There is a widespread misapprehension that damp proof courses (dpc’s) stop all damp, there is no way for them to stop condensation from forming on top of them. Unfortunately dpc’s are by their nature nearly always located at a crucial cold bridge, so too often in timber frame and SIPs constructions there is a risk of condensation forming in very cold weather or with high humidity indoors on top of the dpc, this is in direct contact with the wooden sole plate, repeated dampness from this source will eventually cause rot even where treated timber has been utilised.
I had protracted discussions with a structural insulated panel manufacturer a couple of years ago when I was the sustainability consultant and project adviser on the design team for a low energy new building. I insisted on additional insulation in the cavity to protect the dpc, top block sole plate and bottom rail from getting too cold (it was the coldest thermal bridge in the house). Initially the SIPs technical department stated categorically that no insulation would be allowed anywhere outside of the SIPs on any account and that if we wanted more insulation it would only be allowable if affixed to the inside of their panels. After some discussion and my asking a lot of awkward questions they relented and allowed us to build in a 600mm high sheet of 50mm eps fixed over the wall plate area, covering a band 300mm above and below dpc, the breather membrane was carried over and down the outside of this insulation which had a chamfered top edge so that any water in the cavity would be directed down the cavity to drip away well below dpc. This was a major breakthrough and calls into question the longevity of early SIPs buildings where there could be problems with the sole plates getting damp from condensation on top of the dpc due to thermal bridging. All this also equally applies to timber frame construction too, probably not a significant risk for early poorly insulated frames but as insulation levels increase the risk increases.
Tony Cowling – December 2018