Posted By: borpinqually why does the SIP need and outer breathable membrane?

Because it can get wet during construction and needs to be able to dry.

Paul in Montreal.

I don't like the way the usual SIP external wall detail puts the inner OSB between the polyurethane core & a plastic VCL. However, dispensing with the VCL would require another means of ensuring warm moist air doesn't reach the cold parts of the structure. How do you guarantee this without membrane &/or tape? Is expanding foam good enough? What about the panel joints where insulation is routed out & a timber batten inserted?

I'm sure all these isssues can be addressed, but someone needs to validate details for all of the common joints & a SIP vendor doesn't have a commercial interest in doing so when he can just specify a well sealed VCL.

The breather membrane provides a secondary line of defence to water penetration & improves windtightness, while still allowing the outer OSB to dry should it get wet during construction or due to warm moist air reaching it. I think it earns its keep.

David

Well, firstly, look on the UKSIPs website. They have recently published some simple techincal bulletins on a number of subjects. I would suggest reading them.

A breather membrane, as mentioned by others, help to protect the structure during construction, but more importantly in service, it also shed water that may get through the cladding. IMO a breather membrane is a total must. OSB doesn't really like getting wet regularly.

A VCL is normally required. Some SIPs use an EPS core which is fairly vapour permiable allowing the possibility of interstitial condensation. Even with a PUR core, a VCL helps to reduce risks. The inner face of the SIP (the OSB) should be at a moisture content of 20% or less before being closed in to avoid mould growth or decay. In service the VCL will also help to achieve a good air tightness.