How does SPF compare to fiberglass batts?
Fiberglass will not stop air leakage; what fiberglass really does is filter out dirt, just like a filter on your furnace. You would be better off building a home using straw bales than use fiberglass for insulation. Blown-in cellulose only slows down air permeance, where SPF stops air completely. Fiberglass (in the bundle un-opened) has an R-value of about 3.5 per inch thick. SPF R values range from 3.8 to greater than 6 per inch of thickness with no air leakage and a constant R-value whereas fiberglass significantly loses R-value with temperature changes. Fiberglass will sag in the walls creating a void on the top and cause a thermal loop and heat loss, the same with cellulose, it will settle or if installed in an attic it may get blown around by wind entering the soffit of your home. SPF adheres to wood and metal studs resulting in a permanent barrier to air entry and heat loss.
