Does anyone have a way to achieve ~R-75-80 unvented roof insulation in a cathedral or scissor truss design? I haven't crunched the numbers yet but believe I'll be required to insulate to this degree for my passive solar home to be built next year in an ~8,300 HDD climate (northern Maine-Canada border). Am I right to assume that a cathedral truss is ~12" thick? The scissor option will allow for more volume at the peak to insulate however, as it narrows towards the eves, won't it become thinner than the 12" than the cathedral truss can offer? I think I'll want Zip R sheathing on the roof (R-6.6) to offer water resistive and air barrier with spray foam underneath to completely air seal (so about r-13 so far). Not sure how to achieve the rest and do it in an economically responsible way (maybe blown-in cellulose)?

My understanding is that they can build trusses to any dimensions that you specify. I agree with blown in cellulose, use stabilized cellulose if it isn't flat or the load causes problems for the ceiling.

R75-80 of blown cellulose will be ~22" deep and weigh ~3 pounds per sq ft. You will need more than 5/8" drywall or 1/2" high strength ceiling board on 24" centers to hold up all that weight. You could probably get by with either if you strapped the ceiling perpendicular to the roof trusses with 1x4s on 12" centers.

Posted By arkie6 on 03 Dec 2012 10:56 PM
... You could probably get by ...

If you want to **make certain** that the ceiling will not collapse, should it be entirely covered with sheathing, or is that overkill?

Thanks.