Insulation for Vented Cathedral Ceiling
Hi all. I know the issue of insulation for cathedral ceilings has been addressed before, but I am still confused by what the best approach is for a vented cathedral. I live in Westchester, NY. We currently have 3 large skylights in our great room. One has developed a bad leak, so we have taken down much of the ceiling and are now considering the choice of fixing them or eliminating them. The beams are 2x10s, which currenly have about 10 inches of fiberglass batts. there is no vent channel against the roof, so I assume the air just moves through the fiberglas. The cathedral ceiling runs up into my regular atttic, where I had them put a channel where the cathedral comes in (and seal around the channel) and which has a ridge vent. If we get rid of the skylight (which I am leaning towards), I would open of the rafters so that there was direct flow from the soffit to the attic. Also, though I know this is considered a no-no, I plan to put in sealed recessed lighting (it is unfortunately the only lighting option that works from a design point of view)
From the various ino on this website, it seems like there are 2 possible approaches to get the maximum insulation and possibly reach R 38. First option: staple vent onto the sheathing all the way up to the attic, spray 6 – 7″ of closed cell foam (assumption is that R is about 6.5 per inch). This seems the simplest, though most costly route. Second option: staple vent all the way to the attic, spray a couple of inches of closed cell foam and then dense-pack cellulose after the wall board is put back up. I am a bit worried about the celluose pushing out the wall board.. Also, there seemed to be some concern with using cellulose with a vented cathedral ceiling. My assumption is that if I put the vent and then spray, that should take care of the problem because the air should flow between the sheathing and the vent, not the cellulose.
Any thoughts on the best/most cost-effective approach would be appreciated. Thanks in advance
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Replies
Richard,
If you are willing to install R-38 of spray foam insulation, you don't need to ventilate. (However, you can add ventilation channels if you want -- but the ventilation channels aren't required.)
Most of your questions are answered in this article: How to Build an Insulated Cathedral Ceiling.
If you do decide to build a ventilated cathedral ceiling, I hope you choose something sturdier for your ventilation baffles than a product that is installed with staples. A product that is installed with staples is unlikely to be sturdy, nor will it be airtight.
Finally, you're making a big mistake to install recessed can lights. A good lighting designer will be able to offer you many alternatives that won't ruin the performance of your ceiling. If you want R-38 performance in 2x10 rafters -- and you claim you do -- then you absolutely can't install recessed can lights.
Martin,
Thank you very much. I did read the article, but wans't sure from reading it whether I needed to vent with spray foam. I think I would feel better having the venting. I was planning to use the venting material recommended in the article (accuvent?) Re the recessed lights, don't the sealed cans prevent much of the airflow? I understand they take up space where insulation would otherwise go.
Also, what do you think about the second approach - closed cell foam and dense packed cellulose? I assume that is going to be cheaper
Richard,
Q. "What do you think about the second approach - closed cell foam and dense packed cellulose?"
A. As my article noted, you don't need ventilation if you choose this approach. However, you'll need to be sure that the spray foam meets code requirements: a minimum of R-15 of spray foam for climate zone 4, or a minimum of R-20 spray foam for climate zone 5. Your suggested approach of installing "a couple of inches" of spray foam would not meet minimum code requirements.
Martin,
So I spoke with my insulation guy. Re the vens, he was where I think you were, which is that he suggested using firring strips and 1/8" hardboard to create the venting rather than the plastic vents. He thought that even with the Accuvent the heat and pressure from the foam could be a problem
Re the foam/cellulose route, he was also in agreement with you. he said minimum of 3" of foam and then the cellulose. Given that this approach requiares them to have 2 different trucks, etc.it may be cheaper to just go with the 6" of closed cell foam. I'll see when I get the quote.
I know theoretically that I don't have to vent if I go these routes, but given that the soffits and ridge vents are already in place and the only extra work is tp put the venting in, I see it as cheap insurance re geting rid of the heat and moisture. It sounds like you agree that the venting can't hurt.
Re the recessed lighting, he siad that if I have to go that route, I should go with sealed low voltage LEDs, which give off very little heat.