Spec’ing an air/vapor barrier for row house roof? – GreenBuildingAdvisor

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