure the real cost through the life of a building, it is essential to consider the direct and indirect benefits of using offsite solutions.
Due to a shortage of skilled workers in onsite construction, UK labour costs are likely to rise in future at rates higher than the average for manufacturing. In recent years the cost of all construction in Europe will have remained lower than might otherwise have been the case due to the influx of cheaper European labour. As this influence reduces and the labour cost proportion of construction costs increases, the balance of cost competitiveness will swing in favour of offsite.
Offsite construction can reduce the amount of waste generated onsite from between 40% for simple panelised systems, to 90% for more complete modular building (Wrap, 2007). This waste reduction is not just a shift of waste from site to factory because in lean, offsite manufacturing processes the waste generated and sent to landfill is less than 1% of the total weight of material used. This is achieved with the control system inherent to manufacturing processes which allow better control of the waste generated and promote direct re-use and re-cycling at source. Mtech (Wrap 2007) shows that, in the worst cases, 40% of waste is saved onsite and only 1% is generated at the factory. Waste saving starts from the initial design approach, where waste is ‘designed out’ by optimising the use of materials, rationalisation of the design loads, and better controlling the supply chain.
Offsite manufacturers use their supplier relationship to go beyond the traditional construction procurement approach by involving their suppliers in the reduction of waste from packaging, rationalisation and optimisation of the products to suit the manufacturing process (e.g. plasterboard delivered to fit whole walls). This extends to co-operation between manufacturers and suppliers where waste from the manufacturer is re-cycled or even re-used by the suppliers.
Waste management and improvement systems (e.g. ISO 14000; Kaisen approach) involve employees at all levels in reviews of processes and methods to make them more effective and more cost efficient. Part of the exercise is to identify how to cut wasted material, time and energy! The same studies have demonstrated that the total use of energy is at least 25% better for offsite typologies when compared to the same construction activities carried out on construction site (traditional approach). This considered all the energy used for the maintenance of the buildings, the manufacturing process and the transport to and from the factory for deliveries of supplies and finished goods to site (including journeys by employees).
However, the challenges for offsite are equally substantial, and not the least of these is to devise a solution which balances the manufacturer’s need to make a repetitive solution many times with an industry which generally designs a specific solution for every building.
18.3 Standardisation in construction
Mtech Consult has developed the following definition based on the BuildOffsite Glossary of Terms (2006):
Standardisation is the widespread and systematic use of processes and components with repeated regularity in construction and engineering projects with the conscious objective of achieving optimum economy, quality and functionality.
Unitisation and standardisation of layout and design increases the opportunity for offsite prefabrication of buildings and building components. When repeatable processes are incorporated in offsite production, additional time and resource can be applied to the design process, development, testing and manufacturing set-up, all of which reduces the unit cost and increases the product performance.
This impetus for standardisation must not outweigh product saleability in a blind drive for a high degree of repetition at all costs. Even where standard buildings are not suitable, standardisation of bathrooms, kitchens, tank/boiler cupboards and other elements can realise much of the benefit. These are typically the high value and labour intensive parts of a building but standardisation principles can be applied to any part of the construction where it will create a sufficient volume of repetitive components to make offsite manufacture more viable.
These are the principles increasingly being applied in all industries, often described as ‘mass customisation’, which enables them to reduce design, manufacturing and tooling costs as well as simplifying the processes and reducing the reaction times to new trends and demands. The benefits of this are seen all around us with the reducing cost and improving quality of most of the everyday products we come in contact with and the same approach is what will allow us to see similar benefits in the built environment.
18.4 Types of offsite construction
The following definitions provide a summary of the more common offsite construction types, demonstrating the breadth of approaches being adopted to transfer existing construction processes from onsite to offsite. Offsite is not limited to the prefabrication of volumetric or large scale components; manufacture of complex components or building elements which are highly repetitive also benefit from being factory assembled.
This list is far from exhaustive; it is intended to establish a common, basic understanding of the offsite construction sector, its building systems and its technologies. New technologies are emerging constantly from around the world as creative minds turn their attention to new and better ways of creating the built environment.
18.4.1 Volumetric buildings
These are buildings supplied as a 3D element, volume or box.
Portable and relocatable buildings
These are designed to be easily dismantled, moved and relocated. They may be modular (made up of a number of building slices) or single volumes (where they are transported as complete buildings). They are mostly designed as temporary or semi-permanent accommodation and have a relatively short lifespan of 15–30 years, due to choice of materials rather than workmanship (Fig. 18.1). Typical uses are toilets; office accommodation; canteens; site storage and offices, so internal finishes are generally hard wearing, prepainted steel or pre-vinyl covered plasterboard.
