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Insulation Methods - Cathedral Ceilings

Hello everyone!


One of our community members Kenchan inquired (PM) about methods that can be used to insulate the unvented cathedral ceilings. I thought some of you can use this info at one point too.




Steel Toes, I saw a post of yours regarding insulation and I'm hoping you can answer my question. I have a 12' x 20' room with an open-beam cathedral ceiling. The roof is constructed with a 20' ridge-beam and has 4 x 6 rafters on 48" centers. I want to insulate, hopefully to R-30, and I'm hoping there is an acceptable method to do this without cutting in vents through the stucco siding and building a soffit. I've been reading about unvented cathedral roofs, different types of foam, climate zones, blah blah, and I'm always left uncertain as to what my options are. I've spoken with two different contractors who weren't interested in the job because it's too small. I'm in Torrance, California. Do you know how I can properly insulate an unvented cathedral ceiling to R-30? Any info is greatly appreciated.



Hi Kenchan,


To answer your question first, Yes you can properly insulate unvented ceilings too.


However, with 6” deep rafters and no vented soffit on the outside, you really have only one feasible option and that is to use closed cell spray polyurethane foam (ccSPF).


Depending on the manufacturer, in general ccSPF foam has a density between 1.5 and 2.5 lb. per cubic foot and a nominal thermal resistance of R-6.5 per inch. This said, with your 6” cavity you can technically go up to R39. Closed cell spray foam is still relatively “new” to the marked and considered expensive. But taking in consideration your setup, no venting -6” cavity, I think it will turn out to be your very best option, on all fronts.


Another option is to combine ccSPF with “standard” insulation. In this case, you would apply up to 2” of ccSPF and fill in rest of it with standard denim or fiberglass insulation. At 2” ccSPF is considered to be an effective vapor retarder.


I’d opt out option of deepening/extending the rafters considering they’re spaced out 48” o.c.


Hope this answers you question, and please do not hesitate to post back with any further questions you may have.

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Posted 2013-01-22T21:41:49+0000  by George_HD_CHI George_HD_CHI


I have no idea as to the exterior style of this house or the roofs configuration, but a possible solution would be to leave the interior ceiling intact, and build up the exterior roof with ridgid foam panels.  I would grant that this would be quite expensive, requiring a whole new roof and sheathing.


I have seen such projects done in Fine Homebuilding Magazine where the goal was super-insulating homes - **bleep** the expense!

Posted 2013-01-23T02:55:36+0000  by ordjen
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