Timber structures SIPS
2. Efficient insulation There are a number of reasons why use of SIPs can contribute to the energy efficiency of a building (as calculated using the Government’s ‘Standard Assessment Procedure’ for Energy Rating of Dwellings, SAP). But when it comes to keeping the heat in a building, SIPs score particularly well, offering good thermal insulation (U-values, in W/m2K) for a given wall thickness because the insulation is part of the structure. ‘Fabric First’ is a popular ethos that has grown from the German Passivhaus standard, where energy efficient buildings are created by focusing on the performance of their external envelope. Under this approach, elemental U-values and heat loss are driven down to very low levels, so the building consumes a minimal amount of energy to maintain a comfortable temperature. SIP wall panels can be of any thickness, although most manufacturers will make panels between 100mm and 250mm thick. Most new build developments will have external wall U-value targets of between 0.1 W/m2K and 0.2 W/m2K, which SIPs can easily achieve.
SIPs, showing plywood with foam infill. Photo: Cowley Timber + Partners
In addition, junctions between panels may use insulated splines, which will help to reduce heat loss due to repeat thermal bridging.
Buildings with very low fabric air leakage rates will require additional ventilation measures. Typically these will take the form of mechanical ventilation heat recovery (MVHR) units. MVHR units mechanically control the movement of air into and out of a building while recovering latent heat in the exhaust air, further contributing to energy efficiency.
3. Tightly built – by design
4. Healthy indoor environment
Another major aspect of energy efficiency is airtightness. Building regulations require that air leakage into, or out of, the building is controlled.
The OSB used in SIPs is manufactured from strands of softwood timber and adhesive resins, typically phenol formaldehyde. The off-gassing from this type of adhesive is minimal and the quantity of resin used is very low. Some OSB manufacturers use adhesives which contain no formaldehyde.
SIP buildings are capable of providing very good levels of airtightness due to the panelised construction system. They would normally incorporate a vapour control layer (vcl) onto the warm (inner) side of the external walls. This primarily controls the movement of moisture vapour through the wall, but is also an effective air barrier. The vapour control layer/air barrier can be lapped and sealed at wall junctions, at the junctions with other external elements, such as windows and doors, as well as around service penetrations of the building envelope. With SIPs it is also normal practice to seal the splines at panel junctions with either mastic or expanding foam products during erection. This, coupled with the vcl/air barrier, is the key to the excellent airtightness levels that can be achieved with SIPs. www.trada.co.uk
In the UK, structural insulated panels are available with a number of different insulation cores: expanded polystyrene (EPS), extruded polystyrene (XPS), polyisocyanate (PIR) and polyurethane (PUR). Insulation materials such as EPS and PUR do not contain formaldehyde or any CFCs, HCFCs (which deplete the Earth’s ozone layer and have high values for global warming potential, or GWP). During the manufacture of the SIPs, off-cuts of insulation material may be re-used, if large enough, or more normally are recycled by the manufacturer where the facility exists. Inevitably, some waste will go to landfill, but this is normally be far less than similar insulation waste generated on a construction site. >> Timber 2016 Industry Yearbook
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