Spec’ing an air/vapor barrier for row house roof?
Hey all,
I live in a Brooklyn rowhouse originally constructed around 1875 and converted into four condos in the late 1990s. I usually manage our larger maintenance projects given I have some DIY type experience with construction.
The time has come for us to replace our roof membrane, which is a somewhat daunting project given our varied experience with roofing contractors, the costs involved, and the bad outcomes possible with poor design or install.
Our building has a low-slope roof, and multiple layers of bitumen membrane have accreted over time. There is a cockloft (i.e., a small sloped attic) between the top floor unit and the deck, typical of rowhouse construction of this era. The roof deck is timber, original to the building, and from what I can tell is generally in good condition. I have limited access to the underside of the deck via a hatch, and through some gaps in the deck boards I can see that metal sheeting (tin or lead?) has been laid over the boards. Brooklyn is in climate zone 4A.
We are planning to do a full tear off of the existing membranes, and will install rigid insulation on top of the deck as part of the new build up. To my surprise most of the estimates we received do not specify an air/vapor barrier. This seems odd to me given everything that I’ve read about low slope roofs. Our preferred roofing contractor said that they only install them when explicitly specified by an architect or engineer, and characterized them as more of a “belt and braces” approach than of necessity.
Our building is particularly drafty (the 1990s renovation was done on the cheap) and there’s almost definitely interior air circulating into the cockloft behind the old lath and plaster walls, and via a poorly firestopped chase for chimney B-vents. I don’t see any ventilation openings from the cockloft to the exterior. These factors would seem to make the installation of an air/vapor barrier above the deck all the more important.
I’m curious if that assessment seems correct to others with more experience in this area? And, if you have been down this path, does my experience with the contractors sound typical?
Thanks,
Niall
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Replies
The attic is drafty because it is currently a vented attic. That is a good thing, if the insulation is on the attic floor, you need that space vented to the exterior.
By installing the rigid above the roof, you can now turn it into an unvented attic. For this to work you have to get the insulation off the attic floor and insulate under the roof deck with fluffy. The ratio between the exterior rigid and interior fluffy needs to be kept to 30%, so say R15 above, you can have up to R34 fluffy in the rafter space. You would also have to seal the existing attic perimeter, this is usually simplest with a flash layer of spray foam. Watch around the b-vent as you can't have spray foam on that.
After that, you don't need and specifically don't want any vapor barriers. The above assembly works by keeping the roof deck warm enough that you don't get condensation on it. Any vapor barriers in the stack up will end up trapping moisture.
Akos, thanks for the reply.
I'm realizing that in describing the attic as "drafty", I implied that it's intentionally ventilated to the exterior. That does not actually seem to be the case (i.e., I don't see any ventilation openings at the perimeter walls or on the roof itself). What I should have said is that, given the current condition of the building, I suspect conditioned interior air can easily circulate into the attic space, which is why I'm concerned about the potential of condensation in the new roof assembly.
That said, generally speaking, the whole building is drafty. For example we temporarily removed an electrical box on an exterior wall when painting, and in windy conditions could easily sense air moving inside the wall. Or if you close light drapes over a window, they will sway lightly. I suspect exterior air is blowing in around the original timber window frames (double hung weight-and-chain), and circulating behind the original lath and plaster finish on the exterior masonry walls.
So, back to the roof, if we were insulating the roof from both sides (R-15 sheet insulation above deck, R-34 open cell spray foam below deck), how would the presence of an air/vapor barrier above the deck be problematic?
My understanding is that it would actually be desirable, since while the roof deck would be above the dew point, the polyiso and roof membrane layered above it would certainly not be.
I appreciate an air/vapor barrier below the deck (or closed-cell spray foam) would be undesirable, as they would prevent interior drying, but I can't see how one above the roof deck would be an issue, when insulating from both sides, or just on top.
Niall