If you have designed or built a low-slope roof with a single-ply membrane, at some point you will be confronted with this simple question: Is the vapor retarder necessary?
With considerable costs at stake, the prospect of omitting a vapor retarder is always a precarious notion and one that merits careful consideration of the project-specific dynamics at play.
Consider the Entire Assembly
In order to answer the question, one must first understand the characteristics of every component in the building envelope assembly.
Pictured below is a common low-sloped (i.e., slope less than or equal to 4:12) roof assembly featuring a single-ply roofing membrane:
Now, I could write at length to qualify the permeance values I have indicated above. Depending on the specific product, the permeance can vary quite considerably. However, suffice it to say that in most instances the general classification of vapor permeance for each component as listed above will hold consistent – with the roof boards (i.e., cover board and deck board) and metal deck as two notable exceptions.
You Actually Have Two Vapor Retarders
One should note that this assembly exhibits two vapor impermeable (i.e., 0.1 perms or less) components: the dedicated vapor retarder and the roofing membrane.
Whether considering a thermoplastic polyolefin (TPO), polyvinyl chloride (PVC), or ethylene propylene diene monomer (EPDM) single-ply roofing membrane, for a 60-mil thickness you are probably looking at about 0.05 perm (0.10 max). Classified as vapor impermeable, this layer functions as a vapor retarder.
Omitting the Vapor Retarder
One way or another, moisture will find its way into any building envelope assembly. When this happens in the roof assembly described above, the moisture will not be able to drive out in either direction because the vapor retarder is inboard of the insulation and the roof membrane (another vapor retarder) constitutes the exterior layer of the assembly.
The solution: by omitting the dedicated vapor retarder, vapor drive toward the interior can allow moisture to eventually dissipate.