Re: Spray Foam vs. Blown-in attic insulation
Yep --- the method you describe has definate advantages of providing better thermal regulatiion of the attic by providing a semi-conditioned space.
The reason it's a semi-conditioned space generally there is no connection to providing heat or A/C into this space ---- then would technically be a conditioned space.
By applying insualtion along the roof and all perimeter surfaces along with closing off all vents to the exterior maintains a controlled temperature within the attic. This results in a warmer attic during the winter which reduces heat loss ( reducing the heat load on the heating equipment ) . The opposite during the summer with a cooler attic reducing heat gain ( reducing the cooling load for the A/C )
Doing this will also help reduce vapour drive from the lower living area into an attic space that has signifigant temperature and pressure differentials noramally associated to traditonally insulated vented attics.
I'm not convinced that applying foam to the underside of the roof decking is much of an issue when it comes to roof leaks. However , I would be more inclinded to consider the foam applied directly to the underside of the roof deck being somewhat problematic in the event of repairs to the decking is needed.
An alternative would be apply a backer spaced 1 1/2 away from the deck along the entire distance of each cavity. This should eliminate any concerns regarding roof leaks , allowing repairs to decking , provide the venting to the underside of the roof that some feel is needed for shingles.
The downside to this with spray foam is cost. Consider the square foot coverage for the underside of the roof and the gable walls is considerably more than the floor surface area. Spray foam is at least 4 times the cost as conventional insulation products.
"" an ounce of perception -- a pound of obscure "