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How to insulate cathedral ceiling with standing seam metal roof?

Chad Kotlarz | Posted in General Questions on

I am in the design phase of building a new home, and having difficulty determining how to construct a cathedral ceiling roof with stand seam metal roof. I have read all the articles on this website for different techniques available, but none of the completely address my concerns.

The architect designed the roof with 2×12 rafters, ply-wood roof sheathing, non-vented, with a standing seam metal roof. Insulated with spray in closed-cell foam. The interior will be painted drywall. There will also be an exposed truss with a collar tie on the interior of the home.

The main concern is with the thermal bridging from the 2x12s. I am in zone 6. The roof is a 10/12 pitch, with a dormer, and 2 valleys. I was told that the thermal bridging created from the 2×12 would cause hot/cold spots in the roof and would lead to problems with the metal roof. I’m assuming this is from expansion/contraction or condensation, but Im not certain.

I was told by an experienced roof contractor, that the 2×12 thermal bridging should not be an issue, or cause problems, but I want to be 100% certain. I don’t really want to use rigid foam on exterior of the sheathing, or the interior near the drywall to break the thermal bridging for several reasons.

Will thermal bridging from a 2×12 cause problems in this application over time? Will it cause a rippling effect on the roof metal, or condensation issues/wood rot? Thanks so much for any input! I have really had a tough time getting this question answered.

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Replies

  1. Stephen Sheehy | | #1

    I have cathedral ceilings in my pretty good house, with standing seam roof. We used raised heel scissor trusses deep enough that we could insulate the bottom of the truss space with about 20" of cellulose and allow for a vented space above the insulation.

    A nice benefit to trusses is the ability to put interior partitions anywhere you want, since the trusses carry the entire roof load.

    Without foam on top, you will have some thermal bridging.

  2. John Clark | | #2

    How a roof is insulated has no bearing on rippling (aka Oil Canning) of metal roofs. The gauge of metal, how the panels are clipped together (do they allow for panel movement from heat caused expansion, length of the metal panels themselves (longer panels move expand more) and installer experience are more important.

  3. Chad Kotlarz | | #3

    Thanks for the response. Unfortunately, I don't have the option to use scissor trusses.

    After a bit more research, I found to alternatives to framing the cathedral ceiling.

    Option #1: Use 2x10s and stagger them, as to create open spaces for insulation above and below every other rafter. Im not sure of the structural integrity of this method.

    Option #2: Create a parallel chord truss but using a 2x3 or 2x4 gusseted to the 2x12 with plywood strips stapled on, to create an insulation space. I'm not sure how this would look, I just found the option in earlier posts. It seems like a lot of labor.

    Does anyone have comments any either of the two methods?

  4. Stephen Sheehy | | #4

    Chad. Are you contemplating about a foot of closed cell foam?

  5. Chad Kotlarz | | #5

    I haven't decided about the actual thickness of foam, but I was thinking in the realm of 10 inches. I like the Option #2 above, that Dan Kolbet used. Making extension of the 2x10s with gussets in order to provide an air gap for the foam. It looks like Dan used blown in cellulose, from what I understand is not advised for a non-vented cathedral ceiling.

  6. Expert Member
    Dana Dorsett | | #6

    A foot (or even 10" ) of closed cell foam is the opposite of "green" due to the high polymer content per R and the HFC245fa blowing agent.

    In a zone 6 climate with closed cell foam under the roof deck and fiber insulation under the foam, a minimum of half the total R-value would have to be closed cell foam to avoid moisture accumulation at the foam/fiber boundary over a winter. See:

    http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

    The R25 prescriptive presumes R49 code min for the total. At higher R, the ratio has to stay the same.

    If you install 2x3s perpendicular to the rafters "Mooney Wall" style that would yield a total cavity depth of 14", and there would be an R10-ish thermal break over the rafters. If you installed 5" of closed cell foam, and 9" of cellulose you would be at about R60-R65, and would have sufficient dew point control for the foam/fiber boundary to be fine with a Class-III vapor retarder (standard latex paint on gypsum board) on the interior. Cellulose is able to store and redistribute whatever moisture accumulates without damage or loss of function even if the ratio is slightly off.

    If fiber insulation makes you nervous, comparable thermal performance could be had in that assembly with 5" of closed cell foam and 9" of half pound density open cell foam. That's still a lot of foam, and a lot of HFC245fa blowing agent for the closed cell layer, but it's a lot cheaper and higher performance than 10" of closed cell foam.

  7. GBA Editor
    Martin Holladay | | #7

    Chad,
    1. I agree with Chris M that your use of spray foam insulation, as planned, will not negatively affect your metal roofing or affect oil canning.

