While reading an online article concerning soy-based spray in insulation the following statement was made with no back-up info: "Soy-based insulation may also reduce building costs by decreasing the amount of lumber used." First, if this is the case, I assume many (Icyenene, Demolak, etc.) spray in insulators could do this. But my real questions is...HOW IN THE WORLD could their use decrease the amount of lumber I need to build my house??!!

Perhaps they are assuming that rafter sizes would not have to be unnecessarily deep in order to accomodate fiberglass insulation.

Fiberglass of say R-30 value is 9 inches thick, giving a value of R-3.33 per inch of thickness. Most foams run around R-5 per inch, so R-30 would only require 6 inches to achoieve with foams. So a 2x12 rafter could be reduced to 2x8...not an enormous saving, but enough to make "honest" people of those who make such claims.

AppleBuild...Richard is correct, with the higher R-value per inch provided by spray foam insulation you can achieve an R 21 or slightly higher in a 2X4 wall with 3.5" of spray foam insulation, where before you needed 2X6 framing to achieve over R 19 with fiberglass or cellulose.