Closed Envelope Attic

The only time most people bring the attic inside the envelope is when they want to convert it to a living space.

Closed envelope attic.

Intake vents located at the lowest part of the roof under the eaves allow cool. A fundamental requirement of an unvented attic assembly is the use of air impermeable insulation on the underside of the unvented roof to prevent air infiltration and exclude airborne moisture from the attic. The attic space becomes indirectly conditioned as a result of the air leakage heat transfer and vapor diffusion through the ceiling. When hvac equipment is in the attic insulation and an air barrier at the roof line brings that equipment within the conditioned enclosure envelope thereby allowing it to operate in more favorable temperature and humidity conditions without air leakage and heat loss gain directly to the exterior.

Too often mechanical ventilation ducts from bathrooms kitchens or laundry rooms deposit warm moist air into the attic instead of outside the building envelope. An attic which can get up to 130 f or so on a hot summer day is not a good environment for your heating and cooling system. Attic ventilation works on the principle that heated air naturally rises primarily utilizing two types of vents. Optimizing attics and crawl spaces.

I think the best solution is to keep the envelope at the attic floor with airtight drywall and use more thickness of less expensive insulation. Spray foam insulation is more expensive than batt insulation and needs to be expertly installed to prevent missed areas or pockets that can reduce the efficacy of the building envelope. Reason 1 to move the building envelope upward and encapsulate the attic is to get hvac equipment and ductwork out of the blast furnace. Minimize the entry of dirty unhealthy air.

Poor workmanship during attic and roof installation will result in expensive callbacks and repairs as insulation will have to be replaced. Increasing the thickness and r value will improve the hers index.