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How much foam do I need?? Post New Topic | Post Reply

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Posted: Jun 01, 2011 04:30 PM
How much foam do I need??
I live in Central NJ. I'm doing an addition to my home, and I've gutted the whole house. I'm contemplating using spray foam, but I have no idea how to evaluate how much and what type of foam I need and how to get the best bang for my buck. Any advice? The total square footage is about 4700 sq ft. Thanks very much!
John Shockney
Posted: Jun 02, 2011 01:19 AM
Well there are a lot of things to take into consideration.

1. How much room do you have for insulation and what R-value do you want?
2. Is the space below grade? Closed cell foam is almost waterproof.
3. Do you need structural strength? Closed cell adds strength.
4. How much money can I spend?

With that said if you have the money to spend the best insulation is the 2# closed cell with an R-value of almost 7 per inch, 3 inches gets you an R-20 in a 2x4 wall. But you can spray 5.5 inches of .5# open cell for an R-19 for about half the cost of material but it requires more labor for trimming and cleanup.

But I think that you also wanted to know how to figure how much foam it takes to spray a house.

1. All spray foam is figured in an old lumber measurement called a board foot and one board foot = 12x12x1 inches.
2. So you figure the square footage of the walls and multiply that by the thickness (R-value) wanted in inches will give you the board footage needed. So if you have a wall 10x10 feet =100sqft and you want 3 inches thick that would require 300 board feet of foam.
3. Now an drum set (two 55 gal drums one part A one part B) of 2# closed cell will produce 4,000-4,500 board feet of foam and a set of .5# open cell will produce 15,000 board feet of foam but will have more waste due to trimming.
4. Most closed cell foam has an R-value of around 6.9 per inch, and most open cell has an R-value of 3.8 per inch.

Hope this helps answer your questions

Posted: Jun 02, 2011 06:32 PM
Very helpful! Thanks very much!

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