Mineral Wool Exterior Insulation, Humidity and Windwashing
Every time I think I have my insulation strategy set, I learn something new that makes me question it. That’s what you get for reading the internet.
Anyway, I am planning on 2 x 6 walls with dense pack cellulose with a smart vapor membrane like Intello Plus, an insulated 2″ service chase. Outside of the cellulose would be plywood, housewrap, then probably 2 inches of mineral wool exterior insulation (Roxul Comfortboard is my plan) then strapping, then Hardiboard. I am in zone 3 mixed-humid in the Piedmont of the Carolinas.
I’ve been reading things here for about 3 years, so I have what I think is a very reasonable wall for our climate that is also fire resistant and insect resistant. This week I started reading about limiting humidity in the South by using a less vapor permeable house wrap like Pro Clima’s DA membrane (http://www.foursevenfive.com/index.php?main_page=page&id=15) to prevent inward vapor drive during the long cooling season.
The other issue suggested by this source is that if you have an airtight membrane on top of the exterior insulation, the insulation would be more effective because it is air sealed on all sides. I thought about that for the cellulose, but I did not think about it for the exterior insulation.
What do you guys think about using a membrane on top of the mineral wool? Sounds like a real pain to detail around windows, etc. I know Lucas Durand is building his house with exterior insulation. I am curious what kind of membrane he will use exterior to the mineral wool.
I guess what I am asking is – from a performance perspective, how much do I lose in terms of insulation effectiveness by NOT covering it with a membrane?
Thank you as always. Lucy
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