How Thick Should My Spray Foam Be?

If you are considering having spray foam insulation installed in your home you may be looking at the cost per board foot and trying to decide just how thick you need to apply the foam for the best economy.

The answer to this question varies. The actual thickness of the foam you should be applying depends on the climate where you live, the R-value of the foam you are using, and where you are planning to have it installed.

Each climate zone has different code requirements for the amount of spray foam that should be applied in each area of the home. For example, the thickness of spray foam installed in the attic should be a different thickness than that of foam installed within the walls.

You can calculate the thickness of spray foam required in each area of the home by simply dividing the R-value required by the R-value of the spray foam product you are using. For example, if you need to achieve an R-value of 21 and you are using a spray foam product that delivers an R-value of 7 per inch – 21/7=3 inches.

Is 2 Inches Of Spray Foam Enough?

If you are using a closed cell foam, assuming an average R-value of 6.8 depending on the specific product, spray foam applications that are 2 inches this are only appropriate for floors and wooden walls in climate zones 1 and 2.

Spokane is located in climate zone 6, so here in the Inland NW 2 inches of spray foam would not be sufficient.

How Much Spray Foam Do I Need In My Attic?

Due to the fact that heat rises, the attic is one of the most important places in your home you need to ensure has sufficient insulation. Failure to do so could substantially decrease the energy efficiency in your home, thereby costing you more on your monthly utility bill. 

The thickness of spray foam required in your attic will vary depending on the material used and the climate zone where you live. The recommended R-value for insulation in attics that are uninsulated in climate zones 5-8 is between R49 and R60. That means for homeowners in Spokane that are using a closed closed-cell foam insulation with an R-value of 7 per inch would need to apply at least 7 inches of foam to achieve the minimum recommended values.

How Much Spray Foam Do I Need In My Walls?

Wall spaces tend to require significantly less insulation than the ceiling or attic space. This is simply because less heat tends to escape from the walls than through the roof space. 

For climate zone 6, which includes Spokane and the Inland NW, wall spaces only need 3 inches of spray foam to achieve the recommended R-value of 21. This is assuming you are using the same closed-cell foam that delivers R7 per inch.

How Much Spray Foam Is Too Much?

With amazing benefits and insulation properties that come along with spray foam insulation, you may be thinking that more is better. This is however not necessarily the case. 

While it is definitely important to make sure that you are using enough spray foam in order to achieve the recommended R-values, using too much could just be a waste of money. With spray foam, there is a certain point of diminishing returns. The reduction in conductive heat flow plateaus with the more inches applied. This means after a certain point, you are getting less insulation value out of each additional inch of application.

A reputable contractor likes to help their customers get the most value out of their money. If you are considering having spray foam insulation installed in your home, the best thing to do is find a trusted contractor in your area and work with them to figure just how many inches of spray foam you need for your specific situation!