Closed Envelope Attic

The attic space becomes indirectly conditioned as a result of the air leakage heat transfer and vapor diffusion through the ceiling.

Closed envelope attic.

Attic ventilation works on the principle that heated air naturally rises primarily utilizing two types of vents. Pretty soon you re coughing and sneezing and trying to rub the dirt out of your eyes. You re used to thinking of your car as a closed envelope which is why you weren t worried about the dust. 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. But when there s a hole in the envelope pollution can get inside not to mention lots of hot or cold air wind and precipitation. Reason 1 to move the building envelope upward and encapsulate the attic is to get hvac equipment and ductwork out of the blast furnace. My original plan was to build a conditioned attic and place 1 5 of foam xps board r7 5 installation on top of the roof deck and fill the old true 2x4 rafters and rafter walls with closed cell insulation and if able attach 3 of foam xps board r15 to the rafters in the attic.

To truly be a fully conditioned attic it should be included in the hvac or ventilation system. If it s a nasty old attic vacuum it out and add a high merv filter on the return from the attic. Bringing the attic inside the envelope doesn t mean you can stop addressing the water vapor. 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.

Poor workmanship during attic and roof installation will result in expensive callbacks and repairs as insulation will have to be replaced.