Closed Envelope Attic

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.

Closed envelope attic.

If it s a nasty old attic vacuum it out and add a high merv filter on the return from 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. 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. 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.

Bringing the attic inside the envelope doesn t mean you can stop addressing the water vapor. 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. 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.

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. Pretty soon you re coughing and sneezing and trying to rub the dirt out of your eyes. To truly be a fully conditioned attic it should be included in the hvac or ventilation system. The attic space becomes indirectly conditioned as a result of the air leakage heat transfer and vapor diffusion through the ceiling.

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