Housewrap under outsulation foam
Can you tell me if the housewrap actually serves a purpose underneath foam board?
Second post, I posted here: https://www.greenbuildingadvisor.com/blogs/dept/musings/where-does-housewrap-go , but I wasn’t sure proper etiquette on an article that was several years old. I have been researching for about a year, read every article and link I was able to. Current project is crawl encapsulation and insulation. Replacing 3 exterior doors and 1 window (Innie). I had a quote to encaps from inside, but after researching I like the idea of exterior insulation. I have since set aside a healthy budget and am trying to do this in best practice. I’m excavating to footer, clean stone and french drain, Carlisle 860 peel and stick up to and past rim joist 24″ onto house, 2 layers of 2″ of EPS staggered seams, a protective barrier 12″ below grade and 4″ above, firing strips and corrugated metal or stone veneer for aesthetics. This was an effort to ensure the sill and rim joist was air sealed. In a year or so when I do siding and the rest of the windows I will add the rest of the 4″ of foam up the wall. My original thought was the peel and stick up the walls and on roof, but in reality this may be a little too lofty for my goals and the time I’ll be owning the house. Plus it’s a 12/12 or 14/12 pitch, and the roofers I asked weren’t keen on 8″ of foam. So this brings me to my house wrap question, and it’s pertinent now because I am replacing doors and a window and flashing correctly. So if the housewrap serves a purpose under foam, I will do it on the wall that will be exposed. I just don’t understand the function. I originally thought you wanted a foam wall completely air sealed under foam. But in further research, that doesn’t seem to be critical, just the R value requirement for zone 5 to keep dew point exterior the sheathing (R-5 walls). Tyvek recommends their drainage wrap, to break capillary action if water does get under foam, I am just not sure it would perform. House wrap I think would allow me to ignore air sealing the roof, without causing moisture issues, vs peel and stick on walls and nothing on roof, correct?
Regular house wraps can’t act as a drainage plane if there isn’t a gap and a gap negates the foam insulative properties; and to my knowledge it is vapor permeable, but even EPS foam in 4″ thickness will negate drying to the outside. So is the housewrap at that point not just a waste? What purpose does it serve?
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