Use of pro clima vapour control membranes and SOLITEX weathertightness membranes to encapsulate insulation, eliminating air gap, will prevent moisture accumulation (interstitial condensation) and still allow drying capacity to remain high. Drying capacity required for removal of construction moisture and small amounts of moisture diffusing through the structure in winter will be removed via air pathways between the roof cladding and the roof underlay. This is our standard national and international advice and best practice.
There is no requirement for a 25mm air gap above insulation when pro clima SOLITEX membranes are used.
Older products were porous and allowed wicking of moisture when in contact with insulation whereas new technology is non-porous, watertight, yet still allows for vapour movement (drying).
Sailing a bit too close to the wind
Earlier this week, I read an article by Tom Hunt on Leaky Buildings, Hidden Mould and the Impact on Health, in which he made a comment that "Experts agree that leaky homes are still being built in New Zealand" and quoted a registered building surveyor who thought the 'Building Code had gone a long way toward fixing the problems, but homes were still being built "that are leaking and need significant remediation"'. Maybe this is a case of appearing to do something to avoid leaky buildings but simply not doing enough, or even understanding it well enough.
The installation accuracy of products means designs should never sail too close to the wind — there should be room for some 'error' of overall thermal performance (including accounting for framing and big thermal holes where roof windows sit), without resulting in failure of the structure.
Step away from the risk in New Zealand climate conditions:
For least risk of moisture accumulation within the construction, moisture should be restricted from entering the construction by means of a vapour control layer installed at the inside face of fibrous insulation which can be reliably achieved even where torch-on membranes are installed in skillion roof construction.
Predict performance before building begins with Passive House Planning Package (PHPP), or WUFI (computer modelling for vapour/moisture and heatflow) software to give you confidence in the materials and installation.
In the next blog I will describe a project we are working on in conjunction with Technoform Bautec called 'Triple Connected Windows' (how to ensure standard thermally-broken windows are positioned to succeed).