Volumetric (modular) building systems
These are offsite manufactured buildings that are permanent with a lifespan from 30 to over 60 years. They are generally comparable in performance to traditionally constructed buildings. Certain designs can be fully relocatable (Fig. 18.2). Volumetric modular building systems are currently used in a wide range of sectors including:
• Education – classrooms, storage, laboratories and staff facilities
• Leisure and hotels – rooms with en-suite bathrooms
• Healthcare – wards, laboratories and highly serviced facilities
• Commercial and industrial – garage forecourts, shops, office accommodation, storage facilities, workshops
• Housing – private and social housing
• MoD – barracks and officers’ living quarters
Volumetric modular building systems are generally provided with a temporary external finish prior to leaving the factory and then re-clad and finished onsite by a specialist sub-contractor. However, some systems have factory applied, fully weathered, external cladding, but jointing and some degree of site applied finishing is usually required. Internal finishes are factory applied, generally to a more advanced level, such as tape and jointed plasterboard or plasterboard with a skim finish and may well include factory applied paint coatings.
In the more prestigious/higher quality modular buildings, there is a trend to use lightweight thin screed and concrete floors, supported on lightly profiled metal decking trays and floor joists.
Park homes and caravans
Leisure homes have little resemblance to early static caravans. Today, residential park homes are designed for permanent, holiday, retirement or second home living and look like traditional detached bungalows. Manufactured in factory-controlled environments, usually offering standard ranges of styles and layouts, constructed using a variety of system types, including steel or timber frame and structural insulated panels (SIPs), these buildings offer a high performance specification for their cost. They are completed offsite with kitchen, bathrooms, plumbing, heating and electrical fit out, then transported to their final destination, where they are sited and then connected to mains services.
18.4.2 Pods and room modules
These are a hybrid form of construction where ‘whole rooms’ are prefabricated in the factory but the rest of the building can be finished with either traditional or offsite panel construction.
Pods
These are small, complete volumetric rooms, constructed using light steel frame (LSF), timber, concrete or glass reinforced plastic (GRP), fitted out in a factory including all cabling and pipework, then installed into a pre-engineered space in a building onsite. Pods are commonly used for bathrooms, shower rooms, washrooms and for kitchens. Pods may be non-structural or structural load bearing, depending upon the overall building design strategy (Figs 18.3 and 18.4).
Room modules
Larger and more complex pods are often referred to as ‘room modules’ and may be used for washrooms, hotel rooms or single living accommodation for students or the military. These are self-contained rooms, manufactured offsite with their own superstructure. Again, where possible, all mechanical and electrical cabling and pipework will be factory fitted, allowing final connections onsite.
18.4.3 Panel systems
These are where the elements are supplied in 2D flat panel format.
Light steel frame
These are build systems that use cold-formed steel as the primary structural material, typically formed into storey height panels in the factory, or sometimes stick built onsite. In practice, light steel frame panels are designed to be maintained in warm and dry conditions and corrosion is avoided. This is known as the ‘warm frame’ principle (Fig. 18.5).
Timber frame
These are stick build or panellised systems that use timber as the primary structural material. They consist of pre-engineered frame elements, fixed together to erect a skeletal structure that is then enclosed and finished onsite (Figs 18.6 and 18.7). These are normally delivered in flat pack form, comprising walls, floors and roof panels.
Timber frame construction has won wide acceptance amongst UK developers, specifiers and the social housing market as an effective, cost efficient and environmentally sustainable method of construction:
• ‘Open cell panel’ systems are bare panels of studs with an external sheathing board, delivered to site to have the cladding, insulation, services and finishes installed in situ.
• ‘Filled cell panel’ systems are panels inclusive of insulation, electrical and plumbing services and vapour control layer to support and protect the insulation during transport – but with no internal lining applied.
• ‘Closed cell panel’ systems are pre-engineered panels, inclusive of linings, insulation, electrical and plumbing services, doors and windows.
Structural insulated panels (SIPs)
This is a form of construction used in panel building systems. Structural sandwich panels, typically comprising a core of foam with plywood, oriented strandboard (OSB) or cement bonded particleboard skins, are joined to form the superstructure of the building. Some manufacturers provide panels inclusive of service ducts and voids for cables, but in most cases, mechanical and electrical services are positioned within service voids created by battens fixed to the panels.
SIPs are well established in the United States and a substantial market is being developed in the UK, initially introduced in housing. They are increasingly used in general cladding, education and hotel construction. The glued joints and insulated core of these SIPs naturally result, in good airtightness, thermal and structural efficiency, although care must be taken at connections and around windows and doors to ensure that the detail is good enough to maintain the levels of good performance (Fig. 18.8).
Concrete panel
Concrete can be precast in a mould in a factory to form a sheet or panel material. Typically the bottom, mould face has a smooth fine surface and, with the addition of suitable reinforcement, these panels can be designed to create a panelised system for the construction of offsite buildings.
Single skin walls will always have just one fair face and are well suited to external cladding applications where insulation and other finishes need to be applied to the other face, but forming double skin walls with two fair faces provides a solution for internal walls. Floors can be pre-stressed planks or solid. Joints between wall panels, floors, the flights and landings of the staircases are supported and interconnected by means of support angles, reinforcement and concrete infill and grouted joints (Fig. 18.9).
Applications have included student accommodation, hotels and prisons as concrete can command a premium in applications where robustness has a real value.
18.4.4 Mechanical and electrical services
As the mechanical and electrical (M&E) element of a project can reach 30% or more of the timescale and cost, it is an area that can gain maximum benefit from the offsite approach. Offsite M&E services reduce the number of activities onsite and help speed up the build programme.
