Vapor Barrier on a Low-Sloped Roof
I have understood that in low-sloped roofs, the vapor barrier typically goes below the insulation, right above the structural deck. Is this true for all climates? Or would we treat it similar to a wall, where it goes only on the "warm side"?
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Yes, it's mostly true for all climates in low-sloped ("flat") roof assemblies. But what we often call the "vapor barrier" in those applications is serving a different, more important function as the air barrier. What we really need is for that membrane to keep warm, interior, moisture-laden air out of the roof assembly. If interior air were permitted to enter the roof assembly, it would cause condensation on the underside of the roof membrane in wintertime. (Because warm, moisture-laden air + cold surface = condensation). This problem is worse in colder climates, but can happen in mild climates, too. Best practice is to include it in all climates.
Anyway, architects (and manufacturers!) are in the habit of calling the membrane you're describing a "vapor barrier" even though its primary purpose is to block air. The fact that it's vapor impermeable is often just incidental. This is a case when popular terminology is confusing.
What adds to the confusion is that in some climates (cold and very cold ones) we need to keep both interior air AND interior water vapor from getting into the roof assembly. In some assemblies, this membrane will perform double duty as the air and vapor barrier. In other assemblies, the vapor control comes from something else (like OSB sheathing or fluted metal roof decking). But this is hard to remember. So people just call the membrane a vapor barrier whether they need the vapor control or not and they do the same thing in all assemblies.
This is way more than you need to know for the AREs. So the simple answer to your question is: yes. As to the second part of your question about vapor barriers on the warm side of the insulation, remember that in low-sloped roof assemblies in all climates we typically already have a vapor barrier on the warm side of the insulation because the roof membrane itself is vapor impermeable (because it's fully waterproof). In low-sloped roofs it is common to have a double vapor barrier: One being the membrane you describe and the second being the roof membrane itself.
This is a really difficult topic to describe in words. I'll see if I can find any illustrated explanations and post them below. Hope this helps anyway.
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You're so welcome!
Here's a video explaining the difference between air barriers and vapor barriers. The practical examples I use to explain the difference all happen to be low-slope roof assemblies! The video is about 15 minutes.
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Hi Christine,
Most projects I work on are located in zone 2A, our typical roof assembly is composed of (2) layers of staggered polyiso insulation, gypsum coverboard and a roof membrane. Staggering an air impermeable insulation should suffice as the air barrier - in theory. I am not confident we can maintain a continuous air barrier with several hundred roof fasteners penetrating the metal deck...
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