Do You Put a Vapor Barrier Over Spray Foam Insulation?

Sprayed foam insulation is one of the most effective methods of insulating the home.

Sprayed Foam

Vapor barriers typically come as sheets, membranes or boards.Vapor barriers typically come as sheets, membranes or boards.
Its dense, cohesive structure and ability to fill any space mean it can be used throughout the home to keep a building warm or cool, depending on the outside climate. As effective as it is, there may be occasions, however, when additional barriers are required alongside the foam.

Sprayed foam insulation is manufactured from several different materials. The most common is polyurethane. Typically this has an R-value – the unit of a material’s effectiveness at preventing thermal conductivity (the higher the number the better the efficacy) of between 7 and 8 per inch. Other materials include cementitious foam – which has an R-value of 3.9 -- phenolic foam – rated at 4.8 per inch -- and polyisocyanurate foam, which has an average R-value of between 5.6 and 8. All are installed either via hand-held spray guns or the use of pressurized air machines. They are suitable for insulating basements, walls and attics.

Vapor Barrier?

Because it is applied as a liquid, sprayed foam insulation – of any material – conforms to the shape of any area where it is installed, penetrating crevices, cracks and seams. As such, it is one of the most effective means of insulation, in part because its comprehensive coverage minimizes air and vapor penetration. As such, in many cases, a separate vapor barrier is not required. However, in more extreme climates, where moisture is a constant problem – in areas with heavy, regular snowfall, for example – a separate vapor barrier may be used in conjunction with the foam to protect the building from moisture penetration.

Types of Vapor Barrier

If a homeowner chooses to install a vapor barrier in addition to the sprayed foam, he should choose one with a perm rating of below 1. Perm ratings are the measure of a material’s effectiveness as an impediment to moisture, with those rated 1 or less classified as vapor barriers. Common vapor barrier materials used with spray foam include polyethylene plastic, which at a thickness of 6 mils has a perm rating of 0.06, plywood, rated at 0.7, and aluminium foil, which at.35 mil thick provides a perm rating of 0.05.

Required Barrier

Regardless of whether a vapor barrier is used or not, all spray foam insulation must be covered with a fire-resistant barrier. The chemicals used in spray foam insulation products become toxic if set alight. Drywall is a common fire-resistant covering for sprayed foam insulation. Further, homeowners must ensure that spray foam is not applied over electrical wiring or covering heating pipes.

About the Author

Dirk Huds has been a writer/editor for over six years. He has worked for bookshops and publishers in an editorial capacity and written book reviews for a variety of publications. He is currently studying for his master's degree.