How to Install Rigid Flat Roof Insulation

A turbine exhaust vent on a flat, black roof.
  • 8-24 hours
  • Intermediate
  • 200-900
What You'll Need
Rigid foam board insulation
Staple gun
Spray foam
Tape measure
Utility knife

Installing flat roof insulation doesn’t have to be a very difficult job. Most do-it-yourselfers can actually install rigid foam board insulation on their own. This type of insulation works best for an exposed ceiling directly below a flat roof. Installing it will probably cut your energy bills a bit, but will definitely make your living quarters more comfortable.

Take Measurements

Before you can cut the foam to size, you need to take measurements. First, measure between the joists. Be sure to take perfect measurements, because the foam is going to be held in place by the joists. If the measurements are inaccurate, your foam won’t stay in place.

Cut the Foam to Size

Using a utility knife, cut the foam to size. An easy way to do this is to place a straight edge on top and score the foam. Once you penetrate far enough, you should be able to snap the foam along the line.

Press Foam Between the Joists

Depending on the height of your ceiling, you may need to use a step ladder to install the foam. The 2-inch foam will probably work fine, however, rigid foam insulation is available in various sizes. Depending on the depth of your ceiling, look into the various sizes available. Standing on the step ladder, press the foam against the top of the ceiling. Butt the pieces of foam tightly against one another.

Spray Foam Along the Edges

Along the edges of the rigid foam insulation, apply spray foam to create an airtight seal. Spray foam comes in a can and can be found at your local home-improvement store. Spray along every seam with this foam. If you are unable to find spray foam, you can also use caulk, but you won’t achieve quite the same results.

Add Plastic Sheeting

Once you have placed the rigid foam and created a seal with the spray foam, cover the ceiling with plastic sheeting. Start from one corner of the room and staple the plastic to the joists. Keep the plastic taut as you work across the room. Overlap the plastic one complete cavity. This will create a tight vapor barrier.

An alternative to plastic sheeting is to simply hang a drywall ceiling. This will involve more time, however, it will create a more finished look.