Insulating new cabin build at the attic roof line & attic floor
I’m insulating a new cabin build. Zone 7, northern WI. Main cabin is 20 x 26; Monolithic slab; electric baseboard heat; 1.5 stories; 2×6 wall construction. Exterior walls are flash & Batt with 2″XPS and R19 compressed into the rest of the cavity. I used attic trusses with a 10/12 pitch in order to give me storage in the attic and an over-flow sleeping room upstairs. The upstairs attic is 11′ x 26′ w/ a 7 ft ceiling. A Bessler pull down stairway(these are awesome) will provide access to the upstairs attic/room. Yes, I am insulating the “devils” triangle. As you can see from the attached pic, blocking has been installed, and R38 batts are in the ceiling on each side of the knee walls which are above. You can also see the attic floor which is already installed. Its a vented roof, site-built 1/4″ OSB baffles with 1.5″ airspace to run from the soffit to the ridge. I plan on filling the vaulted ceiling with at least 2 layers of fiberglass batts and then polyiso on the interior to provide a thermal break. With the 1/4 OSB baffles, batts will be enclosed on all 6 sides. No dormers in the roof plane. On the 1st floor all the exposed insulation in walls & ceiling have been covered with drywall for an air barrier, and all insulation batts have been taped at the seams. Eventually a tongue & groove ceiling will be installed on the 1st floor. HERE’s the QUESTION !…and I already know the typically answer which would be to insulate at the thermal envelope; which would make my sleeping loft part of the conditioned space. HOWEVER, the upstairs attic/loft space will probably have less than 15% usage for kids sleeping, or extra place to getaway, etc. During the winter temps can get 20 below. I plan on keeping the heat at 50 even if no one is at the cabin during the winter. Would there be ANY benefit to insulating the attic floor/1st floor ceiling while the bottom truss cord is still exposed? This is the 11 x 26 area between the upstairs knee walls. The thought is to keep electric/heat cost down when most of the time the upstairs attic space doesn’t need to be conditioned. Then…if the upstairs is used, with the pull down stair case and the insulated roof line, upper half of the cabin can be heated or cooled.
Any thoughts would be much appreciated! Dan Musielski
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