At the smallest scale, offsite starts with domestic M&E solutions such as snap-fit electrical wiring solutions and push-fit jointless plastic plumbing systems. More recently there has been an increasing trend toward the prepackaging of boilers, water heaters and storage cylinders into a single unit. Ideally services should be designed so that they could be integrated into one end of a bathroom pod and have the pod supplier complete this element with the primary plumbing to a bathroom (or kitchen).
In order to achieve the higher levels of sustainability targets, there has been a resurgence in the use of centralised combined heat and power (CHP) plants, often run on renewable bio-fuels. Centralised packages can be supplied as part of a modular plant room, complete with distribution networks which can be pre-assembled offsite.
In another vein are modular stair products, a pair of stair flights packaged in a steel frame with robust handrails, landings and half landings all in place from the factory, which allow safe, permanent access to upper levels to be installed as the frame of the building is erected. More recently this thinking has developed to include the lift shaft which can be pre-fabricated offsite to take another key item off the critical path.
18.4.5 Future trends and materials development
In traditional construction most materials perform one role. The structure is distinct from the insulation which is separate from the weatherproofing. Outside of the construction industry, this would be highly unusual, as most manufactured products and components perform more than one role to maximise efficiency and effectiveness of both the materials and the processes. Offsite is one area of construction where this multiple functionality already exists.
An example of this is the use of composite panels for both volumetric and panelised forms of construction, such as SIPs and precast insulated concrete panels. Here, the combination of an external grade sheet material, such as ply, OSB or steel, adhesively bonded to an insulating core material and a suitable internal lining, creates a wall, floor or roof component with many functions. It can provide a water and weatherproof layer; it can take structural axial and racking loads; by the use of the right materials it can provide fire protection and reduce the passage of sound through the fabric of the building; it can provide insulation and airtightness with the minimum of cold bridging and it can, in some cases, provide a visually acceptable, robust surface for the user.
Although composite panel technology has been used for many years, the development of new materials and the tailoring of materials to the specific needs of the system provide plenty of scope for improved performance. These opportunities include:
• improved adhesive bonding to create stronger panels, speed up production or reduce the need for expensive, slow pressing operations to achieve a good bond
• new sheathing materials to offer a better aesthetic appeal or feel at a lower cost, with a longer life
• combinations of existing or new materials to improve performance in fire, sound or thermal performance or the required combinations of all these physical properties.
18.5 Comfort factors in lightweight buildings
Buildings primarily provide shelter, warmth, shading and security in varying degrees depending upon the climate and local custom. After these fundamentals have been addressed, comfort is high on the occupants’ list of requirements.
Comfort is a relative term and, when applied to a building or occupied space, can be defined in a number of ways. Human comfort factors are subjective and can be skewed by age, gender, visual cues, clothing and state of mind. ‘Human thermal comfort is essentially a subjective response or state of mind where a person expresses satisfaction with the surrounding environment’ (ASHRAE 2004).
Human thermal comfort is affected by heat and evaporative heat loss, and can only be maintained when the heat generated by the body’s own metabolism is allowed to dissipate, ensuring a maintained thermal equilibrium within the internal environment. If the internal environment is such that this equilibrium cannot be maintained (body heat loss or gain) then discomfort is felt. Ensuring building occupants remain comfortable does not just rely on air temperature alone. Buildings have to be designed using the 3Ls principles (long life, low energy and loose fit) and with building design life in excess of 50 years, have to be able to absorb and deal with future climate change and change of use without major remodelling.
The ability for any building to cope with the changing environment over such a long period depends upon the makeup and durability of the building fabric, the quality of the workmanship, loose fit design and some measure of scheduled maintenance or refurbishment programme. The necessity to design new materials and systems to cope with the changing environment whilst providing a sustainable approach to both the construction and lifespan of the building is paramount.
18.6 Design led materials for addressing lightweight performance issues
With a ‘solutions’ approach to advancing system design, the potential for offsite to satisfy and exceed the future requirements for sustainable energy efficient buildings is vast. New ‘smart’ materials combined with high quality assembly will ensure designed performance for the life of the building.
Major factors affecting the long-term performance of lightweight buildings relate to the ability to effectively regulate the internal environment in a sustainable way for the life of the building throughout all aspects of climate change, whilst maximising the comfort factors which modern day living demands. The ability to effectively control the internal environment in a sustainable way means that the temperature, moisture and air quality needs to be balanced to provide an acceptable comfort level whilst at the same time ensuring the fabric of the building does not become compromised by external or internal moisture penetration, and the solution is sustainable long term.
18.7 Delivering sustainable comfort: a question of balance
Delivering sustainable comfort in any form of building is complex as the process is dynamic; differing building systems both old and new face these ongoing challenges with every change in season, change of use or occupancy level. All buildings irrespective of their construction type and age are governed by the laws of physics, and by applying the rules of ‘building physics’ early in the design process, the well-being of the occupants and the building envelope can be predicted and ‘designed in’.
