Insulation contractor recommends open-cell foam for cathedral ceiling
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Hi – we are building a new home where the great room section has a cathedral ceiling with dormers on each side. We are planning on spray foaming the underside side of the roof deck, and the insulation contractor is suggesting open cell foam because if the roof ever leaked with closed cell foam, it would go undetected and the roof would rot. We are in climate zone 4A and we used ZIP roof sheathing with shingles applied directly to the ZIP. I would appreciate any input on this.
Thanks,
Brad
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Replies
Brad,
I don't recommend installing open-cell spray foam on the underside of roof sheathing unless you are willing to install an adequate thickness of rigid foam insulation above the roof sheathing. Here are links to two articles on this topic:
High Humidity in Unvented Conditioned Attics
Open-Cell Spray Foam and Damp Roof Sheathing
For information on all of the different ways you can insulate a cathedral ceiling, see How to Build an Insulated Cathedral Ceiling.
--Martin Holladay
HI Martin - thanks for the response. The issue I'm running into is that this advise is contrary to what the installers are recommending. What would your response be to the point that installing closed cell foam in this application could hide a roof leak, which could result in roof rotting.
Thanks again.
Brad
Brad,
There is no evidence that roof leaks are easier to spot when you use open-cell spray foam compared to closed-cell spray foam. If you are worried about leak detection, I advise you to design your building to have a vented unconditioned attic, with all of the insulation on the attic floor.
-- Martin Holladay
Martin - we are already completely framed, so I'm trying to figure out the best way forward for this cathedral ceiling. This cathedral ceiling has dormers on each side that interrupt the potential ventilation from the eave to the ridge for some of the rafter bays. If we were to install ventilation baffles with mineral wool batts for insulation, we would have the issue that some of the rafter bays would not have intake ventilation from the eave. Would it be feasible to drill ventilation holes in the rafters for the bays interrupted by the dormers so they could pull some eave intake air from the uninterrupted rafter bays?
Thanks again.
Brad
Thanks,
Brad
Brad,
I think that you are stuck with an unvented approach. It's probably too late for you to install rigid foam on the exterior side of the roof sheathing -- which in my opinion is always the best approach for unvented cathedral ceilings.
If you install spray foam against the underside of your roof sheathing, I strongly advise you to specify closed-cell foam, not open-cell foam, for the reasons outlined in the articles I linked to. Of course, if you prefer to listen to the advice of your local contractors rather than my advice, you can.
-- Martin Holladay
Vents that don't go the full distance don't really buy much protection at all. Hipped roofs, and dormer valleys would be at risk.
To meet the IRC prescriptives for a code-min R48 unvented roofs without a class-II vapor retarder on the interior you would need a minimum of R15 of closed cell foam against the roof deck with R43 of open cell foam below that. If the total R increases, the fraction of the total R has to increase, but if you're insulating to less than R49 it can shrink proportionally.
It's roughly 30% of the total R that has to be closed cell foam. Whether the remaining 70% is open cell foam or fiber, it still needs Class-III vapor retardency on the interior. Standard latex paint on gypsum board is good enough. If you cheat the ratio by very much you may do just fine using a half-perm "vapor barrier latex" or a "smart" variable permeance vapor retarder on the interior side, but it's getting into a gray area of the code. With the (unrecommended) all open cell foam solution it's strongly advisable to use half-perm paint, which may or may not be enough to protect the ZIP, but would mitigate against the potential for summertime humidity-pumping in & out of the foam & ZIP with daily swings in roof temperature. (That's more of an issue with attic roofs than with cathedralized ceilings.)
With as little as 1" (R6-ish) of closed cell foam the ZIP would be protected from interior side moisture drives, and would be sufficient dew point control for up to R13 or so of fiber or open cell foam at the interior side of the closed cell. If you did the rest a open cell foam it would be pretty safe overall, but you'd still want to use half-perm paint or smart vapor retarders on the interior side.
I have a question regarding your 30/70 ratio of closed cell to open cell, Dana. I'm in Zone 5 and I'm encountering an un-vented second floor with a cathedral ceiling that has 3" of closed cell applied directly to the roof deck. New 1/2" drywall (or possible 1/4" wood planks) will be installed as an interior. While the rafters are exposed, is it worth adding 2 1/2" of open cell foam on top of the 3" of closed cell? Or should I investigate the cost of extending my rafters beyond their current 5 3/4" depth and add a sizable amount of some other insulation to get to a higher R value? (The second option, while appealing, may be difficult as the house is an old balloon frame with no ridge beam and may take some engineering analysis to determine if the rafters can support additional weight.)
Thanks for the input Dana. If I understand you correctly, you are saying all closed cell foam would be best, but (say for cost reasons) we can use a mix as you described? One other factor is that we will most likely apply 1" thick wood paneling after drywall for the cathedral ceiling aesthetics. Would that change the equation at all?
Thanks,
Brad
Closed cell foam is not very green, and using the MINIMUM amount necessary to achieve the design goals/moisture resilience is best. Closed cell foam uses 2x the amount of polymer per R as open cell foam, and uses climate damaging HFCs as the blowing agent (in all but a few cases) in stead of water (open cell foam).
When thermally bridged by rafters, the rafters severely undercut the thermal performance of the foam, so anything more than the minimum needed for dew point control (30% of the total R) would be a waste.
If you want to save money, an inch of closed cell is sufficient to protect the roof deck from interior moisture drives in winter, and half-perm paint or 2-mil nylon on the interior side is sufficient to keep any fiber or open cell foam sufficiently dry when you have that inch of closed cell under the roof deck. It's side-stepping the letter of the code a bit, but it works.
Read this document, pay particular attention to the tables the end:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf
I have a question regarding your 30/70 ratio of closed cell to open cell, Dana. I'm in Zone 5 and I'm encountering an un-vented second floor with a cathedral ceiling that has 3" of closed cell applied directly to the roof deck. New 1/2" drywall (or possible 1/4" wood planks) will be installed as an interior. While the rafters are exposed, is it worth adding 2 1/2" of open cell foam on top of the 3" of closed cell? Or should I investigate the cost of extending my rafters beyond their current 5 3/4" depth and add a sizable amount of some other insulation to get to a higher R value? (The second option, while appealing, may be difficult as the house is an old balloon frame with no ridge beam and may take some engineering analysis to determine if the rafters can support additional weight.)