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:
Example low-sloped roof assembly with a single-ply membrane.
Illustration by Daniel Overbey.
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.
Control the Permeability of the Metal Deck
It may occur to you that metal deck is theoretically impermeable to vapor diffusion. However, according to Oak Ridge National Laboratory, values of metal deck permeance can vary from 0.64 to 10 perm, depending on particulars characteristics regarding gaps between deck components, burn holes, and open air feeds between the deck and walls along the perimeter of the building. Depending on the state of its installation, the metal deck may be vapor semi-impermeable or vapor semi-permeable.
In the interest of allowing vapor-driven drying to occur across the entire roof assembly, it may be good practice to specify a slotted metal deck in order to achieve a consistent vapor semi-permeable condition.
Carefully Specify the Roof Boards
Without a vapor retarder, the "deck board" (sometimes called a "roof board" or "substrate board") under the insulation layers has much less utility. Considering that some deck board products may be vapor semi-impermeable, omitting this component will save cost and may foster greater drying potential toward the conditioned building interior. Unless a deck board is required as a thermal barrier, or to achieve a certain fire rating, or to address high-humidity interior environmental conditions, it could be eliminated.
The "cover board" (sometimes called a "roof board" or "recovery board") is placed just beneath the single-ply membrane. This component is often necessary to achieve a resilient roof surface condition. However, depending on the cover board product specified, its permeance can range quite considerably
Will Omitting the Vapor Retarder Negate the Roofing Membrane Warranty?
Recently, I approached representatives from two major roofing membrane manufacturers about the notion of omitting the vapor retarder in a low-slope roof assembly with a single-ply membrane. Neither would comment on whether an assembly should include a vapor barrier/retarder since their companies do not perform hygrothermal or dew point analyses.
However, they were both in a position to comment on the requirements for their roofing membrane warranties.
One representative stated, "Not only do you not need a vapor barrier for warranty purposes. You don’t need the “recovery” board either. Unless you have a building with high humidity, the roof will perform just fine without a vapor barrier."
The second representative said, "A Vapor Barrier or Retarder is NOT required for our "PVC" or other manufacturer warranties. It is probably considered good roofing practice for many of the types of buildings [where] you are maintaining higher levels of humidity and the NRCA recommends that when the January 'avg' temp is 40 degrees Fahrenheit or colder and you are maintaining an interior RH level of 45 Degrees or greater that a Vapor Barrier should be utilized."
Project teams should carefully approach the issue of whether or not a particular low-sloped roofing assembly with a single-ply membrane needs a dedicated vapor retarder. A building envelope assembly should never be designed in such as way that it could trap moisture. The potential for drying must be afforded. Ultimately, the decision should be based on a careful and comprehensive analysis of the particulars of the project.