Building physics is the cornerstone of designing, constructing, and operating high performance buildings, that is, buildings that are durable, comfortable, energy efficient, affordable and healthy. To avoid or solve many building problems, a unique mix of heat and mass transfer physics, material science, meteorology, construction technology, human physiology, and engineering analysis and design must be applied. This mix of knowledge and expertise is often called building science. It was initially developed and promoted in Europe (especially Scandinavia) where it is called building physics (Building Science Corporation).
The two primary zones requiring controlling are the internal occupied space in which we live or work and the building envelope which encompasses the occupied space and forms the primary enclosure system. The fabric of the building (external walls, roof and floors) comprises the enclosure to the occupied space and is referred to as the ‘building envelope’. The building envelope’s primary objectives are:
1. structural capacity to carry dead and live loads
2. moisture control – in and out of the building
The building must be designed to ensure the building envelope does not suffer from either external or internal moisture-related problems. Often the first sign of building stress is when conditions have reached a critical point and the problem manifests itself visually. The fabric of the building can tolerate some short-term discomfort but in the long term, it needs to be kept dry and ventilated.
Externally, the temperature, humidity and air quality may vary dramatically depending upon the local climate, topography and season. This has a major effect on the internal environment and the extent of the measures necessary to be ‘designed in’ to balance the extremes once the building envelope has played its part. The greater the differential between the external and internal conditions, the harder the building envelope or internal control system has to work to deliver the desired environment and then maintain that balance through the extremes.
18.7.1 Temperature control
Of the three primary factors cited, temperature is the one most easily assessed as we are used to deciding whether we are too hot or too cold; however, we are not very efficient at detecting relative humidity levels unless they are extreme.
Lightweight buildings have perceived advantages and disadvantages when it comes to controlling the temperature of their internal environments. Whilst there are many differing lightweight construction types, most have very fast thermal response times (heating and cooling), high levels of airtightness and thick insulation, which provides highly energy efficient heat storage systems.
However, the ability for a building to modulate the internal temperature fluctuations passively by use of thermal mass is desirable, especially when reviewing the likelihood of continued temperature rises due to global warming over the predicted lifespan of any new building. There have been a number of research projects (Arups DTI report 2004) which, as part of a wider review, question the ability of lightweight construction to perform in the future as climate changes drive up external temperatures; this refers primarily to the lack of thermal mass. The word ‘mass’ implies bulk, weight or both, neither of which are particularly easy to accommodate in lightweight structures. Whilst insulation levels can be improved to gain additional thermal performance giving rise to even faster thermal response times, it is not a complete solution to the lack of thermal mass.
There are two fundamental approaches to controlling temperature and internal air quality in buildings:
Both are valid and apply equally to temperature, moisture and air quality control. Although a strong case can be presented for the individual use of both passive and active techniques on their own, a balanced mix of both may be most appropriate for many building designs and conditions.
Offsite processes can be intricate yet repeatable, leading to high quality component fit which ensures the resulting building matches the design requirements. This is vital to ensure the erected structure has not had its intended performance characteristics breached by poor building processes, or workmanship. High performance buildings require high performance manufacture and assembly, especially important for passive designs where airtightness and insulation fit are of paramount importance.
Passive design promotes the internal environmental control of temperature, moisture and air quality by natural means such as:
Active solutions may be used on buildings which have none, or all of the above and any mix in between. It can be used as the primary control system for temperature and moisture or as a ‘cut in’ for extremes that cannot be coped with under normal conditions.
‘Passive design’ based upon these fundamentals can eliminate the need for both mechanical heating and cooling by adopting all or some of these principles into the design depending upon construction choice and climate in which the building resides. Lightweight buildings can be built using pure passive design principles, or a combination of both. Build quality plays a major role in achieving a true passive designed building.
Many of the ‘Passive House’ designs rely upon very thick walls and roofs to facilitate the high levels of insulation required, which adds to the building’s footprint either externally or internally depending upon how you view it, increasing the visual size of the building or reducing floor area. On a global basis, the additional footprint size may have little significance, but in highly populated countries such as the UK where land is scarce and costly, a reduction in footprint size is preferable to an increased one.
Whilst offsite techniques can accommodate varying structural thicknesses, it makes sense for transportation and ease of erection to make the components, panels or systems as light and bulk free as possible. Techniques and materials which are thin, flexible and high performing are being designed to maximise the offsite principles.
18.8 Thin solutions (insulation and mass)
With growing pressure to keep new building footprints small, the drive for ‘thin wall’ solutions for both refurbishment and new build is growing. Ultra thin materials are also very useful for providing insulation and temperature modulation in areas of buildings where previously the constraints of the design did not allow space for adequate provision of either. Wall penetrations such as window and door openings provide classic examples of where ultra thin solutions work perfectly, as they are able to be fitted around the sides of the window and door reveals internally without it impeding the window frame.
Additionally, the majority of lightweight construction methods rely on a structural framework where the insulation sits in the same plane as the structure (between the structural studs and joists). As most frameworks have little or no insulation qualities, they bridge the gap between the cold and warm surfaces of the building envelope (thermal bridging).
Ultra thin insulation such as Aspen Aerogel can be fitted at the extremity of the framework to provide a thermal break and thus improve the overall performance of the building element without adding to the footprint size.
