Smart MemBrain in Zone 5
As some of you may have followed my other Q&A posts, you may know I have installed the Certainteed brand Smart MemBrain on the interior side of my walls.
I live in Zone 5. My walls are 2×6. Exterior to interior is vinyl siding – Green guard Raindrop 3D wrap – 7/16″ OSB – 2×6 studs w/ R-23 mineral wool insulation – smart MemBrain – future 1/2″ rock – latex paints.
I was under the impression that the membrane is a good idea. However I talked to some older builders who say poly on the inside destroyed homes. They also said either cellulose or fiberglass was used in the walls. So I have two unconventional products that are not time tested here. Mineral wool and the smart membrane. Should I be concerned? Am I doing things proper so far? I am assuming the poly for them was bad because of their insulation type?
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Replies
Nicholas,
You have nothing to be concerned about.
Consulting local builders -- especially older builders like me -- is rarely a good way to get answers to building science questions. (Since I quit building, I have started listening to scientists and researchers; most builders don't have the time to attend conferences or to study, unfortunately.)
The homes with problems attributed to interior polyethylene had factors lacking in your house, and were usually located in warmer climate zones. The disasters included homes with poorly flashed EIFS and homes with brick veneer installed without an effective air gap. Your house isn't like that.
Moreover, a smart vapor retarder is not polyethylene. When necessary, it becomes vapor-permeable. So stop worrying.
Excellent. When I said the membrane can vary it's permeability, I think they just tuned out every thing else. :)
In a zone 5 climate the inherently back-ventilated vinyl siding alone provides sufficient capacity to dry toward the exterior, protecting the sheathing, even if only latex paint was the interior side vapor retarder. That drying capacity would also protect against moisture trapping issues if you had used interior-side polyethylene.
But with the MemBrain you bumped it up a notch on resilience, curtailing the wintertime moisture diffusion by about an order of magnitude below what would have occurred with only standard latex paint as the vapor retarder, but without losing the ability to dry toward the interior as would happen if polyethylene sheeting were used.
Mineral wool insulation is at least as old as fiberglass insulation (I have some rock wool batts in my house that was installed in the1920s, and some loose fill rock wool installed in the 1940s). It is fully time tested, with a very good track record. It has long been the standard insulation in wood framed houses in Europe, where fiberglass insulation is all but unheard of.
MemBrain was initially launched about 12 years ago, and has undergone a fair amount of third party testing. The material itself (2 mil thick nylon) has well established hygroscopic & vapor permeance characteristics as well as good longevity. (It has long been used in food-wrap applications.) The only thing really new about it is the application as a variable vapor retarder in building assemblies, where it has less than 2 decades of real-world experience. But it tests well!