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Vented Cathedral Ceiling Insulation Assembly

Bobby_H | Posted in General Questions on

Location: Nashville,TN
Zone 4 => amendment to IRC allows R-38 and 0.30 U-Factor

Hello – I am a young designer getting a crash course in insulating cathedral ceilings. I have read many “A” artical and I find my self getting lost in the weeds.

– The cathedral room assembly attached is my attempt at retro fitting my already installed 2×10 rafters with a combined U-factor or 0.2985 (see attached U-factor sheet).

If you find a moment please comment and critique my details – This would be greatly appreciated!

Note – I can not do a hot roof system due to the roof sheathing being sub-par. -> the roofers who replaced the roof 6 years ago did not replace a good deal of rotting roof sheathing…

A little background of (new/retrofitted) roof assembly & house:

– Standard 26’x52′ ranch home with 4/12 roof pitch.

– the roof system has been retrofitted from site built trusses -> to an assembly consisting of a structural ridge beam and 2×10’s sistered to the existing 2×4 truss rafters under the roof sheathing (wordy…sry). The cathedral ceiling runs the full length of the house via a system of structural post supporting the structural ridge.

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Replies

  1. Bobby_H | | #1

    The U -factor calculation has a mistake -> the xps insulation yields only R-5. This drops the combined U-Value to 0.02985. See attached. sry for the confusion.

  2. Bobby_H | | #2

    I have one more detail I would like an opinion on. See the attached outward drying cathedral ceiling insulation detail. Will this detail preform better/same/worse than the previously posted detail?

    Major question on this detail is whether or not baffles are needed at the eave of the roof assembly to ensure adequate air flow and to prevent insulation washing? The 1" air gap between sheathing and insulation is tight -> is this adequate?

    Note: The gable roof has no dormers hips or valleys -> simple straight roof from gable wall to gable wall.

    Thank you

  3. user-2310254 | | #3

    Binkley,

    I will offer a few comments.

    Another poster asked a similar questions recently, it seems one inch is pretty much the minimum for for vent channel. One point five to two inches would be better.

    I guess you are relying on the U-factor alternative to rationalize the R-38 target. Everything I am reading on GBA suggested that R-49 is not overkill for Zone 4. Maybe Martin, Dana, or Michael will comment on this and whether it's safe at least until you get around to re-roofing. At that point, you might want to consider exterior rigid foam insulation.

    In the meantime, maybe the following article would be helpful:

    https://www.greenbuildingadvisor.com/articles/dept/musings/three-code-approved-tricks-reducing-insulation-thickness

  4. Expert Member
    Michael Maines | | #4

    Binkley,

    It’s too late for you, but for anyone else reading—when you re-roof is your best chance to install exterior insulation. Check out the next Fine Homebuilding for some how-to tips. If your sheathing is sub-par, it’s a good time to replace it, and you can use either nailbase or SIPS to provide a structural over-insulation system.

    I don’t see it noted but the math says your rafters are 24” on center.

    Although it’s sold and rated as R-5/in, XPS ages to around R-4.2 to R-4.4 per inch. It’s up to you which value you use, but R-4.2/in is the safest, especially when dewpoint calculations are important. Conscientious designers and builders don’t use XPS because its blowing agents are potent, persistent global warming agents (aka carbon polluters). Used XPS is ok, but for new material, EPS or polyiso are much more environmentally friendly. EPS is available in different densities, and is more vapor-open than similar densities of XPS. Polyiso usually comes with a foil facer that is vapor-closed, but can be found with fiber facings that also allow it to be somewhat vapor permeable in thin samples.

    I didn’t double-check your math but it looks about right—you add proportional R-values, then convert to U-factor. Your assembly with foam on the interior will provide better drying to the exterior. It will also make your fiberglass perform much worse than its rating, due to wind washing, so I would recommend a vent baffle of some sort, sealed to the framing.

    As Steve noted, a 1” space is the minimum to meet code, but thicker is better. I regularly spec Brentwood Industries’ Accuvent in 1 ½” thickness, as opposed to their standard 1” product, and sometimes use the top flange of an I-joist rafter to create a 1 ½” space. Yes, baffles in a cathedral ceiling need to be continuous from eave to ridge, and should be sealed to the framing as well (though it’s less important with dense insulation like cellulose or mineral wool than it is with typical "filterglass").