18.9 Thermal mass in offsite potential
Thermal mass has been described as a ‘thermal battery’ as it can moderate fluctuations in internal living temperature by absorbing and releasing heat as internal conditions allow, whilst working passively. Thermal mass is a component of sustainable building design that acts passively to help control temperature fluctuations, and ideally should be used as part of the holistic design of the building once all of the factors affecting the performance of the building have been considered.
Many old buildings have thermal mass ‘in built’ which occurs as an emergence factor from the original design as often at the time of construction thermal mass was not part of the design considerations. It just happens that the construction type had ‘mass’ due to its construction material choice. Optimising where and how the mass works is vital to accurately predicting how the temperature in the building will be modulated over a variety of temperature ranges and conditions.
With refurbishment and thermal upgrading of old buildings (especially the range of ‘hard to heat and hard to treat’ single skin buildings) many are having the thermal mass benefit reduced by ‘decoupling’ the mass from the living environment by the introduction of an air gap as often the wall will have ‘plasterboard on dabs’ applied.
18.10 Phase change materials (PCMs)
Phase change materials (PCMs) are materials which change their physical characteristics when absorbing or releasing heat/energy. The materials vary in the ‘phase time’ and temperature range at which they melt or freeze. (Ice is a prime example of a PCM.) PCMs enable the principles of thermal mass to be applied to lightweight buildings and building elements without affecting the major benefits of the system approach.
Lightweight and thin PCMs provide the designer with the potential to incorporate thermal mass in the most appropriate locations of a building, to maximise the benefits of the thermal capacity, thus delivering the possibility to ‘fine tune’ the building envelope to maximise the potential for passive storage and subsequent release of energy. (A factor which is often not possible with traditional mass as it is often a secondary function to its main purpose of providing part or whole of the support structure.)
PCMs are not a new concept, but developments in technology have enabled them to be incorporated within the range of existing materials currently used in lightweight buildings. Wallboards, shutters, external cladding and internal plasters can be manufactured to contain PCMs; the resulting element will remain ‘thin’ and relatively lightweight. The term ‘lightweight thermal mass’ may appear to be a contradiction in terms, but effectively it does describe these characteristics – ‘virtual thermal mass’ may be a more appropriate term.
Rechargeable batteries store and release electrical energy; phase change materials perform a similar function for thermal energy. For example, PCM wallboard can remove excess heat from a room during the day and release it at night, with minimum use of energy and CO2 emissions; unlike air conditioning units. Global legislation on climate change is driving sustainability issues and the development of phase change materials technology can help to ensure that stringent environmental targets are met. There is increasing research effort into phase change materials for both passive and active systems for room temperature regulation and the storage of both heat and cold (Ian Biggin, Ciba 2009).
PCMs are also being used in furniture, kitchen work surfaces and other areas where temperature balancing or buffering may be desirable or localised. In normal use, these types of elements may not be considered as part of the structure, but with modular construction techniques and volumetric pod systems, these types of items may be standard fitment and be used as part of the overall comfort strategy.
18.11 Advancements in phase change materials for buildings
The surface spread of flame on construction materials is of prime concern with all building types; none more so than with lightweight structures as often the primary structure comprises timber or light gauge steel. One of the problems associated with the most common types of PCMs was its tendency to burn, as it is based on encapsulated liquid wax technologies. Recent developments amalgamating PCMs into a wood-based matrix have enabled the benefits of micro-encapsulated liquid wax to be added to internal lining boards, shutters and cladding whilst being fireproof, offering real potential to provide virtual thermal mass for lightweight buildings to be used in design-specific locations without fear of fire spread.
18.11.1 Moisture control
Moisture vapour is a gas and remains as a gas until it condenses; in the liquid form it poses a threat to the building fabric whereas if it remains in the vapour state whilst passing through the building fabric it is inert. If saturation point or ‘dew point’, as it is commonly known, is reached within the building fabric, vapour condenses and becomes liquid, referred to as ‘interstitial condensation’. In this form it is harmful to the fabric of the building if it cannot escape or revert back to a vapour again.
Moisture vapour laden air can enter the fabric of the building externally or internally and requires climate sensitive designs to dictate where the air or vapour barriers should be placed. Climate dictates external temperature and relative humidity, and in turn the building envelope needs to be designed to cope with extremes experienced and predicted over the lifetime of the building. Any form of water penetration into the fabric that has not been designed for is damaging. Water ingress can occur for many reasons:
• Envelope failure – workmanship, bad detailing or inadequate materials.
• Service failure – leaking water pipes, washing machines or drains.
With global warming giving rise to increasing wind speeds, variant temperature extremes and horizontally wind driven rain, the likelihood of water ingress through poor detailing or bad workmanship is growing. Offsite can mitigate some of the potential for water ingress by raising the level of workmanship; however, the tighter the external envelope is sealed, the greater becomes the chance of water entrapment should it gain entry, as its ease of exit has been impeded. Whatever the reason for water penetrating the fabric, it has to be able to dry out and return to its designed state.
18.11.2 Airtightness
The control of air leakage and airtightness are both one and the same, although confusion exists between the two. Confusion also exists between vapour barriers and air barriers. It is vital that lightweight buildings are designed with a full understanding of each and the appropriate mix of the two are used in a design-specific way according to the existing and future climates in which the building has to function.
