The Best Way to Insulate Across Rafters


Homeowners are increasingly looking for the best way to insulate across rafters. For typical home, attics have the most extreme temperature of any parts of the house. During the summer it is the hottest part of the house. In winter, the attic is usually the coldest space. This extreme variance in temperature, between the conditioned living space and the outside, means the heating and cooling system has to work harder to maintain the comfort level in homes that are not properly insulated.

Installation Methods

  • The attic is crucial for allowing your home to breathe and for excess water vapors to escape. The attic must be adequately ventilated. Speak to a building department inspector to get information on attic ventilation requirements, recommended R-values and techniques for completing the job in the correct manner. Also, get specific information on the use of vapor barriers.

    Perform a visual inspection of the attic; make sure that all cracks and openings are sealed and weather tight. Use spray foam to seal any openings. If you are using spray foam insulation, you can skip this step.

    Various types of insulation can be used to insulate the rafters, including Fiberglas batts, rigid foam boards and spray foam insulation. Foam insulation is classified as open-cell or closed-cell. Foam insulation is more expensive than Fiberglas insulation; however, closed-cell foam insulating products have significantly higher R-value per inch at 7.0 to 8.0. Fiberglas insulation yields an R-value range of 3.1 to 3.8 per inch.

    The primary difference between open cell and closed-cell foam is that closed-cell foam is dense and prevents air or water from penetrating the material. The R-value of open-cell foam insulation is comparable to Fiberglas insulation. Nevertheless, open-cell foam insulation is a better-insulating product.

    Typical R-values for attic rafter range from R-19 to R-38. However, if you are working with 2-by-6 inch or 2-by-8 inch rafters, you can only install up to 10 inches of batt insulation. To fit more insulation in a cavity, consider increasing the depth of the space by installing 1-by-2 inch or 2-by-2 inch furring strips on the rafters. Another approach to getting the desired R-value is to install rigid foam board on the underside of the roof.

    When working with limited space, you may want to consider installing high-density insulation. High-density insulation gives you as much as 35 percent more insulating value for each inch of material compared to standard insulation.

    In ceilings with 2-by-12 inch rafters, you will need to install vent baffles between the insulation and the roof deck. This will give you the required ventilation channels required for the adequate flow of air into and out of the attic.

    If you decide to insulate across the rafters using a closed-cell foam product, speak with the building department inspector first. Some codes prohibits the use of closed-cell foam between rafters, especially if the HVAC is located in the attic. Remember, closed-cell foam does not allow air or water penetration. Also, if the heating and cooling equipment is located in the attic, you will need to insulate the ductwork.

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