SIPs (structural insulated panels) are pre-insulated panels that can form part of a building or whole buildings. The panels are made of either cement particle board (CPB) or, more commonly for self-build, engineered wood-based sheets known as oriented strand boards (OSB). Two sheets of this are sandwiched together with a rigid insulation (such as polyurethane).
Tests on Siptec’s panels, for example, by the University of Surrey, show that they are up to five times stronger than a timber-frame structure. SIPs have thermal and sound-insulation properties that comply with, or often significantly exceed, current Building Regulations and some incorporate a vapour-controlling membrane.
SIPs come as individual panels for walls, floors and roofs, or in kit form, ready to be joined together to make a whole house. They can come with windows and doors already installed, plasterboard lining and first-fit electrics. A finished house built with SIPs can be clad with bricks, timber, render or stone.
Major manufacturers of SIPs for houses are Siptec, Sip Building Systems (who supply Sipit), Kingspan Tek and Build It Green. Some manufacturers just supply kits for self-builders, some supply and appoint their own regional installers, some offer a design, build and installation service.
For a complete four-bedroom detached house of 200m², Andrew H Wilson, one of Kingspan TEK’s system suppliers, can supply a turnkey option (you don’t have to do anything) for £240,000. This is the equivalent of £1,200 per m². Panels only would be much cheaper.
SIPs have a high U-value relative to their thickness, so maximising available room space. They can be precision-cut to specification in the factory, so minimising the work that needs to be done on-site. Although SIPs can be used as part of a conventional build, this is not as efficient in terms of time and materials, or as economical, as building a whole house from SIPs.
Although it might look as though it is an expensive way to build, there are savings. As the house can go up in a matter of days, fewer tradespeople are needed and for a much shorter time than with a conventional build. A four-bedroom house can be erected and weatherproof in a minimum of five days. There is also less waste (you only get the materials you need) and mess to clear and dispose of.
Heating costs will also be lower – a whole SIP house will be very airtight and thermally efficient. As a result, standard heating system choices include heat recovery and air ventilation systems, particularly good for asthma sufferers. Siptec claims a SIP house will give energy savings of 40 – 60 per cent more than a timber-frame house of equal size. Airtight SIP-built houses easily comply with the air-leakage test for all new builds that is part of new building regulations due within 12 months.
Some mortgage lenders are beginning to reduce their rates for borrowers with SIP-built houses. A potential disadvantage for some is there cannot be any on site changes: the client has to make all the decisions about the self-build and sign off the drawings before the build begins. It is vital that the dimensions of the foundations are accurately measured and constructed, so that the SIPs fit perfectly.
SIP builds are booming. The government is keen to encourage building with SIPs as it is quick and economical – it uses about 60 per cent less timber than conventional timber-frame construction.
BRE publishes An Introduction to Building with Structural Insulated Panels by J Bregulla and V Enjily and Modern Methods of House Construction by K Ross, both available from www.brebookshop.com
Caption: Build It readers the Malcolmsons combined SIPs with an outer skin of rendered blockwork, timber cladding and glass to create their modern Scottish longhouse, completed for just £205,000
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