Vapor Retarders...to use or not to use

When in doubt, leave it out - this seems to be the best rule of thumb when contemplating whether or not it's necessary to use a vapor retarder. But what is a vapor retarder?

A vapor retarder is a roof component designed to restrict migration of water vapor molecules from areas of high pressure to areas of low pressure. In a cold climate, this vapor migration is in an upwards direction. In a warm climate, the vapor is reversed or in a downwards direction. In coolers and freezers, vapor may move in both directions depending upon exterior temperature and humidity.

For a material to qualify as a vapor retarder, its vapor permeance rating should not exceed 0.1 perms.* (* A perm is a unit of water vapor transmission defined as 1 grain of water vapor per square foot per hour per inch of mercury pressure difference; 1 inch of mercury 0.491 psi). A variety of materials qualify as vapor retarders:

• Polyethylene film
• PVC (polyvinyl chloride) sheets
• Aluminum foil
• Coated Kraft paper or laminated Kraft paper with bitumen filler
• Saturated felts (2-plies) with two or three asphalt moppings. Coated base sheet with one or two asphalt moppings.
• Built-up roof membrane

The Manual of Built-Up Roof Systems, (second edition), lists the following points for and against using vapor retarders respectively:

Advantages:

• A vapor retarder can ensure the continual thermal resistance of insulation sandwiched between the vapor barrier and built-up membrane.
• A vapor retarder is a good safeguard against vapor migration if a building's use changes from "dry" to a "wet" use. "Dry" is defined as an interior relative humidity under 20 percent. "Wet" is defined as an interior relative humidity over 45 percent.
• A vapor retarder is advisable over wet decks (poured gypsum and lightweight insulating concrete) to prevent vapor flow upward into the insulation. For this purpose, the vapor retarder should be a venting base sheet, with mineral granule underside surfacing, mechanically fastened to the decks.

Disadvantages:

• The vapor retarder, together with the built-up roofing membrane, inevitably seals trapped moisture within the roof sandwich, which can eventually destroy the insulation, split or wrinkle the built-up membrane, or in gaseous form, blister it.
• In the event of a roof leak through the membrane, the vapor retarder traps the water below the insulation and releases it through punctures that may be some lateral distance from the leak, making it difficult to locate. A large area of insulation may be saturated before the punctured membrane can be repaired.
• A vapor retarder is a disadvantage in summer, when vapor migration is generally downward through the roof. (Hot, humid air can infiltrate the roofing sandwich through the vents, or by diffusion through the membrane. It may also condense on the vapor retarder itself).
• A vapor retarder may be the weakest horizontal shear plane in the roofing sandwich. Failure at the vapor retarder-insulation interface can split the membrane. The vapor retarder introduces an additional component where shear resistance may be critical to the membrane integrity.

When should a vapor retarder be used?

The old school of thought was, "If in doubt, include a vapor retarder." However, the modern policy is, "If in doubt, omit the vapor retarder." The NRCA recommends that a vapor retarder be considered when one of two conditions are anticipated:

1. The average January temperature outside is below 40 degrees Fahrenheit.
2. The expected winter relative humidity is 45 degrees or greater on the interior.

Even if both these conditions exist, an analysis should be performed to determine the need for a vapor retarder.

Design Considerations:

The vapor retarder is located on the warm, humid side of the roof assembly. Generally, the vapor retarder will be placed on the deck with the insulation and roof membrane subsequently installed. An exception to this installation is a roof application over a freezer.

A built-up roof application does not require a vapor retarder, as the roof membrane serves a dual function.

If a single-ply roof application requires a vapor retarder, it should be located between the insulation and membrane. If a single-ply roof application is installed over an existing built-up roof, a vapor retarder is not required as the existing built-up roof will serve as the vapor retarder.