In the Southeastern US the climate is often hot and humid. Air conditioning is assumed. And the dominate vapor drive is inward – unlike the outward vapor drive of cold and mixed climates farther north. Let’s also assume you’re not interested in wrapping your building in foam – utilizing instead more sustainable cellulose, mineral wool or other fibrous insulations. So what kind of air barriers and vapor control materials will create the most resilient high performance assembly in such a climate?
On the exterior – when it is hot and humid for long periods at a time – it will be beneficial to have a class III vapor retarding weather resistive barrier on the exterior. At 1.4 perms (Sd-value 2.3m) the Pro Clima’s DA membrane limits the moisture drive into the wall/roof, while also allowing the exterior to be absolutely airtight (unlike conventional housewrap materials and details). Both factors will keep the moisture content in the insulation and other construction materials on the inside of the insulation within the acceptable norms (less than 18M% or 80% RH). If one would use conventional house-wrap, which is (intended to be) vapor open, it would lead to increased vapor finding it’s way into the insulated enclosure and could lead to increase moisture content there, leading to more humid interior conditions as well – especially if this exterior wrap is not part of a durable air-sealing system (that includes gaskets, adhesives and tapes).
In these climates the interior air-barrier shouldn’t be a class I or II vapor retarder – this because for the majority of the year the building will have a vapor drive that is directed inwards (higher vapor pressure outside than inside, especially when the house is airtight and air-conditioned/dehumidified) – the materials should be more vapor open on the inside than on the outside. Yet in colder periods, when the vapor flow will reverse, the interior air-sealing layer ideally will become vapor retarding – INTELLO Plus is the most suitable candidate because it’s large vapor variability will allow inwards drying in hot and humid weather, while preventing moisture build up in colder times.
In hot and humid climates the combination of DA membrane outboard of the insulation and INTELLO membrane inboard of the insulation can provide a robust and resilient assembly for fibrous insulations. For more assembly example suggestions see our blog post Foam Free Assembly Guidelines by Climate Zone.
If you have an assembly in a hot and humid climate that you would like us to do a WUFI analysis on, please contact us.
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