Q. Do I Need to Use a Vapor Barrier with Spray Foam?
A. According to code requirements, a vapor barrier is required to be used with open-cell foam, but is not necessary with closed-cell foam. A vapor barrier, for the purposes of satisfying code, is a retarder with a permeance rating of 1.0 or less. In the Northeast, the vapor barrier is required to be placed over the interior side of the insulation (the warm side of the wall). This is because vapor typically follows heat flow – heat flows from warm to cold. Since the Northeast is predominately a heating climate, we try to stop or slow the migration of vapor through the wall or roof cavity at the interior surface. Recently, however, a significant amount of building science has been devoted to studying this vapor issue. Many researchers believe the use or misuse of vapor barriers may play a significant role in the cause of moisture related problems. There is a debate whether a vapor barrier is necessary with open-cell foam.
The latest science has separated the classification of vapor barriers into three categories: Class I, Class II, and Class III. Open-cell foam is considered a class III vapor retarder at normal thicknesses (sometimes as low as a class II retarder at greater thickness); while closed-cell foam is considered a class II vapor retarder. Further, the latest International Residential Code breaks the climate zone map for the U.S. into 7 zones. For instance, Connecticut is Zone 5, Westchester is Zone 4.
Vapor barrier requirements will vary depending on type of building assembly, use of the building, and climate zone. Some advanced computer modeling has shown that a vapor barrier may not be required with open-cell foam in typical wall assemblies in climate Zone 5 (since open-cell is already a Class III retarder). However, every wall and roof assembly is different and will have different requirements. With spray foam, vapor migration becomes less of a concern since most vapor is transported with air flow, not through diffusion. This is why the latest code language is primarily concerned with an air-impermeable insulation being applied to the roofdeck and does not discuss vapor permeability (the IRC explains this in its code commentary). Both open and closed-cell foam meet the requirement of an air-barrier.
As such, open-cell foam does retard the transfer of vapor both through air-movement and diffusion. In fact, The U.S. Dept of Energy issued a vapor barrier journal paper that provides vapor barrier guidelines for specific climate zones. Our climate zone, zone 5, requires a class III vapor retarder on either wall or roof cavities. Even basic 0.5lb open-cell foam will satisfy this requirement. Plus, southwestern CT is on the cusp of zone 4, which does not require any vapor retarder according to the D.O.E. guidelines. Moreover, studies conducted by the University of Waterloo, based on field testing and hygrothermal WUFI modeling software, concluded that an additional vapor retarder is necessary over open-cell foam only in climates above 7200°F heating degree days – well north of CT. (However, an additional vapor retarder is recommended in areas where the relative humidity will be maintained above 50% – i.e. wine cellars, indoor swimming pools, etc).
Confusion with the vapor barrier requirement often results in double vapor barrier assemblies – never recommended. For example, if the outside of your roof has a polypropylene waterproof underlayment with a vapor permeance rating under 0.05 perms (class I vapor barrier) and you then install a vapor barrier over the inside face of the insulation. Further, by installing a flash-and-batt insulation system in the rafter bays with FSK or kraft-faced paper, a double-vapor barrier situation is present. If excessive vapor becomes trapped within the fiberglass, it can
lead to unusual hygrothermal behavior and a dew point may occur. This is even more likely if heat producing air-handlers will be trapped behind insulated knee-walls and attic areas – the heat build-up and moisture loads in these areas can lead to extreme conditions. For instance, if these enclosed areas reach a temperature of 80°F with 50% relative humidity, then the dew point level would be 60°F! Remember, the code criteria requiring an R-20 minimum of air – impermeable insulation to the roof-deck is based on maintaining the surface of the foam above 45°F. This condensing temperature is based on normal attic conditions – 75°F interior temperature at 35% relative humidity. If the attic will be operating outside of these ‘normal’ parameters then additional measures will need to be taken. See the dew point chart below for more information.