How to insulate ceiling and walls in zone 3 - new construction?
I am in the framing stage of a new house near Raleigh, NC (zone 3). Our strategy for an air barrier is to rely upon placing the air barrier on the outside at the sheathing using the Zip system on all walls and the roof with all seams taped. The house is situated on a concrete slab that is insulated with rigid foam (R-5). The sill plate is air sealed using Protecto-Wrap sill sealer gasket. Our plan (right now) is to insulate the walls using fiberglass batts (2x6 walls). I know that is not popular, but due to our region and the air barrier on the outside, we are willing to make this cost-based trade-off. The house has a utility room located in the center of the home (inside the conditioned space). All plumbing and HVAC components will be run only in the conditioned space of the home (never in an exterior wall or the attic - we are employing 20" floor trusses that make this easy). The house will employ separate, dedicated ducting for the heating and air duties from the ventilation system which will be an ERV.
My big question is the roof insulation. The roof is not "simple" enough to vent it properly (2 large gables, 2 large dormers, multiple valleys), consequently, I feel like I should be building a non-vented roof. The roof design calls for cathedral ceiling from the floor/rafter junction up to the 9 foot ceiling height. The remainder of the roofline will be attic (I have no intention of letting anyone go up there, no storage needs, etc.). The rafters are 2x10's.
My thought right now is to move the insulation barrier to the outer edge of the envelope throughout the roof (ie - the rafter bays) and not just in the areas of the cathedral ceiling. Our goal is R-30 for the roof structure. We are considering 3 possible methods: fill the rafter bays with 9" of open-cell spray foam, spray a 5" layer of closed-cell foam, or employ a flash and batt strategy (could be 1" of closed-cell with 8" of batts or 2" of open-cell with 8" of batts.
Cost is obviously a factor, but I want this to be a "safe" installation. Based upon the climate zone and my air barrier being the sheathing, what is the best route?
Posted Sat, 12/28/2013 - 14:09
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