Can I Add Styrofoam Insulation to a Drywall Garage Ceiling?

Attic Access

• Most garage ceilings have an attic access framed in during the construction of the ceiling. If your garage ceiling does not have an access, you’ll have to cut one between the ceiling joists to reach the space. There is no decking above garage ceiling joists, so you'll need plywood panels, temporarily positioned over the joists to provide a safe platform on which to stand or kneel.

Fitting the Insulation

• Rigid foam insulation should fit snugly between joists or studs to block air transfer between the interior and exterior of a structure. From the attic, you have access to the joist spaces, but it can be difficult to measure and fit the individual panels. Styrofoam insulation comes in 4-by-8-foot panels, so you’ll have to cut the panels individually. At the edges of the garage ceiling where the rafters meet the top wall plates, it may be difficult to insert the panels into the joist spaces.

R-Value

• A material’s ability to resist the flow of heat is its R-value, and the higher the value, the more insulating factor the material has. Thicker Styrofoam panels have greater R-value. A panel that is 1 inch thick has an R-value of 5 and a 2.5-inch-thick panel has an R-value of 12.5. Unfortunately, R-value decreases when there are gaps between the rigid foam and the joists. Because studs and joists often have some measurement discrepancies, cutting all the panels the same dimension may result in some gaps.

Considerations

• Installing Styrofoam insulation in a drywall garage ceiling is possible, but it’s labor intensive and the results may not be a good as if you were installing the rigid foam between wall studs where you have easy access to measure and fit each panel. If your garage ceiling has lights, you’ll have to notch the rigid foam around the light boxes. Fiberglass batt insulation is easier to fit between the ceiling joists. Blown-in cellulose fiber insulation is the simplest to install in limited attic spaces, and it can be blown in to custom depths when additional R-value is desired.