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Garage cathedral ceiling insulation

Kraken | Posted in General Questions on

I am insulating my old “shed” and not sure what to do for the rear part of the roof. The roof is ~12ft in front and 24ft  in the rear (low slope) with a ridge vent. It was shingled but just finsihed putting on a metal roof with 1″ thick strapping. The front 18ft of the shed has a standard flat ceiling insulated with batt insulation. The rear of the building is cathedral open 2×6 rafters. No soffit vents and it does not go as far as the ridge vent.  Not sure what to do with insulating it. No real access to spray foam where I am. I am in Climate Zone 7A (Northern Minnesota) so can get quite cold here! I have a wood stove for heat. Any suggestions would be appreciated.

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  1. Expert Member
    BILL WICHERS | | #1

    If you can't use spray foam, the next option to get a high R value in a vented assembly is to cut'n'cobble polyiso panels to fit. In your assembly, what I would do is tack some 1x2 furring strips up along the inner edges of the rafters tight to the roof sheathing to give you a stop to form a vent channel under the sheathing. Use roofing polyiso (which is usually a little cheaper) cut to fit in between the rafters, press the first piece of a "thick-ish" sheet up against the furring strips and secure it in place with canned foam. Put another piece over that and secure that in place after the first canned foam has cured. Keep doing that until you fill the rafter bay even to the lower edges of the rafters. Finish things off with a layer of foil faced polyiso with seams taped over the interior edges of the rafters, then drywall (or T+G) over that with your finished ceiling materials.

    That will get you the highest R value in the rafter bays without spray foam. You could also use mineral wool, which is the next best option. I'd still use the "under the rafters" interior polyiso though with seams taped. The taped polyiso on the inside helps to keep moisture from getting up into the assembly, then the vent channel deals with any small amount of moisture that does manage to sneak it's way up there. Note that cut'n'cobble like this is NOT reliable if NOT vented -- you NEED the vent channels. You also need some soffit vents so that the vent channel can allow for convective air flow from soffit to ridge. Roof venting is really a system, in at the low point and out at the high point. Omitting either the soffit "in" vents or the ridge "out" vent severely hampers the functioning of the system as a whole.


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