John Roy is building a house in southeastern Massachusetts, and at least part of it will have a cathedral ceiling. He’s thinking of insulating the ceiling with dense-packed rock wool.
The president of a local insulation company tells him there’s no need to install air chutes in the rafter bays before the insulation is blown in because the insulation does not absorb water. The local building inspector is prepared to go along with the recommendation providing soffit vents are installed.
Is Roy getting good advice? That’s what he’s asking in this Q&A post, the subject of this week’s Q&A Spotlight.
First, will that be enough insulation?
Whether unvented cathedral ceilings should be dense-packed at all is a question that’s generated plenty of discussion in the past. Leaving that aside for the moment, a key question for Roy is whether mineral wool packed into a 2×10 rafter bay will provide enough thermal insulation for his locale, and the answer is no, it won’t.
As GBA senior editor Martin Holladay points out, a 2×10 is really 9¼ in. deep. Using the values published by the National Association of Home Builders Research Center, mineral wool at a density of 2½ pounds per cubic foot would have an R-value of about 34 if it completely filled the rafter bay.
The 2006 International Residential Code requires ceilings in Roy’s area (Climate Zone 5) to be insulated to at least R-38, Holladay says.
If part of each rafter bay is dedicated to an air chute, reducing the amount of insulation that can be added, the numbers get worse. This means that whether Roy adds air chutes or not, he’ll need to find a way to add more insulation if he wants to meet IRC requirements.
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