It makes perfect sense to do away with the mess, bother and lead times of traditional building methods and embrace the emerging world of off-site manufacture, but is it a safe option? For prefabricated buildings to make a significant impact on our built environment, the concept must first be proven to present reliable solutions.
After all, many of us grew up with the view that prefabs were sub-standard buildings. We knew of damp community halls, musty classrooms and cold prefab houses that had outlived their original design life and were literally rotting from the ground up.
Over 150,000 prefab buildings were put up at a cost of £2m in the difficult years following the war, when urgent need for housing dictated the need for speed. Surprisingly however, prefabs were not necessarily cheaper than traditional buildings, with many types embodying the latest technology including electricity and central heating, contemporary fitted kitchens and modern bathrooms. In fact, the average cost of a prefab was twice that of a terraced house – but at the time such spending was necessary for the authorities to meet their needs.
It is apparent to us now that the key weakness of prefabs was down to damp, which caused unhealthy mould growth and rot. These failings can mostly be traced to failed damp-proofing and insufficient insulation. However, flaws in the design cannot usually be held responsible, as generally the design life of a prefab was only 10–20 years.
Fast-forward to today, and we again find ourselves with a serious housing shortage, and as before, the government is encouraging the uptake of buildings constructed off-site or using off-site manufactured components to guarantee quality and decrease lead times and cost. The new industry government expert Off Site Construction Group announced recently by Housing Minister, Max Prisk, will seek to use off-site construction to ‘revolutionise the way we deliver our housing, providing a swift, high quality solution to creating cost effective, zero carbon homes’ [i]
Their reasoning is that using quality controlled processes in a factory, high standards can be achieved alongside more efficient use of materials. Also, technology is able to augment individual productivity and reduce labour costs per unit of production. We should be clear, however, that the cost efficiencies obtained in this way will not necessarily undercut the latest mass site-build systems employed by established developers. In such situations, the entire building site effectively becomes a factory with the implicit efficiency of repetition and scale.
It is undeniable however, that off-site manufactured, engineered systems can produce higher quality buildings with far quicker site-assembly times, and whilst mass developers are often aiming to meet building regulations at the minimum level, some modern prefab systems are striving for true sustainability and ‘eco’ status.
Thus, modern prefab buildings come into their own for small scale developments, where lead times must be short, and costly site disruption minimised. Here also, eco-credentials and excellent building performance are likely to be highly valued.
Today’s prefab market provides plenty of options to meet customer budgets and design aspirations by essentially providing three categories of product: fully-fitted turn-key solutions, site-finished solutions and self-build kits. Prices range from around £500/sqm at the bottom end, for self-build kits, up to around £1,800/sqm at the top end, for turn-key solutions.
The UK market is unusual in that our heritage of vernacular homes creates an aspiration for something unique or personally tailored, a fact that has had to be reflected by modern prefab designers. For example, the Suffolk based offshoot of a Swedish prefab manufacturer, Svenskhomes, has developed a range of styles which its Swedish market simply didn’t require, as Swedes were often happy to live in essentially identical homes.
German design has famously given us the Huf house, which arrives on a lorry and is assembled by employees from the company’s single factory. The high cost of this prefab with its trademark look makes it attractive to an aspirational market that might otherwise employ architect design. This is not typical of German prefabs however: Hanse Haus and WeberHaus have made inroads into the UK in a similar way to the Swedes by providing a range of styles and kit-build options.
The UK itself has struggled to enter the prefab market at an aspirational level, with some suppliers focussing on practical buildings, and the long-established static caravan market offering low-cost solutions under the Caravan Act, which exempts the units from the need for building regulations approval.
However, a number of recent entries to the market are showing that British ingenuity does have something to offer to meet the challenge taken up by the Germans and Swedes. Perhaps the two most exciting examples being Dwelle, who produce clever micro-homes that can be built together in modular fashion, and Green Unit, who have developed a barrel-vaulted modular system that can be configured for seemingly endless uses.
Both companies have rethought building design from the ground up. Green Unit, for example, carried out a prototyping and development cycle lasting over three years, then scientifically tested their prototypes to achieve grade A/A under SAP testing and code 5 of the Code for Sustainable Homes.
Prefab design is particularly exciting in providing the opportunity to redefine buildings in accordance with the goals of sustainability and energy efficiency. Materials and process can be selected on the basis of carbon-neutrality, and energy-saving systems can be used in manufacture. The building itself can bring together a number of innovative sustainable technologies and concepts from renewable energy to water efficiency.
Nevertheless, the key areas of focus are always insulation levels and airtightness: insulation prevents cold-bridging through the building envelope, which not only allows heat to escape, but causes condensation and mould and leads to overheating in the summer. And airtightness prevents air movement, which is traditionally a major cause of heat loss, and employs a heat recovering ventilation unit (HRV or MVHR). These units also filter pollen, preventing allergies, and provide summer cooling. They work well with in-duct heating.
Insulation comes in many forms, with sheep’s wool proving to be among the best performing of sustainable products, but cold bridging through structural elements is always a challenge. Green Unit, for example, uses wool in an overall wall thickness of around 280mm to prevent cold-bridging through structural elements of the timber frame.
Similarly, glazing and external doors can severely impact building performance, and it goes without saying that triple glazing is essential for long term best performance.
Ground-up rethinking and modern technology have made it possible to produce these new, attractive prefabs with a design life well in excess of the 40–50 years expected of many traditional new-build homes.
In offering this degree of designed-in sustainability, the new prefabs are certainly the current front runners in green building design.
Article written by Green Unit Director Phil Clayden for Adjacent Government, May 2014.
Philip Clayden, Designer, Director and Co-founder of Green Unit Limited, a company which off-site manufactures the Green Unit, a modular, sustainable, timber-engineered, pre-fabricated building.