1

I recently hired pros to spray foam my attic, essentially sealing the soffits, roofdeck, ridge vent, and the removed all cellulose insulation to allow air exchange and balance the conditioned space below with as of now unconditioned attic.
Question: adding fiberglass insulation over the garage (no HVAC) - should there be a vapor barrier face down ? (SC southern warm climate). Basically the vapor barrier between attic joists resting against the drywall ceiling below.

4
  • 2
    Why did the pros seal the soffit and ridge vents?
    – SteveSh
    Oct 15 '21 at 9:23
  • I am with SteveSh, it sounds like you blocked all attic ventilation, which is usually not good.
    – crip659
    Oct 15 '21 at 12:31
  • SteveSh, it’s my understanding that it’s common practice particularly in the south, to completely seal the envelope in the attic when professional spray foaming the roof deck and sealing the soffits and ridge vent. My attic has full stairs going up to it, it’s a modular home, it already has a plumbing feed, so with the spray foam done, I hope to have air conditioning added in the next year. I will put my music studio in the attic, I may still have some problem areas like behind the new walls if I drywall my music room up there so I’m thinking maybe I’ll put a couple low amperage air movers.
    – guitar guy
    Oct 15 '21 at 15:50
  • You've converted unconditioned space to conditioned space. This would be similar to a cathedral ceiling. You are really supposed to have some ventilation below the roof sheathing to prevent the shingles from heating up too much. I wouldn't have sealed it like that - hope you have a metal roof. Oct 17 '21 at 3:11
0

A vapor barrier is wonderful because it is an excellent air flow stop. The key to where to put and not to put a vapor barrier lies in the fact that there can be only one vapor barrier between two spaces that breathe. The existence or lack of an air temperature changing unit is of no consequence.

  • Temperature changes cannot be avoided. Day/Night happens.
  • Water cannot be totally removed. Water, usually in the form of vapor, is in the air everywhere.
  • The temperature of the air determines how much water it can hold. When life gets cold, the water cannot remain as a vapor in the air, but comes out of the air as water deposited on everything (the cause of morning dew).
  • Air trapped between two vapor barriers will wet every morning and dry every night. Everything between two vapor barriers will mold, rot, etc.
  • A vapor barrier stops water. Water resting on a horizontal vapor barrier will not flow away. Therefore, a vapor barrier is usually never installed horizontally. Unfaced insulation is for your attic because it should not have a vapor barrier at all.

If your attic has a vapor barrier, then leave it there, but don't add another one. If your attic does not have a vapor barrier, then don't add one.

Foam insulation is a vapor barrier. But unlike anything assembled, foam is not subject to internal molding and leaves no air gaps at all.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.