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Still confused about cathedral ceiling insulation and vapor barrier with a T&G ceiling

Karen Loo | Posted in Energy Efficiency and Durability on

Had massive roof rot over master bedroom which has a vaulted ceiling that was constructed with: T&G cedar -> plastic vapor barrier -> mostly unfaced fiberglass batts -> air channels (cardboard I think) from soffits to ridge -> roof sheathing. Recessed can lights of course, not air tight back then.

I’m in the process of tearing out the T&G cedar to fix this, to put up sheetrock first before T&G cedar. Do we still need the plastic vapor barrier?

i.e. should layers go like this? T&G cedar -> sheetrock -> plastic vapor barrier -> mostly unfaced fiberglass batts -> air channels from soffits to ridge -> roof sheathing.

or like this with NO plastic: T&G cedar -> sheetrock -> -> mostly unfaced fiberglass batts -> air channels from soffits to ridge -> roof sheathing.

Also plan to install the LED air-tight retrofits into the existing cans, sealing tight around the can and sheetrock.

Really appreciate any answers to the plastic yes/no question! Or anything else I might be missing on the layers…

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  1. user-2310254 | | #1


    Where are you located? How deep are your rafters?

  2. Karen Loo | | #2

    Am in Washington state, west side we don't get super cold weather. The rafters are smaller at the soffit end and larger at the ridge end, have R38 batts in there. Don't have the exact measurement (yet).

  3. Karen Loo | | #3

    More space than I thought, about a foot or so from the soffit end, 4' - doesn't seem right, but as I recall those were scissor trusses. So I guess technically this is a vaulted ceiling, not cathedral :)

  4. GBA Editor
    Martin Holladay | | #4

    It sounds like you are in Climate Zone 5 or Climate Zone 6. [Later edit: Oops! I misread your comment, and thought you were in the eastern section of Washington. My mistake. You're probably in Zone 4 marine.]

    Building codes require the installation of an interior vapor retarder in your climate zone. That requirement can be fulfilled with vapor-retarder paint, polyethylene, or a "smart" retarder like MemBrain. For more information, see this article: Vapor Retarders and Vapor Barriers.

    Since you had "massive roof rot," you know the risks of air leakage. I think it is absolutely nuts to install LED retrofits in the existing recessed can fixtures, in light of your disaster. Recessed can fixtures displace insulation, creating hot spots in the roof, and an LED retrofit kit, even if it is advertised as "airtight," will usually leak air. What is likely to happen is that you will need to tear everything apart again in 6 or 7 years, due to a recurrence of "massive roof rot."

    The correct solution is to remove all of the recessed can fixtures, and to come up with a different approach to lighting this room -- an approach that does not require any electrical boxes in the ceiling.

    To avoid a recurrence of your problem, you need meticulous attention to details. You need airtight ventilation baffles; perfect installation of insulation (with no recessed cans); and a perfect interior air barrier.

  5. Karen Loo | | #5

    Thank you so much, so YES to a layer of plastic (or Membrain type material) between sheetrock and insulation on ceiling because of building code.

    re:roof rot - the original roof only had a few box vents, it now has a full ridge vent so that should help right?, along with the new sheetrock layer. Plus all the plywood that rotted was replaced with pressure treated so I've got longer :)

    re: airtight ventilation baffles - a bit confused on this, will have to research, There is over 4 feet of space near the smaller soffit end of the bays, sort of a mini-attic if you will due to the scissor trusses, so I believe there were just short baffles installed at the soffit ends to keep the insulation away from those soffit vents and there is just air space above the batts the rest of the way. Will pull out some insulation today to get a better view. But with that much air space on top of the batts are full length sealed baffles necessary?

    re: lighting - well, what about those LED slim lights 2" thick like these airtight ones:
    The room is 24' long there are 6 of the can lights (as well as 3 skylights and 2 sprinkler heads.)

