Is it safe to put a poly vapor barrier over an old roof when insulating?
GBA Editor
| Posted in Energy Efficiency and Durability on
I have a 2 X 6 T&G wood cathedral ceiling, with 20 year old asphalt shingles on it. I built a new steeper roof with space for insulating above it. I prefer not to remove the shingles which were left on to protect the house while the new roof went up. I am insulating and air sealing and am putting a poly vapor barrier over the old shingles before adding about 18″ insulation. Is there any chance the old shingle layer could create indoor air quality problems being under the poly layer?
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Yes.
To elaborate. My previous answer was to your last question: "is there a chance..."
There is no good reason to leave the old shingles behind. Eventually, they will end up in the landfill and it may as well be now. In fact, it's almost always problematic to leave an existing layer of asphalt shingles when doing any sort of reroofing.
Whats needed in conjunction with the T&G ceiling is an air barrier, not a vapor barrier (unless you're in climate zones 7 or 8). And a secondary weather barrier would be advisable as well. So taped housewrap would be the more appropriate barrier. The existing roof could have been temporarily covered with roofing felt to protect it during reconstruction.
I would also advise incorporating a ventilation channel in the new roof, along with continuous soffit and ridge vent (with external wind baffles).
At this point, since the new roof above (a 7/12 slope) and the attic space above the ceiling (a 2/12 slope) are complete, it will be difficult to remove the shingles and felt paper through the attic hatch. I had thought about laying housewrap down first, over the old shingles before the FG batts were laid but assumed it should be on the outside, or in this case the upside of the insulation, since it is normally installed on the outside of the envelope. The space above the insulation, the attic space, is ventilated with continuous soffit and ridge vents. So if I understand correctly, your advice is to remove the shingles and felt paper, put down a taped layer of housewrap as an air barrier, then insulate. Where would the secondary weather barrier be placed? Would this be another layer of housewrap draped over the FG batts? Thanks for your suggestions. I am in the Denver, CO area, zone 2.
The most important thing to have above a T&G cathedral ceiling is an air barrier, uninterrupted by ceiling lights and other penetrations, or you're almost certain to have moisture problems in the attic. Ideally, that would be located immediately behind the wood ceiling. A vapor barrier, like poly, is no longer required by code in the US, only a 1 perm vapor retarder like latex VR primer. Poly creates as many problems as it solves because it traps moisture.
Since you're probably not going to remove the T&G ceiling to put a proper air barrier above it, the second best would be to install a complete air barrier above the old roofing (since it's probably impractical to remove it now. But the new roof framing may make it impossible to install an edge-to-edge air barrier, so you may have to do the best you can with material that stops air but not water vapor, such as housewrap. This will NOT be a secondary weather barrier, but rather an air barrier.
The secondary weather barrier will be whatever underlayment you install under the new roofing. Make sure the ventilation channel is isolated from the fiberglass insulation or you can get "wind wash" convection which will significantly diminish the effective R-value.
By the way, Denver is IECC zone 5.