Closed Envelope Attic

Too often mechanical ventilation ducts from bathrooms kitchens or laundry rooms deposit warm moist air into the attic instead of outside the building envelope.

Closed envelope attic.

A combination of air sealing and insulating the attic floor while providing ventilation considerably reduces the potential for condensation as warm moist air is less likely to enter the space and condense on cold surfaces. The attic space becomes indirectly conditioned as a result of the air leakage heat transfer and vapor diffusion through the ceiling. Poor workmanship during attic and roof installation will result in expensive callbacks and repairs as insulation will have to be replaced. 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.

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. Optimizing attics and crawl spaces. Insulation gets blown placed or stuffed between the ceiling joists at the base of the attic but the space above those joists is left unconditioned and intentionally vented to the outside. Increasing the thickness and r value will improve the hers index.

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. 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. By air sealing all wiring penetrations in your attic and sealing and encapsulating your crawlspace more on that in a moment you can further tighten your home envelope and maintain greater control over indoor temperatures all year round. Attic ventilation works on the principle that heated air naturally rises primarily utilizing two types of vents.

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. Reason 1 to move the building envelope upward and encapsulate the attic is to get hvac equipment and ductwork out of the blast furnace.