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.
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.