Insulating duct attic
I have a 2 story house in Houston TX that has a 2nd floor unconditioned “duct room” below the attic and next to a unconditioned space above a 1st floor cathedral ceiling. This area is also adjacent to multiple finished/conditioned rooms/areas. The builder insulated the cavities of the interior walls with batt insulation but left the unfinished side exposed. There is also a wall between the area over the cathedral ceiling and the walls next to the conditioned space. This wall is not insulated. Looking for advice on the best way to increase the efficiency of the insulation on the walls next to the conditioned space. Should I place thermax foam board on the unfinished side and air seal these walls or attempt to insulate and air seal the wall between the area over the cathedral ceiling and these walls? Or would another option work? The room with the letterbox window next to both of these spaces stays consistently 3-5 degrees warmer than the rest of the upstairs areas. Not the best design but trying to do something that’s hopefully not too significant and help keep the room cooler.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I would insulated the exterior walls by removing the batts from the interior walls and moving them to the exterior walls. I’d install either a sheet of Thermax or a sheet of “regular” polyiso over those studs on the interior with a sheet of drywall over that for the required fire rating. I would detail the polyiso as the air barrier.
Doing it this way brings the “duct room” into the conditioned building envelope, and insulates the exterior walls as they should be. Make sure there are no missed air sealing details connecting your now-conditioned duct room to any other unconditioned spaces.
It looks like you have enough space in that duct room to do the work without too much trouble.
Bill