Can Foam Insulation Be Applied to the Underside of a Roof?


Attic roofs coated with foam insulation create a sealed space that becomes part of the living space in your home. You can apply two basic types of foam insulation to your roof deck. Spray foam insulation is applied using a pneumatic sprayer that evenly distributes the insulation across the underside of the roof. Rigid foam insulation comes in boards that are attached using fasteners.


  • Roofs with an insulated deck are designed to incorporate the attic space into the thermal envelope of your home. Unlike traditional attic ventilation systems, an attic with this type of insulation is unvented. If you decide to install foam insulation underneath your roof, you will need to block any soffit or ridge vents in your attic. Sealing other gaps in the roof where it meets the walls, a chimney or around your plumbing vents will improve the energy efficiency of your attic.


  • Moisture infiltrating your attic can damage your insulation and the structural members of the attic. Typical attic insulation systems put the vapor barrier over insulation in the attic floor. Insulated roof decks do not use a vapor barrier over the floor since it is unnecessary, and could impede the flow of air between the attic and the rest of your home. Since the insulation on the underside of the roof acts as a vapor barrier, it is important to fully seal the attic against air leaks. This will prevent moisture from condensing on the area around the leak and damaging the attic.


  • Many forms of foam insulation are flammable and require additional fireproofing. Rigid foam board insulation often requires a layer of gypsum board or another flame-retardant material. Consult your local building codes to determine what type of fireproofing is required for foam insulation in your area.


  • Including the attic in the conditioned space of the house will reduce energy losses from heating and cooling ducts in the attic. Airborne moisture from the outside can damage your insulation and promote the growth of mold in your attic. Sealing the attic against outside air will help mitigate these issues. Before you begin work, consult your local building codes to determine the legal requirements for converting your attic to a ventless insulation system.

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