Sip it up

When is a timber frame not a timber frame? When it’s a SIP. Imagine a timber frame wall with an OSB racking board fixed to the inner and outer face. Now remove all the timber studwork between the two racking boards. Fill the void between with a solid mass of insulation and you have a structural insulated panel (SIP) every bit as strong as the original wall.

The downside is that you need a second racking board - timber frame walls only use one. The upside is that you have done away with the timber skeleton between the boards, and in its place you have a lightweight shell with much improved U values. SIPs can be used for floors, roofs and walls, and work well either as part of an entirely prefabricated shell or loose fit.

The nearest equivalent construction forms to date here are the sandwiched masonry panels used widely in Germany and available in the UK from Swedish company ThermoneX and Marshall’s Panablok. But this year, the UK has seen the launch of Jablite’s Roof Element, a Dutch SIP system, and the use of a similar Dutch roofing product, Opstalan, on Southern Housing Group’s Nightingale Estate in Hackney, east London.

Now the first UK whole house manufacturer, BPAC has set up a production plant in Kirkcaldy and is currently working on a demonstration house for Bellway in the North East.

BPAC director Derek Cumming says: “We’ve studied the way SIPs have taken off in the US and are convinced the system has a big future in the UK. Technically, it leaves conventional timber framing far behind.”

US player, Pennsylvania-based SIPs maker Murus, is also looking to set up a UK plant (see box). “We reckon we need to be building no more than 50 houses a year to make a UK plant feasible,” says Steve Keller, president of Murus.

For SIPs to become more widely accepted in the UK, some regulatory hurdles have to be crossed. In particular, UK Building Regulations concerning vapour barriers and roof ventilation need to be addressed. It is unclear whether an OSB-clad SIPs wall should be classed as a timber frame wall, but in the US they are not built with vapour barriers and there seems little reason to insist on them elsewhere. With roofs, there has to be some ruling as to whether a SIPs system qualifies as a warm roof or a cold roof (thereby requiring a ventilation channel). Again, ventilation channels are not required in other countries using SIPs roofing. Other pressing issues to be resolved are fire performance and sound insulation. In both cases, the polyurethane-based panels outperform the polystyrene ones.

While SIPs may be set to become a major player in the transformation of British housebuilding, it is by no means clear which variety of SIPs will prevail.