Spray-in-place polyurethane is the only commonly used solid insulation that absolutely protects itself from air infiltration. When it is properly placed between two studs or against a concrete block wall or wherever, the bonding of the spray plus the expansion of the material in place creates a total seal. It’s almost impossible to overestimate this total seal. In my opinion, most of the heat loss in the walls of a home has to do with the seal, rather than the insulation.
Heat does not conduct horizontally nearly as well as it does vertically. Therefore, if a home had no insulation in its walls, but did have an absolute airtight seal, there would not necessarily be a huge difference in heat loss. But this would not be the case if ceiling insulation was missing.
Spray-in-place polyurethane can most effectively stop air infiltration. It is the only material that properly applied fills in the corners, cripples, double studs, bottom plates, top plates, etc. The R-value of a material is of no interest or consequence if air can get past it.
3 inches of foam insulation is plenty
Nearly 100% of the heat loss from air infiltration is stopped with the first one-fourth of an inch of urethane foam. The second inch of spray-in-place urethane stops about 90% of heat loss, and the third inch stops about 95% and so forth.
We sprayed all sides of our substructure. Subfloor, all 4 walls, and the ceiling. This creates an airtight building envelope which will dramatically reduce heat loss.