Insulation: Monolithic Thermal Blanket
If you are looking for the most efficient way to insulate your attic and protect against moisture issues, using the monolithic thermal blanket insulation system is the easiest and best way to accomplish this. Notice that I did not say least expensive. The above picture is a corner of a fairly large total job that I performed in Wisconsin for roughly $8,000. However, I also reduced their energy bills from an average of $795.00 per month to $365.00 dollars a month on average. Here is how this system works:
The rafter cavities should be filled to a minimal nominal depth of 3 inches. Then the spray foam is sprayed on the rafters also in order to nullify the thermal transfer of energy through the rafters. This is a skim coat, but will make a huge difference in terms of thermal protection. It is very important that all of the rafters be sealed where the top plate meets the rafters. Any air leakage in these areas can cause issues with heat loss, heat gain, and moisture.
If the finished ceiling is supposed to have drywall attached to it for a vaulted ceiling, fill the rafter cavities with a minimum of 3 inches of closed cell spray foam and trim any areas so the foam does not stick past the rafters. Attaching a 1 inch polyicynene foam board or equal grade foam board will create a thermal break and keep the surface flat so as making it easy to install drywall over.
This system should only be used with closed cell spray foam as open cell spray foam is not appropriate for application such as this because of the absorption qualities of the product.