The primary reason to seal and control the movement of air into and out of a building is to achieve airtightness, as a ‘leak free’ building envelope aids internal temperature control and energy conservation. There are a number of ways in which airtightness can be achieved, by applying the principle airtight membrane either on the external or internal faces of the envelope. Whilst both provide an airtight solution, they have differing effects on the fabric of the building.
Externally applied air barrier systems control the air passing into the structure itself, which has benefits where external moisture laden air is present or the building fabric comprises open cell insulation which could be cooled by air infiltration. Internally applied air barrier systems also control air infiltration into the building, but applied at the internal face usually behind the internal lining board. Both externally and internally applied air barriers have perceived advantages and disadvantages when considering the way the building envelope is designed to function and, if viewed globally, legislation may force the use of one or other type of system.
18.11.3 Membranes
Lightweight buildings rely heavily on membranes to provide both air and moisture control either to the inside face, external face or in some cases both faces of the building envelope. But before considering future membrane development, some basic understanding of membrane types is required, as their differences and functions are often misunderstood – there is confusion between air and moisture permeable membranes, what combination of each are required and at what location on a building they are to be placed.
Depending upon the design characteristics of the building envelope, one or more of the above can be applied to external, internal or both faces to control the effects of moisture vapour and air passing through the building envelope. Vapour permeable does not mean that it will allow water to pass through it, only water vapour, so all are watertight if poor workmanship and detailing are ruled out.
Ideally, the combination of membranes should be designed to stop unwanted air infiltration (into either the building element or the occupied space) and at the same time allow moisture vapour to flow into or out of the building envelope or occupied space freely, without it ever reaching dew point within a part of the envelope that it was not intended to deal with it.
18.12 New membrane developments
Membranes are primarily preventative rather than reactive and usually single function. But new technologies and advancements in glues and resins have enabled the development of a new range of membranes that are dual function, and able to actively ‘draw out’ entrapped moisture from the building fabric.
Partnering between Apollo Adhesives, Ciba Chemicals and the offsite industry has led to the development of smart glue which contains moisture ‘swellable’ polymers that actively draw moisture into the glue matrix and pass it through the glue and out of the membrane harmlessly as moisture vapour when conditions permit. This is factory applied to the inner face of the membrane, which is then factory or site applied to the building fabric where it functions as an airtight but vapour permeable layer.
In similar vein, the same technology can be applied to rubber seals for use around window and door apertures, allowing free air and moisture vapour transmission in their natural state, but which will swell once water breaches the detail. The rubber seal returns to its original state and size as the entrapped water is released as water vapour when conditions permit.
The ability of adhesives to provide more than one function is evident with adhesives containing nanoparticulate hydrophobic particles. These systems have been proven to allow the passage of moisture through the adhesive layer. These adhesives can be used in combination with breathable membranes to remove moisture out of water damaged buildings (Darrell Tibbins, Apollo Adhesives).
18.13 Composite design
Offsite is beginning to take full advantage of mixing materials to obtain the maximum from the design. Green roofs and green walls have great sustainable credentials, but their system design did not fit well with offsite construction techniques. One of the difficulties has been the provision of a structural waterproofing system which could carry the loads, be flexible enough during construction, delivery and assembly and yet remain totally waterproof throughout. Traditional reinforced concrete would fill this need if it were not for its weight and thickness.
18.13.1 Ultra thin flexible reinforced concrete membrane
Thin, lightweight products which have flexibility make ideal components for preassembled panels or pods. Materials with additional attributes such as fireproof, waterproof, strength and a long life add to that desirability. Lightweight offsite buildings are often ‘finished’ externally with lightweight cladding and roofing materials (either applied in the factory or onsite). These light systems do not all have the durability, fire and security performance of heavier solutions, such as masonry, which makes them vulnerable to physical damage and vandalism.
One new material solution which can address this is Krete, a thin (6 mm) reinforced concrete membrane which is completely airtight, watertight and yet vapour permeable. It can be used on flat or vertical surfaces as a complete waterproofing system to provide total encapsulation of flat roofs, upstands and walls. It is flexible and is applied ‘jointless’, which removes the problem of site assembly weaknesses. It provides a totally waterproof and strong flat roofing layer, making it ideal for use with green roofing systems, enabling lightweight structures to have complete roof assemblies factory fitted in readiness for the greenery to be added onsite. Furthermore, its inherent strength characteristics negate the use of additional root barrier protection.
This material also has considerable potential for use in bathroom and kitchen pods, as its strength and flexibility along with the fact that it is totally waterproof makes it ideal for use in ‘thin floors’, wetroom floors and walls.
RoofKrete has been used to make boats in the past, so it seems only natural to me that it is gaining interest from those seeking lightweight and flexible building solutions which have long life, good sustainable credentials and are totally waterproof. RoofKrete has been fully tested and given a functional maintenance free life of over 100 years (Dr Roy Jenkins, Krete Sustain Ltd).
