How to Make More Room in the Roof for Insulation

Furring Rafters

• 1

  Measure the dimensions of an existing roof rafter, including the length and the angle at the top where the rafter meets the ridge beam. Cut a 2-by-2 board to match with a circular saw.

• 2

  Fit the 2-by-2 board over the edge of the rafter and nail it in place with a framing nailer and 12d nails at the rate of one nail per foot. Insert the nails through the center of the 2-by-2 into the rafter beneath.

• 3

  Cut and attach 2-by-2s to the edges of every rafter in the same method and install regular or high-density fiberglass batt insulation in the deeper rafter spaces.

Adding More R-Value

• 4

  Glue lightweight rigid foam insulation sheets over the furred rafters if you need more R-value. The sheets come in 4-by-8 foot sizes, and every inch of thickness adds R-5 to the roof. Standard thicknesses range from 1 inch to 2-1/2 inches.

• 5

  Position the insulation sheets so the edges meet at the center of a rafter edge. This is called “breaking on center” and will ensure that every sheet has a sturdy base. Cut the sheets to fit along the edge of the roof with a utility knife and a straightedge.

• 6

  Run a 1/4-inch bead of foam-safe construction adhesive along the rafters and mount the insulation sheets in place. Generally, the adhesive is all that you need to hold the lightweight sheets in place, but for extra reinforcement, you can tack the sheets at the corners of the rafters with ring shank nails. Insert the nails only until the flat shank head is flush with the foam surface.

• 7

  Apply foam-safe seam tape over the joints where the sheets meet to seal the insulation for the greatest R-value.