While most attic spaces in Minnesota are insulated with fiberglass or cellulose insulation, there is a relatively new product that provides superior performance: spray foam. Spray foam is the best way to insulate homes, especially old one-and-a-half story homes, and I'm such a firm believer in this that I had it done on my own home a couple years ago. Spray foamed roofs are commonly referred to as 'hot roofs'.
Why are they called 'hot roofs?' Traditional attic spaces have insulated floors and are ventilated. Air comes in at the soffits and leaves at the top of the roof, creating a cold attic space. This helps to prevent ice dams, keeps the roof cooler in the summer which helps to prolong the life of the shingles, and may help to prevent the accumulation of condensation. Spray foamed attics have foam applied directly to the roof sheathing, and the attic space isn't ventilated. The lack of ventilation is why we call them hot roofs.
Are they really hot? No. Studies have shown that color differences in shingles will actually have a larger impact on the temperature of roofs than the difference between a ventilated spray-foamed. A hot roof will typically only be a couple degrees warmer than a ventilated roof.
What are the benefits? Spray foam has a higher insulating value (R-Value) than anything else. Sprayed Polyurethane foam insulation has an R-Value of 6.8 per inch, while fiberglass batt insulation is about half that. Foam insulation also makes for a perfect seal - no gaps, no air leakage, no attic bypasses. If ductwork is located in the attic space it won't need to be insulated, elimating energy loss here, which can account for up to 10% of total energy loss. One more benefit that I personally love is having a warm attic area for extra storage! Note: My old house is designed in such a manner as to support extra storage in the attic, but most newer homes are not. This might be another blog topic some day.
What are the downsides? The only one I know of is cost. Spray foam insulation will typically cost thousands more than fiberglass or cellulose. I paid about $3700 to have my own attic spray-foamed with polyurethane, but I could have spent about a third of that to have fiberglass installed, along with proper vents.
Will spray foam void my shingle warranty? No. All the major manufacturers of shingles will still warrant their products when used with a spray-foamed attic.
If spray foam is so great, why isn't it used on walls? It is. I have a friend who insulated the walls in his home when he built it in 1980. Spray foam is also used at the rim joist in almost every new construction home that I inspect.
Additional Information - technical, dry reads can be found below.
Spray-in-Place Polyurethane Foam Insulation Opion Paper, by Craig DeWitt, Ph.D., PE
Vented and Sealed Attics in Hot Climates, by Joseph W. Lstiburek, PE
Reuben Saltzman, Structure Tech Home Inspections