18.13.2 The human factors
Most people with an interest in construction and materials naturally concentrate on the ‘hardware’, the product and the materials, at the expense of the process. But the process, particularly the onsite process which completes the use of offsite elements, is at least as important to saving time, increasing control and so reducing costs and improving quality. The benefits of using offsite elements within a primarily onsite build process are certainty of product design, quality and delivery to the construction programme. It allows site professionals to develop standardised, rule-based approaches to construction issues, such as the order in which tasks are carried out.
18.13.3 Knowledge sharing
Knowledge is often secured and secreted within pockets of industry, and in the past an unwillingness to share this knowledge has prevailed. The adage ‘knowledge is power’ is giving way to a greater understanding that by combining differing knowledge bases, great advances can be made swiftly for the benefit of all, both commercially and environmentally.
Partnering between industry and universities, chemists and all manner of facilitators is giving rise to development of new materials and systems with specific characteristics ‘built in’ to satisfy a weakness in the building envelope, either as a single material or component part.
18.14.1 Offsite construction – reading list
ARUPS Research and Development Report for DTI Partners in Innovation Programme 2004.
Blismas, N., Pasquire, C., Gibb, A.G.F., Benefit evaluation for off-site production in construction – Construction Management & Economics. 2006:121–130. [24 February].
Egan, J. Rethinking Construction: the report of the construction task force. http://www.constructingexcellence.org.uk/resources/publications/deault.jsp, 1998. [The Office of the Deputy Prime Minister].
Everett, J.G., Design fabrication interface: construction vs manufacturing. Proc. 10th ISARC. 1993.
Gibb, A.G.F.Off-site Fabrication: prefabrication, pre-assembly and modularisation. Whittles Publishing Services, 1999. [ISBN: 978-0-470-37836-6].
Gibb, A.G.F., Isack, F. Re-engineering through pre-assembly: client expectations and drivers. Building Research & Information. 2003; 31(2):146–160.
Goodier, C., Gibb, A. Future opportunities for Offsite in the UK. Construction Management and Economics. 2007; 25(6):585–595.
Gosling, J., Naim, M., Sassi, P., Iosif, L. Flexible buildings for an adaptable and sustainable future – 24th ARCOM Cardiff. 2008; 1:115–124.
Horner, M., Duff, R. More for less: a contractor’s guide to improving productivity in construction. CIRIA. C566, 2001.
Kose, S. Building Research Institute in Japan: past, present and future. Building Research and Information. 1997; 25(5):268–271.
Li, H., Guo, H., Skibniewski, M.J., Skitmore, M. Using the IKEA model and virtual prototyping technology to improve construction process management. Construction Management and Economics. 2008; 26(9):991–1000.
Manley, K. Against the odds: small firms in Australia successfully introducing new technology on construction projects. Research Policy. 2008; 37(10):1751–1764.
McEvatt, W. Modular co-ordination. Building Research and Information. 1987; 15(1):17–21.
Nam, C.H., Tatum, C.B. Leaders and champions for construction innovation. Construction Management and Economics. 1997; 15(3):259–270.
National Audit Office. Modernising Construction. http://www.constructingexcellence.org.uk/resourcecentre/publications, 2001.
Pan, W., Gibb, A.G.F., Dainty, A.J. Perspectives of UK house builders on the use of Offsite modern methods of construction – Construction Management and Economics. 2007; 25(2):183–194.
Pasquire, C., Gibb, A.G.F., Blismas, N., What should you really measure if you want to compare prefabrication with traditional construction?. Proceedings IGLC 13, Sydney, Australia. 2005:481–491. [July].
Pries, F., Janszen, F. Innovation in the construction industry: the dominant role of the environment. Construction Management and Economics. 1995; 13(1):43–51.
Reinschmidt, K.F., Griffis, F.H., Bronner, P.L. Integration of engineering, design and construction. J. of Const. Eng & Management. 1991; 117(4):756–772.
Ross, K., Cartwright, P., Novakovic, O. A guide to modern methods of construction – HIS BRE Press on behalf of NHBC Foundation; 2006;Vol NF1. [Issue December].
Stirling, C.Off-site construction: an introduction. Building Research Establishment Publications, 2003. [GBG 56, ISBN 1 86081 624 X].
Tam, V.W.Y., Tam, C.M., Ng, W.C.Y. On prefabrication implementation for different project types and procurement methods in Hong Kong. Journal of Engineering Design and Technology. 2007; 5(1):68–80.
Tatum, C.B. Process of innovation in construction firm. J. of Const. Eng & Management. 1987; 113(4):648–663.
Winch, G. Zephyrs of creative destruction: understanding the management of innovation in construction. Building Research and Information. 1998; 26(4):268–279.
Bottom, D., Gann, D., Groak, S., Meikle, J.Innovation in Japanese Prefabricated House Building Industries. Westminster, London: Construction Industry Research & Information Association (CIRIA), 1994.
Brock, L., Brown, J. The Prefabricated House in the Twenty-First Century: What Can we Learn from Japan?, A case study of the KST-Hokkaido House. http://timber.ce.wsu.edu/Resources/papers/4-2-3.pdf, 1999.
Burns, C.J. A manufactured housing studio: home on the highway. Journal of Architectural Education. 2001; 55(1):51–57.
