How to Install Foam Roof Insulation

What You'll Need
Foam insulation
Air baffles
Staple gun
Heavy staples
Utility knife
Work gloves
Polyethylene vapor barrier

-Foam roof insulation will help guard against thermal weak spots and increase your home's energy efficiency. Foam insulation is resistant to rot from moisture and therefore can be run from underneath siding, down the wall of the foundation and into the ground. When adding any insulation, it is important to remember to fill any gaps that don't match standard sizes. This often occurs in areas of uneven roof framing or headers over doors and windows.

Step 1: Attach Air Baffles

Air baffles are stapled to roof decking to help moderate temperature and protect against damage from water condensation. Staple each of these to underside of the decking before installing the foam insulation itself. Roof insulation itself is designed to create a continuous thermal barrier that reduces air flow through the internal roof structure.

Make sure the foam insulation you purchase is of good quality. It should be labeled with the measurement of depth you should use as a guide for installation. Following this will ensure the most effective use of foam insulation for your roof.

Step 2: Check for Gaps

Look for any spaces between the rafters where air can circulate outside the baffles. Fill in these gaps with appropriately-sized foam insulation batts. A large percentage of roof insulation defects result from incorrect installation with gaps or compressed foam. Paying careful attention to this the first time will do much to avoid costly and time consuming fixes, along with increased heating bills and less ability of the roof to retain heat in cold weather.

Step 3: Position Foam Insulation

Flatten each panel of foam insulation in the same method you would when installing it between wall studs. Make sure that each insulation batt does not overlap or compress any of the air baffles. Carefully trim off any insulation that does not fit, but not so much that you accidentally create more air flow gaps.

Be sure to always use a dust mask or respirator when working with foam insulation materials. Airborne matter from them can remain in the lungs and cause health problems later on. Wearing the proper protective gear is a simple step to avoid this issue.  

Step 4: Insulate Headers

Roof framing often includes headers, which are designed to bridge door and window openings. This is especially true for doors with a plywood core and low natural insulation. If your local building codes allow this, replace the plywood core with a layer of ½-inch thick foam insulation as an easy improvement.

A useful guideline to insulation work is the law of diminishing returns; the first foam layer offers the best thermal air flow improvements and better comfort inside the home. Adding more layers of insulation or repeatedly fixing poorly installed panels will gradually provide fewer and fewer of these initial benefits. A sensible rule to follow is cut measure and cut exactly enough insulation the first time to fill the space in between the roof sheathing members, no more and no less.