How to Get a Durable Finish on Outdoor Rigid Foam Board

  • 3-6 hours
  • Intermediate
  • 50-200
What You'll Need
Vapor diffuser retarder or heavy plastic
Caulk and caulk gun
Gypsum wallboard
Foam board finish

Rigid foam board is a type of insulation that can be used for many different types of buildings and structures. It provides a barrier that reduces the heat conductivity between wood and metal. To give your rigid foam board a durable finish, you need to know what kinds of problems the insulation will face as a result of the elements. Just because you purchase exterior rigid foam board doesn’t mean you’re getting as much protection as you need.

Step 1 - Treat the Rigid Foam Board

Check your local hardware or home improvement store for different kinds of finishes you can apply directly to the foam board insulation. You might be able to find waterproofing or insecticide solutions that can be applied directly to the rigid foam board. It’s important to use these finishes before you actually install the rigid foam board insulation because you want to make sure you cover the entire area of the foam board. Most exterior foam boards come with some kind of built-in protection. If you read about the foam board carefully, you’ll be able to know what kinds of treatments have been done already.

Step 2 - Install the Rigid Foam Board

Rigid foam board is usually installed in attics, roofs, basements, and foundations. It’s important to waterproof the cement before you install the foam board. Use caulk and a caulking gun to seal spaces between separate pieces of rigid foam board. This way, nothing will get into the cracks and crevices. Usually, the foam board is glued directly to the cement, so you need to make sure the surface is clean.

Step 3 - Protection from UV Rays

Sunlight is one of the enemies of the rigid foam board. If you’re using the foam board for roofing, a layer of roofing felt and asphalt will usually protect your foam board from UV damage. If you’re using foam board for other parts of your home, you’ll need to use a layer of heavy plastic rubber and attach it to the surface of the foam board. You can staple it directly to the foam, but make sure it is tight because it can also be useful in keeping out water.

Step 4 - Protection from Insects

Insects can bore holes through rigid foam board insulation, especially when it is along the foundation of your home. Most exterior foam boards come with a layer of some sort of insecticide to keep these pests out. If you apply a layer of plastic, it will be harder for insects to get through. Another alternative is to install the insulation on the interior of the cement, so the insect cannot get through the cement.

Step 5 - Protection from Fire

Installing gypsum wallboard up against the existing rigid foam board, you’ll help keep the foam board safe from fire. Certain fire codes require you to use a half-inch layer of gypsum wallboard if you install foam board. The extra wallboard can also be useful in keeping water out.