Rockwool Comfortboard 80 as a interior thermal barrier for spray foam
I am looking to insulate the cathedral ceiling of our 3rd floor walk up attic that contains the HVAC for the 2nd and 3rd floor. I am considering doing some form of flash and batt approach, but was looking at options for continuous interior insulation to eliminate thermal bridging.
It looks like 2″ comfortboard 80 is approved as a thermal barrier for up to 4″ of spray foam. Has anyone used this as an interior thermal barrier?
I was generally planning on drywall or blueboard, but this might be a bit easier to put up while getting some additional insulation benefits. Could drywall be later screw directly through the comfortboard or would it be better to put up strapping? I am trying not to loose too much interior space if I later finish this space.
The unvented approach would be R27 3.75″ closed cell foam against the roof deck, R15 rockwool batts, R8 2″ Comforboard 80. This is in 2×10 rafters, so I would be left with 2″ of air space. Is that okay? I could go ahead and vent the roof, but not sure the best way to hand the 3 dormers.
I am located just north of Boston.
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Newton's,
You should be able to use mineral wool insulation as an ignition barrier, but not a thermal barrier. To learn the difference between these two types of barriers, and to figure out whether you need a thermal barrier or an ignition barrier, you should read this article: "Thermal Barriers and Ignition Barriers for Spray Foam."
You should also talk to your local building code official to make sure that your planned approach is acceptable in your community.
An unvented approach makes much more sense if your roof has three dormers.
Thanks for the reply. I am familiar with thermal barriers vs ignition barriers.
The document I link to in my original post seems to clearly state that this product has been approved as a thermal barrier.
The only caveat I see is that Comfortboard 80 is described as a exterior sheathing product. This may be recently performed testing as the document is from this year.
Maybe I am missing something in the document. I was hoping someone else had done this, but if it is new, than likely not.
The link you provided does show that the comfortboard 80 is rated as a thermal barrier at 2" thick, and an ignition barrier at 1-1/2" thick. However, the Intertek listing is not one of the most common listings in the US (ESR reports are more common). You should bring the technical bulletin to your local code agency before doing this to make sure they are OK with it. When you do finish the space, adding a 1/2" drywall layer will certainly meet the requirements.
Yes, you can screw the drywall right through the comfortboard.
I would not leave any airspace in the rafter cavities. Fill them with more fluffy stuff, maintaining the right ratio of spray foam to fluffy stuff for condensation control.
The 2018 IRC refers to NFPA 275 in R316.4, so it appears to be the right test (but local requirements may differ).
Hey Matt
Did you ever find your answer? I was thinking of using rock wool as a thermal break on the interior.
Thanks John
I didn't use this approach and didn't ask our inspector about it, so not exactly. It is certainly not a widely done approach at this point. Bring the test data from rockwool to your inspector review it, and get their okay.