Foam Board Sheathing Basics

Foam board sheathing makes good insulation. It’s easy to install, easy to keep in place and it’s readily available and relatively cheap to buy. It comes in different thicknesses and fits outside the house with siding applied over the top. Before buying foam board sheathing, it’s important to know about the different types and how it works.

Common Types

You can buy foam board sheathing in thicknesses ranging from 3/8 inch all the way up to 4 1/4 inches. Generally, if you’re using foam board sheathing to cover siding that’s already on the house, you’ll want to buy either 1/2 inch or 3/4 inch thickness. it’s possible to buy foam board sheathing in fan-fold panels that can be 50 feet long but generally, you will usually buy it in panels. These will be in standard sizes of either 2 feet by 8 feet, 4 feet by 8 feet or, 4 feet by 9 feet.

Foam board sheathing will usually either be made of extruded polystyrene or polyisocyanurate. The latter has a better R value per inch and will always be much higher than the polystyrene. Foam board sheathing is resistant to both air and moisture. The energy savings mean that it offers a good return in investment. The initial costs might be higher than other forms of insulation but the return on your original investment actually comes very quickly.

With foam board sheathing, there’s no need for another building wrap or felt barrier and the fact that the material is rigid and impact-resistant eliminates the necessity for other strengthening materials. Additionally, the fact that it uses a high level of recycled materials makes foam board sheathing very environmentally friendly. Since it makes wall construction easier, it also lowers labor costs and fits well with new window designs.


One of the biggest advantages to foam board sheathing is that it’s easy to cut by using a utility knife. Making it fit takes just a few minutes and no special tools are required. To put foam board sheathing on the wall, you only need to nail it in place on the wall studs with galvanized nails. You do, however, have to make sure the nails are long enough since they need to penetrate the wood to a depth of at least 1 inch. The nails should generally be 12 inches apart although the instructions from some manufacturers might vary. The nails have to be flush and you’ll need to take care that you don’t crush the panels with the hammer.

What to Use

If you’re going to have vinyl, aluminum or wood siding, you should use foam board sheathing that’s not faced with foil. Where you’ll have brick or stucco on top of the sheathing, use foam board sheathing that has either a matte facing or reflective aluminum. You need to be aware of the fire codes in your area regarding the installation of the sheathing since it’s fairly combustible. Failure to follow the codes could result in you having to tear it all off and start over.