Question 1:
I was looking at a home in northern illinois and a few question came up. The home currently has fiberglass insulation in the walls and I believe cellulose on the attic floor completely covered by plywood but the owner has recently had bat issues and is exploring foam insulating the attic (in the roof rafters and I assume any gable walls).
Was wondering what the correct way to do that would be since the rest of the house would not be spray foam and what the correct way to vent the attic would be (if applicable). I’ve read that attic ventilation is not required with spray foam insulation but since the whole house would not be foamed wondering if any ventilation is required and what should be done with insulation in the attic floor if anything. Currently the roof gables are vented. Also, I think this is counterintuitive but should any venting be applied between rafters and spray foam?
Question 2:
The current roof is plywood sheeting with cedar shingles. They are exploring replacing the roof as well.
With the new insulation dynamics what is the proper way to vent the new cedar roof and what materials should be used over sheeting? How will the foam insulation affect the roof? If an asphalt roof was applied instead how should the roof be vented and how would the foam affect the life of the roof?
I have seen people use a hybrid approach with foam insulation (foam the rafters and attic walls, and ventilate the space), but this makes no sense to me. It’s like putting beer in a cooler and leaving the lid open.
From my home-building experience, there are two ways to use spray foam in an attic: One is to spray a 2" or so layer of foam on top of the attic side of the ceiling drywall to seal it, and then finish with blown fiberglass. The attic is then ventilated as normal.
The second is called “full encapsulation”. The attic is entirely sealed - no ventilation at the soffits, ridge, or anywhere in between. No insulation is applied to the attic side of the ceiling drywall, but the soffits, walls, and rafter areas are sealed with foam insulation. The attic in effect becomes “conditioned space”. This boosts the efficiency of the system tremendously since the condenser, furnace, and ductwork are not exposed to extremes of heat or cold. I have been in such attics during the heat of the summer in Texas and they are quite comfortable. People with such systems boast of VERY low utility bills. If the furnace is gas, it must be a high-efficiency unit, and combustion air has to be brought in from outside. If the living space is well-sealed, a fresh-air intake is usually part of such a system.
The “correct” way to do it is to either insulate the living space from the attic and ventilate the attic, or completely seal the attic. There’s nothing to be gained from insulating the attic ceiling and walls then introducing unconditioned air.