Gary N Smith.net

For at least the last twenty years or so, the most common way of insulating attics in Minnesota is to use loose-fill insulation – either cellulose or fiberglass. This is a huge improvement over fiberglass batts, because batts are nearly impossible to install in attics and they cost more money. Despite the decline of fiberglass batts in attics, I still find a small section of fiberglass batting used above the attic access panel at about 90% of the homes that I inspect, even on new construction.

Installing rigid foam on the outside of a house is a great way to double the R-value and eliminate air leaks. Large sheets of foam insulation, screwed and sealed to the walls or roof, can cover or replace conventional sheathing and create a continuous barrier to heat loss or gain—something you don’t get with cavity insulation alone. We wrapped the house in this video with 4 inches of polyisocyanurate in two staggered 2-inch layers. This thickness strikes a balance between ease of installation and maximum R-value. If the foam is any thicker, it’s hard to hit studs when driving screws. Plus, combined with 5-1/2 inches of cellulose in the stud bays, 4 inches of rigid foam brings the wall up to a respectable R-40.