How to Insulate a Metal Roof From the Underside

Put on your safety gear. Even if the closed-cell urethane spray foam you purchased may be nontoxic, it is important to take measures to avoid inhaling it or having the foam come into contact with your skin, because it is extremely sticky and difficult to remove. Your safety gear should include chemical-resistant goggles, a hooded chemical-resistant jumpsuit that covers you from head to toe, nitrile gloves and respirator that the National Institute of Safety and Health approves.

Prepare the closed-cell urethane foam canisters or tanks according to the manufacturer’s instructions. Spray foam insulation generally comes in a kit that contains aerosol canisters of foam or small tanks, one or more dispensing guns, hoses that connect the dispensing guns to the tank, and mixing nozzles. Some manufacturers provide fanning nozzles that you can attach onto the end of the dispensing gun.

Spray the perimeter of the underside of the roof by depressing the trigger on the dispensing gun. The end of the nozzle should be 1.5 feet to 2 feet away from the underside of the metal roof.

Spray the underside of the roof with foam, working from top to bottom. Use a steady back-and-forth motion with your wrist as you spray. Bear in mind that if you move your wrist quickly, the layer of foam will be thin. The foam will expand about six times its initial volume, so you should have a layer of expanded foam that is 6 inches thick. Instead of spraying a large quantity of foam in one area, it is best to apply it in multiple 1-inch layers so you build up to the amount you need. Allow each layer to cure before applying the next.