How to Use Fiberglass With Polyisocyanurate Foam

Fiberglass insulation is frequently used in home construction.

Polyisocyanurate foam is an insulating foam sold in liquid, sprayed foam and rigid foam board forms. The foam has a higher R-value per inch than fiberglass insulation, but is not cost-effective in some applications such as insulating the interior of your home. It is commonly used in boats and cars to provide a strong insulation that also buffers sound. When used in construction, the foam is most frequently used on the exterior side of the walls of a structure while fiberglass insulation is used on the interior side.

Things You'll Need

• Polyisocyanurate foam board
• Heavy-duty stapler
• Utility knife
• Caulk
• Fiberglass insulation

Instructions

1. Finish the exterior studs of the building, but do not begin to apply the siding, brick or other exterior finish. Instead, first line the exterior studs with polyisocyanurate foam board.

2. Place the boards close together over the studs, with less than 3 mm between them. Use heavy-duty staples to secure the foam boards into place, stapling them into the studs. Cut them with a utility knife if necessary.

3. Seal the seams between the boards with a wide bead of caulk to prevent air infiltration and water absorption. Apply tape or plastic over any areas not waterproofed by the exterior finish.

4. Apply the exterior finish, such as siding or brick.

5. Finish the interior studs of the building. Unroll fiberglass insulation, and cut it to a length equal to the height of the room using a utility knife.

6. Tuck the fiberglass insulation between the interior studs, with the paper side facing into the room. Staple it into place with a heavy-duty stapler. Finish the interior walls with drywall or plaster.

References

• Green Building Advisor: Rigid Foam Insulation
• Johns Manville: SP Foil Faced
• US Department of Energy, Energy Savers: Foam Board Insulation