Chowdery, M., et al Homing in on Excellence: a commentary on the use of off-site fabrication methods for the UK house building industry. The Housing Forum, London, 2002. http://www.constructingexcellence.org.uk/resourcecentre/publications [URL:].
Gann, D.Flexibility and Choice in Housing. The Policy Press, University of Bristol, 1999. [ISBN 1-861340893].
Gann, D.The Housing Demonstration Projects Report: improving through measurement. London: The Housing Forum, 2000.
Malpass, P. The Wobbly Pillar? Housing and the British Postwar Welfare State. Journal of Social Policy. 2003; 32(4):589–606.
Meiling, J., Johnsson, H. Feedback in industrialised housing: why does it not happen? 24th ARCOM. Cardiff. 2008; 1:145–154.
Parliamentary Office of Science Technology. Modern Methods of House Building. No. 209, December http://www.parliament.uk/documents/upload/postpn209.pdf, 2003.
Venables, T., Barlow, J., Gann, D., Manufacturing Excellence: UK capacity in Offsite manufacturing. The Housing Forum, Innovation Studies Centre, Imperial College London, 2004. http://www.constructingexcellence.org.uk/sectors/housingforum/publications.jsp
Lawson, R.M., Grubb, P.J., Prewer, J., Trebilcock, P.J.Modular Construction using Light Steel Framing: an architect’s guide. Steel Construction Institute, Publication, 1999. [P272, ISBN 1 85942 096 6].
Steel Construction Institute, Case Studies on Modular Steel Framing, Publication P271, ISBN 1 85942 095 8.
Steel Construction Institute, Case Studies on Light Steel Framing.
Neale, R., Price, A., Sher, W.Prefabricated modules in construction. Chartered Institute of Building (CIOB), 1993. [ISBN 1 85350 061 9].
The Housing Forum. Modular construction for process efficiency and product quality. http://www.constructingexcellence.org.uk/sectors/housingforum, 2001. [The Construction Best Practice Programme].
Pre-assembly and standardisation
CIRIAStandardisation and pre-assembly – adding value to construction projects. London: Construction Industry Research and Information Association, 1999. [Report No. 176,].
Gibb, A.G.F.Standardisation & Pre-assembly – Clients Guide and Toolkit. London: Construction Industry Research & Information Association (CIRIA), 2000.
Gibb, A.G.F. Standardisation and customisation in construction: a review of recent and current industry and research initiatives on standardisation and customisation in construction. http://www.ncrisp.org.uk, 2001. [CRISP Consultancy Commission – 00/20, May].
Gibb, A.G.F. Pre-assembly in construction: a review of recent and current industry and research initiatives on pre-assembly in construction. http://www.ncrisp.org.uk, 2001. [CRISP Consultancy Commission – 00/19, May].
Mechanical and electrical services
Wilson, D.G., Smith, M.H., Deal, J.Prefabrication & Pre-assembly: applying the techniques to building engineering services. BSRIA, 1998. [ISBN 0860225054].
Goffin, K., Mitchell, R.Innovation Management: strategy and implementation using the pentathlon framework. Palgrave Macmillan, 2005. [ISBN: 1-4039-1260-2].
Herbert, G.Pioneers of Prefabrication – the British contribution in the 19th century. The Johns Hopkins University Press, 1978. [ISBN: 0-8018-1852-4].
Hvam, L., Mortensen, N.H., Riis, J.Product Customization. Springer, 2008. [ISBN: 978-3-540-71449-1].
Myers, D. Construction Economics – a new approach, 2nd edn. Taylor & Francis, 2008. [ISBN: 0-415-46229-0. (Economics of OSC, Chpt 9 p154 ‘Offsite Construction Methods)].
Ortiz, C.A.Kaizen Assembly – Designing, Constructing and Managing a Lean Assembly Line. Taylor & Francis, 2006. [ISBN: 0-8493-7187-2].
Powell, C.G.An Economic History of the British Building Industry 1815-1979. The Architectural Press Ltd, 1980. [ISBN: 0-85139-194-X].
Stone, P.A. Building Economy – Design, Production and Organisation – a synoptic view, 3rd edn. Pergamon Press, 1983. [ISBN: 0-08-028678-X (Part II Chpt 7 ‘Innovation in Construction’, Chpt 8 ‘The Industrialisation of Building’, Chpt 9 ‘The Economics of Industrialised systems’ and Chpt 10 ‘The Mechanisation of Construction’)].
Building Research Establishment
(URL: http://www.bre.co.uk/)
Building Services Research and Information Association
(URL: http://www.bsria.co.uk/)
(URL: http://www.constructingexcellence.org.uk/)
Construction Industry Research and Information Association
(URL: http://www.ciria.org.uk/)
Construction Research and Innovation Strategy Panel
(URL: http://www.ncrisp.org.uk/)
(URL:http://www.lboro.ac.uk/research/immprest/)
The Modular and Portable Building Association (formerly the National Prefabricated Building Association)
(URL: http://www.mpba.biz/)
PrOSPa – Promoting Off-site Production Applications
(URL: http://www.prospa.org/)