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
    Why did the pros seal the soffit and ridge vents?
    – SteveSh
    Oct 15, 2021 at 9:23
  • I am with SteveSh, it sounds like you blocked all attic ventilation, which is usually not good.
    – crip659
    Oct 15, 2021 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, 2021 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, 2021 at 3:11
  • Consider this space more of an unfinished 2nd floor rather than an attic - at least now that's what it is. When I add my home theater walls, which will be insulated for sound, then I will create spaces behind the knee wall and between collar ties/ridge that will lack air exchange. I will place a supply/return of 4" flex duct in these three spaces in order to hook up to a dehumidifier after construction. My new HVAC ductwork will of course cool my theater space. Sounds like a preventative remediation?
    – guitar guy
    Feb 19 at 12:11

1 Answer 1


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.

  • "A vapor barrier stops water" - more precisely, a vapor barrier stops water vapor. "Water resting on a horizontal vapor barrier will not flow away" - true, but there should never be any such water. "Therefor a vapor barrier is usually never installed horizontally" - nonsense! Almost all plasterboard (sheetrock) boards in the UK have a metal foil vapor barrier on one side. These are often horizontal installed on ceilings. Jul 12, 2022 at 13:45
  • The difference between "what should be" and "what is" is the difference between an ideal world which does not exist and the real world which does exist. Unfaced insulation exists and is labeled "attic" so that horizontal vapor barriers can be avoided. A slow leak which sits upon a horizontal surface might not be discovered until the ceiling trusses are rotted. The commonality of a practice may prove physical location, but commonality does not prove correctness. An overly insulated house grows mold. One horizontal vapor barrier on the roof and no more is very sensible. Do what you want.
    – Paul
    Jul 14, 2022 at 0:14

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