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Rigid board for cathedral ceiling

I just tore out the drywall on my cathedral ceiling on my lean-to addition and found no insulation at all...just the underside of the roof sheathing. I want to insulate before drywalling but can't seem to find any consistent information on how it should be done. I was going to go with spray foam but $1k for 96 sqft is crazy. I am thinking about going with ridged board against the bottom of the roof sheathing...maybe an R30. Will this work? Will I have any concerns with moisture? The spray foam guy said that he could spray the foam right up against the roof with no moisture problems at all...I am guessing this would be the same for rigid...I hope. Any advise?

Asked by B W
Posted Dec 7, 2012 3:49 PM ET


5 Answers

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Getting to the "right" answer is climate specific. Got a ZIP code?

Plank sheathing, plywood, or OSB?

Roofing type & color?

You may want to read this too:


Cut'n'cobbling rigid foam between rafters & joists is a PITA, and the difference in thermal performance between that and a fiber cavity fill pretty minimal. But unless you get it extremely air tight you run the potential of accumulating moisture in the roof deck during the winter if you live in a cold area.

The IRC doesn't recognize all possible insulation stackups that work, but they do have prescriptive standards for unvented roof assemblies, specified by US climate zone:


If you're planning to re-roof any time soon, using rigid foam on the exterior does a WORLD more good than putting it between rafters where it's higher-R performance is undercut by the thermal bridging of the rafters. A combination of rigid above the roof deck and fiber between the rafters can be the best of all worlds, since you get the full-R out of the more expensive foam, putting the roof deck closer to the drying effects of the conditioned space, but there has to be sufficient R above the roof deck to limit wintertime moisture accumulation to acceptable levels.

Answered by Dana Dorsett
Posted Dec 7, 2012 4:17 PM ET


Zone 5a, osb, asphalt black.

Answered by B W
Posted Dec 7, 2012 4:21 PM ET


This article may answer many of your questions: How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay
Posted Dec 7, 2012 5:54 PM ET


In zone 5 you can't just blast open cell foam in there and expect OSB or plywood to make it, but with plank sheathing and a black roof it'll go for quite while ( maybe until you can re-roof and add rigid foam above the roof deck), but not with OSB.

With your OSB deck the safer option would be to give it at least a 1" nominal flash of closed-cell foam and either dense-pack it with cellulose or put in high-density "cathedral ceiling" or rock wool batts in there. (Rock wool has fewer chemical sensitivity and micro-particle issues than fiberglass, and costs about the same as high-density fiberglass.)

Look at the simulation results the 1" cc foam + fiberglass column or Chicago and Boulder in table 3, p12 of that RR-1001:


The dark shingles make it, the light metal roof not so much. But with 2" cc foam it does.

You can get 2" coverage on 96 square feet out of a 200 board-foot DIY foam kit for about $350, with a few extra spray tips. (eg: http://www.tigerfoam.com/products.php There are others if you go web-surfing for them.) If you find a pro that would even consider a 200 board foot job you can probably get them do to it for about $250-300- depends on how competitive and hungry your local market is.

If you have 2x10 rafters and do a 2" spray foam job you'd then have about 7" of room to fill, which would accomdate an R28 or R30 rock wool or high-density fiberglass batt (Lowes carries Roxul rock wool products now, if you can't find it elsewhere.) If they're 2x8s you'd be left with about 5" which could accomodate R22/R23 rock wool or R21 high-density fiberglass, which would run about $100 for 96 square feet: http://www.lowes.com/pd_89017-1278-RXCB5523_0__?productId=3285114

When using batts stuff the rafter bays completely even if you have to compress the batts- any air gaps will cause the performance of the fiber to suffer.

While 2" of DIY foam + batts will add up to $400-500 for the 96 square feet, it'll be a rock-solid-safe solution for less than half the cost of your spray foam estimate, and more moisture safe than a full-fill of open cell.

Answered by Dana Dorsett
Posted Dec 7, 2012 6:19 PM ET


Thank you guys so much for all of your information...it has been a great help to me!

Answered by B W
Posted Dec 8, 2012 7:15 AM ET

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