    Thanks again

  6. GBA Editor
    Martin Holladay | | #6

    Improving the ridge vent sometimes makes a roof rot faster -- since it can encourage air flow through fluffy insulation (if the ceiling is leaky). For more information on this problem, see these two articles:

    How to Build an Insulated Cathedral Ceiling

    All About Attic Venting

    To prevent this problem, an airtight ceiling is essential -- and your light fixtures will undermine that goal. I'm not saying that airtight fixtures are impossible -- just difficult. Perfect air sealing is required. I'm steering you away from installing ceiling light fixtures because of your history of problems.

    I understand that with scissors trusses, the ventilation baffles don't extend the full length of the roof. That said, it's always a good idea to make your baffles airtight, to reduce wind-washing and to reduce unwanted air flow paths through the insulation. For more information, see this article: Site-Built Ventilation Baffles for Roofs.

    Most cathedral ceiling insulation jobs are flawed. Sometimes, you can get away with imperfect details. But when you are repairing a roof that had "massive roof rot," you don't want to cut corners.

  7. Karen Loo | | #7

    OK thanks, I found these interesting LED fixtures that sit on the outside of the ceiling:

    They simulate the look of a recessed light but are installed on the outside. Still have to have the wires thru the ceiling tho :) but these should be better for airtight purposes yes?

  8. GBA Editor
    Martin Holladay | | #8

    The electrical code requires an electrical box for each fixture. If you insist on installing ceiling lights, (a) purchase and install airtight electrical boxes, (b) choose the shallowest airtight boxes you can find, and (c) pay meticulous attention to the required air sealing details.

  9. Karen Loo | | #9

    OK, thanks again - really appreciate it.

  10. Expert Member
    Dana Dorsett | | #10

    Most of Washington State is zone 4C (everything west of the Cascade). Most of the rest is 5B, with the exception of the northeastern tier counties from Okanongan, Ferry, Stevens,and Pend Orielle counties which are zone 6B.

    Current WA code requires R49, but there are exceptions. It can be R38 as long as the R38 extends fully over the top plates of the exterior walls, with no taper. Unvented roof assemblies are only allowed if there are NO interior side vapor retarders at the ceiling level. See:

  11. GBA Editor
    Martin Holladay | | #11

    Thanks, Dana. I misread Karen's comment, and somehow concluded she was in eastern Washington. My mistake.

  12. user-2310254 | | #12


    Would Karen be better off converting the attic to conditioned space? Could she seal the ventilation openings and install a thin layer of closed cell foam against the underside of the sheathing and then install the fiberglass batts against the close cell to reach the code minimum r-48?

    Or are there reasons to stick with the ventilated attic approach?

  13. GBA Editor
    Martin Holladay | | #13

    I focused on answering Karen's questions. She didn't ask, "Is there a better way to do this?"

    But since you're asking: Of course there are better ways to do this. Fiberglass batts are the worst-performing type of insulation available. Fiberglass batts are hard to install correctly, and easily allow air to pass through them.

  14. Jon_R | | #14

    > NO interior side vapor retarders at the ceiling level

    Not clear what they mean here (everything retards to some extent). Perhaps "Class I"? Hard to say since I didn't find this in the code.

    Also interesting is "Unvented crawl spaces are not prescriptively allowed." Perhaps they want to see "Heated plenum crawlspaces".

  15. Karen Loo | | #15

    thanks for the further comments and ideas. This installation met code back in '97 and the R38 does extend over the top plates of the exterior walls so I think we're good there.

    Also, I just discovered the existing shallow Lithonia LCP 6" recessed housings are actually air tight and I may risk sticking with them, sealing exceedingly well around them (which wasn't done before.). I don't think I can use their newer 2" LED housings because 5/8" sheetrock + 1/2" T&G cedar width will exceed the width the installation clips can span.

    We will be using the Membrain, will be here tonight.

  16. Karen Loo | | #16

    Hey, getting rid of the old recessed cans after all, the clips have enough adjustment we figure we can make it work particularly if we drop down from 5/8" to the 1/2" "ultra light" sheetrock they sell these days for ceilings. Those Lithonia WF6 "wafer thin" lights are pretty sweet, getting the 2700k version off of Amazon.

    So we'll have much better insulation over those lights.

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