Is this “double vapor barrier”..?
Hello,
We are coming late to a new construction low income housing project and are trying to add a few energy efficiency measures. The house has Kraft paper faced R-30 insulation in the attic and no other surface for attic floor but drywall.
We are thinking of adding 6 mil poly sheets, taped at seems and edges and around electrical fixtures etc. before drywall install.
Would this constitute a double vapor barrier to attic? (a.k.a. something one should not do)? We are also thinking of using low-perm (0.6 perm) paint primer on the ceiling and walls. Again, is this a double barrier or – rather – a double measure (something that enhances the positive effect of vapor and air barrier?
We do know to seal all holes leading up to attic and going about it diligently.
The walls are 2×6 with fiberglass batts, OSB, Tyvek, and vinyl siding. We were too late to make any improvements there except air sealing penetrations and electrical outlets.
Thanks
Lucy
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Lucy,
Before we can answer your question, we need to know your climate zone or geographic location.
Also: What is the purpose of the polyethylene? Are you trying to meet code requirements for a vapor retarder, or do you intend the polyethylene to be an air barrier?
In general, the taped drywall is usually the air barrier. The kraft facing meets code requirements for a vapor retarder in cold climates. So the polyethylene is unnecessary.
That said, adding the polyethylene wouldn't create any problems -- having two adjacent interior membranes that are both vapor retarders won't cause moisture problems in your ceiling.
Lucy,
For an in-depth discussion about the misconceptions surrounding "double vapor barriers," see this article: Worries About Trapping Moisture.
Thanks Martin, I'm still in Pittsburgh PA, zone 5. With Passive House background I am primarily thinking of air barriers... but vapor, going into walls and toward attic is an obvious concern too. If we proceed with poly sheet, what (readily available) tape would you recommend for that?
Lucy
We are attempting to air seal the house to at least the Energy Star level (4 ACH/hr at 50P) and will be providing mechanical ventilation of the whole house.
Lucy,
In Pittsburgh -- a climate where air conditioning is common -- I don't recommend the use of interior polyethylene. Interior polyethylene occasionally causes problems in air conditioned buildings.
As I wrote in my earlier answer, detail the drywall as your air barrier. The kraft facing on the fiberglass will easily handle any outward diffusion in the wintertime.
If you ignore my advice and install polyethylene, try to find a brand without a chalky coating. For most brands of polyethylene, housewrap tape works. For a better quality tape, choose from these three high-performing tapes: Siga Wigluv; Siga Sicrall; and Pro Clima Tescon No. 1.
For more information on tapes, see Return to the Backyard Tape Test.
If you live in PIT, CZ5, you are required to install R49 insulation in the attic, not R30, low-income or not.
Lucy,
Q. "We are also thinking of using low-perm (0.6 perm) paint primer on the ceiling and walls. Again, is this a double barrier or - rather - a double measure (something that enhances the positive effect of vapor and air barrier?"
A. Low-perm paint will reduce the rate of diffusion through the drywall, but if you have kraft-faced batts (or kraft-faced batts plus poly), the paint is unnecessary (i.e., a waste of money).
There are absolutely no cases where low-perm paint has a positive effect on the performance of an air barrier. To put it another way: you can't use paint to repair air leaks.
A more relevant question might be air-tight drywall vs some vapor permeable air-tight film. Or both.
A layer of cellulose over attic floor fiberglass works well.
Armando has it right- R49 is code minimum for Pittsburgh.
But code-max air leakage for zone 5A is 3ACH/50 (not 4). How does sub-code air leakage performance of 4 ACH/50 qualify as Energy Star?
Hi Martin, could you say few words as to how the poly sheet on the ceiling may be bad in the air condition house?
Lucy,
Q. "Could you say few words as to how the poly sheet may be bad in the air conditioned house?"
A. Problems are admittedly rare (and are usually restricted to walls, not ceilings). The classic example concerns walls subject to inward solar vapor drive. (For more information on this phenomenon, see When Sunshine Drives Moisture Into Walls.)
In the summer, your interior drywall (and the polyethylene immediately behind the drywall) is quite cool. If the stud bays of your walls are filled with humid outdoor air, condensation can occur on the cool polyethylene, and the water can drip down to cause puddles on the bottom plates of your walls.
Like I said, the problem is rare. But ideally, you won't have a condensing surface like polyethylene on the interior of your walls.
Lucy - Infiltration rates for ES V3.1 (Rev. 08) in CZ1-2 is 4ACH50 and CZ3-8 is 3ACH50 for homes permitted after 7/01/16 same as IECC Code.