Closed Envelope Attic

The dormers and the attic space over the outside porch were sealed off from the rest of the attic by making a wall of plywood then insulating the wall with closed cell spf.

Closed envelope attic.

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. 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. Intake vents located at the lowest part of the roof under the eaves allow cool.

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. When determining how to insulate this attic space various assemblies were taken into consideration and a hybrid combination of vented and unvented attic space was planned. 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. You re used to thinking of your car as a closed envelope which is why you weren t worried about the dust. 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. Reason 1 to move the building envelope upward and encapsulate the attic is to get hvac equipment and ductwork out of the blast furnace.

Attic ventilation works on the principle that heated air naturally rises primarily utilizing two types of vents.