Choosing insulation for a cathedral ceiling
This is my situation: I am located in Colorado, where is is generally dry, gets hot in the summer and cold in the winter. I am in process of building my own house and want to make choices that wont come back to bite me down the road. With that in mind I want to avoid any moisture issues inside my ceiling cavities.
What I have so far is a stick framed cathedral ceiling using 2×12 dimensional lumber. On top of the rafters I installed 1/2″ CDX sheathing and covered that with grace tri-flex (I wish I had used felt paper). On top of the first layer of sheathing I installed 2″ rips of 1 1/8″ CDX fastened through the 1/2″ into the rafters, thus creating ventilation channels from eve to ridge. Then I finished with a layer of 5/8″ sheathing, Grace Ice and Water HT and a standing seam metal roof. All of that already exists.
Now I want to insulate in a way that won’t lead to moisture accumulating on the underside of the lower layer of sheathing that would lead to mold and rot. I will not be using any canned lights. There will be some parts of the house where I will drop the ceiling down flat but am still planning on keeping the envelope at the cathedral ceiling and thus having mini conditioned attic spaces.
What is the insulation of choice? I am afraid that any small failures in a closed cell foam will allow moisture accumulation on the underside of my “insulation backer” (the first layer of CDX) that will never get out. Maybe the moisture will be able to move through the 1/2″ ply and out of the lapped seams in the tri-flex (which has a perm rating of around .5 I believe)…. thoughts?
I would like to use a dense pack cellulose, but am worried that will allow too much moisture up. With that, the installer I am familiar with and generally does excellent work uses a reinforced plastic to contain the cellulose on ceilings – which also concerns me as far as possibly introducing a double vapor barrier situation.
Anyway – what would you do in my situation?
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Replies
Sounds to me like closed-cell spray foam is your best choice at this point. No matter what material you use, any 'small failure' will lead to condensation on the under side of your first layer of roof sheathing.
Sprayfoam should have, by far, the least number of air leaks that you are concerned about. Basically, you will have the same system (unvented "air-proof" foam) that many builders are using today, with the added bonus(?) of your upper vent system. If the system fails in ten years, you won't be the only one in that boat!
Patrick,
You can't install cellulose or any other type of air-permeable insulation between your rafters unless you have a ventilation channel that can help ventilate the roof sheathing in case you get condensation or moisture build-up. You almost managed to include such a ventilation channel -- but you messed up some of the details. So now you have no choice but to install spray foam.
If you had installed asphalt felt or a vapor-permeable European membrane on top of the lowest level of roof sheathing, your ventilation channel would have helped keep your roof sheathing dry. But since you chose to install Grace Tri-Flex, which is a vapor barrier, you shut down that avenue of drying. (For more information on roofing underlayment options, see Synthetic Roofing Underlayments.)
For information on different ways to insulate a cathedral ceiling, see How to Build an Insulated Cathedral Ceiling.
The following information won't help you, Patrick, but I'll provide some advice to GBA readers who are considering a similar approach: while it's always a good idea to include a ventilated air gap under your roofing, especially in areas where ice dams are possible, Patrick missed a chance to include a layer of rigid foam above the lowest level of roof sheathing. Considering how elaborate this roof assembly is, the lack of a rigid foam layer is a real shame. Rigid foam would have stopped thermal bridging through the rafters, and would have allowed Patrick to choose from a wider range of insulation options (including cellulose) between the rafters.
Thanks Martin and Eric. That is what I was expecting. Yes unfortunately I followed some bad advice on the use of the Tri-flex and realized the downsides apart part way through it's install, but since I had been planning on using closed cell foam at the time I continued on. And also because I genuinely didn't know if the lack of tapped seams might not be sufficient in making the material act as vapor permeable, especially considering that the plywood underneath doesn't have a particularly hight permeability - what is it about 1?
And I still believe that the vented assembly was worth the extra time I spent installing it as it will help to prevent ice-dams and help to dry out the upper layer in the event of any bulk moisture intrusion from the top - though in that case perhaps not enough help.
What are you thoughts on a hybrid system with closed cell foam and cellulose on top? Where there is enough foam sprayed to keep it's surface above dew point and then some. One concern I have with this is about the areas in connect with framing members that via thermal bridging might bring that area down to the dew point even with a margin of error built in by the amount of foam sprayed.
Patrick,
It's perfectly OK to install a mixture of insulations -- with closed-cell spray foam in direct contact with the roof sheathing, and then cellulose underneath -- as long as the closed-cell spray foam is thick enough to meet the minimum R-value requirements in the code for this type of hybrid insulation.
Colorado has several climate zones (5, 6, and 7). The minimum R-values for the spray-foam layer would be R-20 in climate zone 5, R-25 in climate zone 6, and R-30 in climate zone 7.
Q. "What are you thoughts on a hybrid system with closed cell foam and cellulose on top?"
A. As long as you remember that the cellulose goes on the bottom, not on the top, this system will work.