    2. I agree with Dana that your plan to use lots of closed-cell spray foam is not very environmentally friendly.

    3. The fact that your roof has dormers and valleys makes the vented approach quite difficult or impossible.

    4. If you are dead-set against installing rigid foam above your roof sheathing (always the best solution), you should use a combination of spray foam against the roof sheathing, supplemented by a fluffy insulation (cellulose, mineral wool, or dense fiberglass batts) on the interior side of the cured spray foam, as Dana suggests. Adding plywood gussets and 2x4s below the rafters is one way to deepen your framing cavities; another is to install 2x4s, 24 inches on center, at 90 degrees to the rafters.

  8. Expert Member
    Malcolm Taylor | | #8

    Chad,
    Dan kolbert, who posts here, made extensions for his framed roof to increase the depth much as you describe:
    http://www.kaplanthompson.com/_images/publications/09.06-jlc.pdf

  9. Chad Kotlarz | | #9

    Thanks for all the great responses.

    Questions: If I just go with the original 2x12 construction, and closed cell foam 5 inches, and cellulose the rest, with no method of breaking the thermal bridge of the 2x12s, will this lead to any condensation issues, or mold/rot?

    If it would lead to problems, would the "Mooney Wall" method of applying 2x4s perpendicular to the rafters, provide enough of a thermal break to solve any issues that might arise?

    Is staggering 2x10s as rafters, just a bad idea due to the amount of wood needed for structural support and an anchor for drywall?

    Thanks much for all the help.

  10. Charlie Sullivan | | #10

    Q1: will 2x12 bridging lead to mold or rot? I don't think so. The thermal bridging in this case is mostly a heat loss issue, not a durability issue. You might be worried about the problem occurring on the surface of the drywall inside, if the thermal bridging was bad enough to make that surface cold. That is not a concern--the wood is about R-1/inch, so 12 inches of it is about R12. You'd need to be a lot worse than that to have condensation there. The other place you might worry about it would be at the top of the cellulose-filled cavity, where the rafter pokes out of the foam. There you have a shorter bridge through the foam to the cold outside, so you might worry about it, but there are two things that mitigate that. One is that you also have a thermal bridge to the inside, helping keep that spot warm. Also, wood is vapor permeable, so it can dry to the outside a bit.

    Q2: Will a Mooney Wall method solve this problem. Moot point, as I don't see a problem.

    Q3: Staggering 2x10s does sound like a bad idea to me. You could probably use something a lot less than 2x10s, maybe 2x6s, for supporting the drywall--it doesn't need to take the snow load--but you'd need nearly the full original number of rafters to support the roof, so you'd end up with more wood in the ceiling.

  11. Charlie Sullivan | | #11

    An idea that I haven't seen done before that might be good for an application like this--curious to see what Dana and others think of this: Instead of 5" of closed cell SPF (ccSPF), how about 4" of open cell spray foam, up against the roof decking, followed by 2" of ccSPF sprayed onto the open cell. That reduces cost, global warming impact, and polymer content, while still providing the vapor barrier at the point where we transition from fluff to foam.

    As others have pointed out, there are several reasons to minimize the use of spray foam, and particularly ccSPF. One is cost--particularly as you get to large thicknesses, it needs to be applied in many layers. You might legitimately find the cost acceptable or worth it. The quantity of polymer per R value is a philosophical issue, that you might reasonably decide to care about or not care about. And there's also the risk of a lingering odor if the foam doesn't cure properly--a rare, but devastating problem. You can minimize that risk but choosing a skilled installer.

    To me, the biggest reason to avoid ccSPF has been the global warming impact of the blowing agent used to make the bubbles in it, which is 1400X worse than CO2. But there is now a real way to avoid that problem: there is one spray foam material company, Lapolla, which offers a low global warming impact system, their "4G" product: http://www.lapolla.com/foam-lok-2000-4g

  12. C. B. | | #12

    Chad, one thing to consider is drying potential of the sheathing under the standing seam metal roof. It will need to dry outwards since you need to insulate with low-permeable insulation on the inside. I'd look at using:
    http://www.roofaquaguard.com/synthetic-underlayments/dry-tech/
    or:
    http://www.cosella-dorken.com/bvf-ca-en/products/roof/underlayments_metal/products/trela.php

    These will provide an airspace under the standing seam roofing since solid, continuous metal doesn't allow moisture to escape.

  13. Chad Kotlarz | | #13

    C.B. , Thanks, those look like great products. Do you recommend one, over the other? Have you had any experience with them?

  14. C. B. | | #14

    When I was putting on my metal roof 18 months ago, the first one wasn't on the market and the second one was impossible to find. I got lucky because my my roof is metal shingles (Classic Metal Roofing Systems' Oxford Slate Rock in grey) which have an inherent airspace underneath which allows drying. My porch roof is standing seam, but it can dry downwards since there is no insulation underneath.

    If I was installing my roof today I'd get the Delta Trela from:
    http://www.spycor.com/DELTA_TRELA_Roofing_Membrane_by_Cosella_Dorken_p/dltr.htm
    but I'd price compare using the Dry-Tech product over Brea-II:
    http://www.roofaquaguard.com/synthetic-underlayments/brea-ii/

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