    Wall assemblies and roof assemblies need to be separated for fire reasons, and it’s also a good chance to compartmentalize them for air-sealing reasons. What are you considering your air barrier on the exterior wall? The sheathing or the drywall/plaster? You should show a continuous air barrier around your entire envelope. It can transition from interior to exterior as necessary.

    As Steve wrote, if you can get there without great expense, you will probably see a reasonable payback on increased R-value. Providing a thermal break with continuous insulation will help with ROI. But if you’re just trying to meet code and don’t care about energy or environmental costs, it looks like your math works out as is.

    You note a concern about having two vapor barriers but you are not showing any vapor barriers. 1” XPS or EPS (assuming un-faced) is a vapor retarder, as is painted drywall or kraft paper. You note “is cutting of the kraft faced insulation required?” As long as two vapor barriers or retarders are directly next to each other there is no danger of moisture accumulating between them. Even if there was, the old technique of cutting slits is completely useless. You can buy unfaced fiberglass, or use a different/better insulation.

    That's a lot of writing, and I still probably didn't answer all of your questions. Digest what Steve and I wrote, read his link and this: https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling, and the related content linked in those articles, and let us know what you have for specific additional questions.

  5. Bobby_H | | #5

    Steve and Michael – Thank you for your QUICK replies!

    Steve - since I have already under designed my roof with the 2x10 rafters I am looking for the most cost effective and efficient design to meet the “amended code” -> R38/0.030 U factor. As you suggested – when the time comes to re-roof (if we stay in this house that long) the exterior foam will be a no brainer.

    Michael – I have heard/read of polyiso losing its “potency” but I did not realize that XPS would do the same. This is good information on insulation types https://www.greenbuildingadvisor.com/articles/dept/musings/choosing-rigid-foam.

    I specified XPS since it is stiffer @ 1” than the typical beadboard. Thank you for the heads up on the blowing agents used – not very green at all… -> I ran across this article on the GBA website. https://www.greenbuildingadvisor.com/community/forum/green-products-and-materials/101779/closed-cell-spray-foam-and-xps-green-or-not

    Again I am very “green/new” on insulation and roofing assemblies. The tidbits of knowledge/information is based on the weeks of reading and researching how to insulate a cathedral ceiling.

    For my vapor RETARDER, I had planned on kraft face insulation for my walls – there is existing foil faced sheathing on the stud walls followed by the brick exterior. For the ceiling assembly I am leaning towards the outward drying roof -> the kraft faced insulation with 1” polyiso below as my vapor barrier. See attached detail.

    The attached detail shows site built baffles at the eaves. These baffles will not be continuous to the ridge – will this be an issue? The plan is to use R-30C kraft faced Batts due to easier install -> Will wind washing be an issue with the non-continuous baffles (baffles only at the eaves)? If wind washing is an issue would using mineral wool insulation solve the washing issue?

  6. Expert Member
    Michael Maines | | #6

    Bob, Martin's foam overview is indeed excellent. (You might have noticed I was even quoted in it; I also wrote a similar piece for Fine Homebuilding: http://www.finehomebuilding.com/2018/01/08/get-right-rigid-foam.)

    There is a lot to know, and a lot of variables, when it comes to roof design, so don't feel bad. I work regularly with experienced architects and builders who don't understand the fine points either. I can barely keep up with the latest research and details for the two climate zones I work in, 5 and 6.

    Generally speaking, I don't see a problem with your latest assembly, but I don't know the specifics of drying in zone 4. I would imagine that outward-only drying should be ok in a vented roof there.

    As I wrote above, insulation baffles in a cathedral ceiling need to be continuous from eave to ridge, and should be sealed to the framing as well (though it’s less important with dense insulation like cellulose or mineral wool than it is with typical "filterglass"). Plenty of roofs exist without continuous vents but for best performance you should include them, especially if you are using fiberglass.

    Martin was more direct in the cathedral ceiling article I linked to: "If you are installing air-permeable insulation like fiberglass, mineral wool, or cellulose, the ventilation baffle isn't optional; it's required. Air-permeable insulation materials need to be enclosed by an air barrier on all six sides. If you don't install a sealed ventilation baffle above the insulation, the thermal performance of the insulation will be degraded by wind washing." (https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling#ixzz5CDzEOVYE)

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