While getting estimates for spray foaming my attic, the sales person stated that research has determined that you do not need more than 4 inches of spray foam insulation. He stated the heat loss/transfer was negligible after 4 inches. He stated that z4 inches would stop 95% of the heat transfer. He stated to go from 4 inches to 5 inches was not worth the money. I live in Delaware where the code requires R38 in the ceiling. If 4 inches is approximately R26, how can I only use 4 inches of spray foam insulation? Is the contractor correct?
We went straight to the experts on spray foam insulation to help us answer this one. Paul Duffy, VP Engineering at Icynene, tells us that spray foam is different from conventional fibrous insulation in that it not only limits conduction heat loss/gain but it also limits convection and air leakage. Now it does cost a little more upfront than traditional blown or batt insulation, but it offers a lot more, so your cost/benefit on installing additional amounts of spray foam is reduced at higher thicknesses. The air leakage control and convection control benefits are maximized at 4” of thickness. Any benefit in adding more than that is is quite small because so much of the heat loss/gain is already controlled. Do keep in mind that heating and cooling savings do vary by climate zone, so this isn't a “one size fits all” solution.
A RESNET HERS rater can accurately pinpoint what the savings would be in your home in your climate by carrying out a computer analysis. The building code actually allows a little deviation on recommended values, because it recognizes that some products (like spray foam) provide more effective performance at reduced thicknesses.
To learn more, visit http://www.Icynene.com.