How about sealing and insulating both the roof deck and the attic floor?
I just read Joe Lstiburek's excellent Crash Course in Roof Venting. It caused me to have an unorthodox idea about which I seek your opinion: sealing both the attic floor and the roof deck. Please read on before you dismiss me as nuts.
My wife and I live in a 100 year old four-square in Joliet, IL (Zone 5). It has a hip roof with about eight feet of main ridge, eleven feet on each of three dormer ridges, and three feet of eave overhangs all around. I had dense-pack cellulose blown into the balloon-frame stud cavities, and I removed the old attic floor and the old and sparse mineral wool, sealed all openings and cracks that I could find, reinstalled the mineral wool and supplemented it with cellulose to the tops of the joists, and replaced the old flooring with ¾" plywood. The attic floor joists are 2x8s, so the insulation, although greatly improved, is still not adequate for the climate zone. Gas usage is down from over 12 BTUs/HDD/sq. ft. when we bought the place in 1995 to just under eight now.
Heat is via hot water radiators, but the CAC air handler and ductwork is in the unconditioned attic. Rafters are 2x6 except in the three dormers, where they are 2x4. We put a new roof on in 1996 and ventilated the attic with about 35 feet of ridge vents, twelve one sq. ft. holes in the eaves, and a powered roof vent fan.
So. What harm might there be in increasing the total amount of insulation above the conditioned living area, with an intervening air cavity (i.e. the attic), by sealing the roof deck and insulating its underside, thereby moderating the attic temperature summers and winters and reducing the temperature differential between the conditioned living space and the attic and reducing the load on the AC air handler? We could have at least 14 inches of insulation (six inches or more between and beneath the rafters and eight between the joists) and dead air in the attic.
Okay, am I nuts?
Posted Aug 12, 2011 9:09 PM ET
Edited Aug 12, 2011 9:09 PM ET
Other Questions in GBA Pro help