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How Do You Figure Board Feet in Spray Foam?

One Square Foot Multiplied By One Inch Depth Equals One Board Foot.

This is a question we get a lot. If you are buying carpet or most other building products, it is pretty straight forward. The price is $X.00 per square foot. Why is spray foam insulation measured in some mysterious way? We can assure you this isn’t an an attempt to be ambiguous about pricing. The fact is that spray foam is sold by the board foot and not the square foot because your desired final thickness can be different. It takes a certain amount of the raw chemicals to create the finished product. 10 inches of spray foam per square foot could never cost the same as 1 inch of spray foam per square foot.

Since sprayed polyurethane foam is made by mixing two chemicals and manufactured on the job site, the other ways it can be measured are in gallons, or pounds of chemical required. This would be even more complicated than board foot measurement. Imagine hearing you need 100 gallons of something to insulate a wall. It would be very confusing.
Lets give some real world examples. Building codes vary by region. We are going to use our local energy code region, referred to as zone 4, in the Delmarva area, for this example. These codes spell out the minimum insulation amounts required in residential homes as well as commercial buildings. In Delaware the energy code is different than say Minnesota which is a much colder climate or Texas which is a much warmer climate.
Our local energy code dictates walls are insulated with a minimum of R-20. Attics on the oth-er hand require R-49. This is because heat rises, so an attic is where most of your heat is lost. We will cover this more in depth in another article. Back to our example, as you can see at-tics by code require more than double the insulation thickness compared to walls no matter which product you use. Whether you choose foam or fiberglass, rock wool or cellulose, it takes more thickness to achieve a higher R value.

Considering the R value of closed cell spray foam is roughly R-7 per inch and open cell around R-3.5, the depth or thickness required to achieve the desired total R value is going to differ between a wall or an attic. If you are seeking R-20 for a wall it would require 3 inches of closed cell foam. On the other hand, insulating an attic to R-49 would require 7 inches of closed cell foam.

This is where board foot measurement comes in. It simplifies the math for everyone. One square foot at one inch depth is R-7. This is whats called a board foot. You can figure how many square feet need to be covered, at what R value, and then determine how many board feet you need. A 1000 square foot area at 1 inch depth is 1000 board feet. The same 1000 square foot area at 2 inch depth is 2000 board feet, and so on. You get the idea.

Hopefully you found this short article useful to you. We plan to publish many more topics on questions we commonly receive which are a little more in depth than can be covered in our frequently asked questions section, although this is definitely a question we get frequently. If you have questions you would like to see covered, or if you question about your specific application, feel free to send us an email or give us a call. Even if you are outside of our service area we are more than happy to help.