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Community and Q&A

Fixing Unvented Cathedral Ceiling Insulation

RobJ13 | Posted in Energy Efficiency and Durability on


We’re in the process of doing a major remodel project in our house and would like advice on our proposed ceiling solution. I’m in Boulder, CO (Zone 5B I believe). The house was originally setup as a vented cathedral ceiling with fiberglass batts in the early 70s. However, 5 years ago, while replacing the roof, we thought it would be a good idea to remove the batts and spray in insulation (open-cell in this case). To achieve this, a contractor cut open the roof decking from above (2 foot strips the length of the roof) and *tried* to fill the space with open-cell insulation. However, as we’re now finding out, the coverage is poor and there’s at least a 2-3 foot strip that runs the length of the ceiling (approx. in the middle) on both sides where insulation is sparse or missing. It looks like there’s other parts near the ridge and eave that may also be only partially covered). One good indicator has been areas where snow melts much quicker.

After reading a number of articles it’s also become clear that the lack of a vapor barrier may also be risky and could cause roof rot (so far, fortunately, there’s been no indication of that).

Now, while I would’ve preferred to leave the ceiling (which was just scraped to remove popcorn and finished) alone, I would rather do this right and ensure the house is well insulated. I can see several options to fix this:
1. Use a thermal camera to find areas where insulation is missing. Cut holes in the ceiling in said areas and fill with additional open cell insulation. Apply a painted on thermal barrier to drywall after fixing drywall.
2. Remove ceiling altogether. This will ensure we can easily spot areas where insulation is missing and ensure it gets filled completely with open-cell. Before installing drywall, add some type of rigid foam, such as polyiso (1″) to add additional R-value and form a vapor barrier on the inside.
3. Remove ceiling altogether. Painstakingly remove all the open-cell foam. Re-spray with closed cell. Apply drywall.

The area between roof decking and drywall is approx 7.25″ deep.

I would love some advice on which approaches make sense or consider what I missed. Anything else I should take care of while I’m at it?



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  1. user-2310254 | | #1


    How deep is the cathedral ceiling? Probably not deep enough for R-49 worth of insulation. Would you be open to installing rigid foam on the exterior of the sheathing? You would need for 41% of the r-value to come from the rigid foam. (You can read more about this approach here:

    I assume you made the cathedral ceiling unvented when the spray foam was installed. (You might also want to read this article on damp sheathing:

  2. Expert Member
    Dana Dorsett | | #2

    Boulder's building codes have somewhat more stringent R-value requirements than the IRC and would require R54 (or U0.020 whichis R50 "whole assembly") on a new build. See TABLE N1102.1.2 on p.50, p.54 in PDF pagination, and TABLE N1102.1.4 on p.51 (p.55 in PDF):

    For retrofits & repairs some amount of leeway is likely to be given by the inspectors.

    Even if gutted from the interior and filling the cavities with 7.25" of an R7/inch HFO blown foam wouldn't get you there, but adding 5" of 2lb roofing polyiso above the roof deck and leaving the existing insulation in place would.

    The insulation itself can be pretty cheap if usiing reclaimed goods, far cheaper than 7" of HFO blown foam (which would run about $11-12 per square foot in my neighborhood.) This building materials salvage outfit in Henderson always seems to have stock in polyiso of various thicknesses:

    There is the additional cost of a nailer deck and the ~7" pancake head timber screws for securing the nailer deck to the structural roof deck, and a 1x6 facia board to cover the ends, as well as the new roofing, which will be more expensive than new ceiling gypsum.

    But it's a higher performance, lower risk solution that actually gets you to Boulder's code requirements, and has significant dew point margin at the roof deck.

  3. user-723121 | | #3

    I would start over from the inside as the roof is only 5 years old. You will have to make sure of course the roof decking is OK. Open the rafter bays completely and make a dedicated airspace right below the roof deck with some 1 1/4" firing strips. I use fiberboard sheathing as it is easy to work with and inexpensive. Fill the remaining void with the insulation of your choice that is suitable, forget the expensive spray foam. Now add layers of Polyiso foam attached to the underside of the rafters to a level agreeable to the city of Boulder, CO. This system works and I have used it for 35 years in Minnesota for cathedral ceilings.

  4. RobJ13 | | #4

    Thanks for the feedback and insight Steve, Dana, and Doug! My hope is to leave the exterior alone, as Doug mentioned, since the roof is relatively new.

    If I were less concerned about getting to Boulder's stringent requirements for new buildings, but concerned about ensuring the roof is solid, would it be possible to do the following?

    - Remove the existing ceiling drywall. Fix the spots where open cell foam was missed and ensure there's a good 7.25" of open cell against the roof sheething. Followed by 3" of poly iso with foil to form a vapor barrier. That would give me approx (7.25" * 3.5 + 3" * 6) R-43.

    As you can probably tell, I'm trying to find a cost effective solution. If not, Doug's solution seems reasonable as well, but would require the removal of the open cell foam and creating a vented roof.

    Here are some details I could find about the ~5 yr old roof: Asphalt shingles: Owens Corning Duration Storm with a combination of Feltex and Owens Corning Deck Defense synthetic felts installed (after carefully studying a few old pictures, I could make this out).

    Thanks for the help!

  5. RobJ13 | | #5

    ^^ I made a couple of edits to the answer above. I noticed that the roofers used synthetic underlayment when installing the roof. I don't know if this greatly affects my options. Whether poly iso on the warm side of the roof will cause issues as a result.



  6. user-723121 | | #6


    I think the decision to work with the existing open cell foam or replace it will be made when you remove the drywall to see what lies beneath. I have no problem with unvented roof assemblies as long as they are highly insulated and airtight. The Polyiso layer on the warm side, foil taped and edges sealed will provide high R-value, minimize thermal bridging and make for a very airtight assembly. As has been suggested, check out used rigid foam board availability.

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