GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Unvented cathedral ceiling interim remodel

user-6883899 | Posted in Energy Efficiency and Durability on

I have a roof problem on a small section of 4/12 above the bathrooms. It is only about 15 feet by 10 feet of insulated ceiling. Background information…. We are in Enumclaw, WA, which is zone 4C, I believe.

My home was built in the early 80’s as a Justus Homes kit. All of the ceilings are unvented cathedral construction as follows:
2×6 tongue and groove on the interiors
15 pound felt
2×6 on edge at 24 inch spacing
Rafter spaces are “cut and cobbled” with 2 layers of foil faced 2 inch isocyanate and one inch of white styrofoam beadboard. I think the foam was put down as they set the 2×6’s, creating a tight fit from what I can see.
1×4’s at about 9 inch centers
30 pound felt
Cedar shakes

Most of the roof is 12/12 pitch. A small section above two bathrooms is 4/12, with two 12/12 pitches joining two sides with valleys. In that section, a skylight was cut in after initial construction, maybe in the late 80’s. (I have owned the home for 5 years). The 1×4’s and a small part of the tops of the rafters are damaged by rot, particularly badly around the skylight.

I’ve read your discussion of cut and cobble and how to build unvented cathedral ceilings in new construction. With the two 12/12 sections of shake roof coming in to this small section, I can’t see how to increase the thickness of this part without involving the 12/12 sections.

I am considering pulling all of the 1×4’s.
Installing 1/2″ OSB.
Sealing the top of the OSB with a full cover of ice and water shield as my air seal.
Along the two edges where the OSB will be under the existing valley metal, sealing with spray foam.
Installing 30 pound felt.
Installing standing seam metal roof, unvented. (the 5000 square feet of shakes on 12/12 remains)

Inside, below this section, I have improved the bathroom fan capacity covered the tongue and groove with sheet rock.

In the long run, it is possible that I am going to have to have the entire roof replaced, which will allow adding the R10 or greater rigid foam and completing the other details, at a very large cost……

For an interim solution on this small section of roof, what improvements to my plan does anyone have?

(I’ll be removing more roof today and will update on the conditions I find)

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    First, care to tell us your name?

    Second, can you describe the roof configuration? For example, "The main roof is a 12-in-12 gable. The bathroom is a shed dormer" -- or perhaps "a gable dormer -- with a 4-in-12 pitch."

    Or perhaps -- "The bathroom is a later addition that intersects the main roof in a way I will describe to make things clear to you."

    Or perhaps -- "Here is a sketch that I have photographed with my smart phone camera."

  2. GBA Editor
    Martin Holladay | | #2

    Two questions:

    1. Why do you propose both Ice & Water Shield and #30 asphalt felt?

    2. Are these 1x4s what most people call skip sheathing?

  3. user-6883899 | | #3

    Thanks for the quick response!

    Sorry about no name, I guessed that the system would publish it... My name is Chuck Kramer. I'm not a contractor (you guessed that, I'll bet....)

    There are two main 12/12 roof pitches, forming a tee. Off the stem of the tee, there are two 4/12 dormers, one on each side. The dormers tie in to both of the main roof pitches, with one gable end open on each. This section is over bathrooms. The other is over bedrooms.

    To your second post....

    1. I thought I had read that as a proposed approach in one of the articles... A water/air barrier on top of the sheathing, with "your favorite material" on top of that.
    2. Skip sheathing sounds right. It is about 50% coverage of the total area.

    I am, at this moment, pulling off more of the roofing. I will add a photo in a couple of hours, and will know the full extent of the water problem. So far, it looks like it may be related to the skylight only. They literally cut a hole in the entire roof structure, added no framing, screwed the deck mount skylight on and covered it with trim. No insulation at the cut, and this was directly above the bathtub/shower....

    More soon.

  4. user-6883899 | | #4

    The damage is throughout the roof, as you can see in the photos. Not sure what the fix is, but for now I'm just going to remove all the sheathing remnants and ponder if there is a way to move the transitions and valleys up enough to add the rigid foam at R10. Any suggestions are appreciated.

  5. user-6883899 | | #5

    I have no experience at this, so this might be way off base....
    1- remove the 1 inch styrofoam
    2 - replace with 1 inch closed cell foam boards and spray foam all edges.
    3- tape over the rafters and all joints, creating a complete air and water seal
    4 - install the roof deck - 5/8 inch OSB for 24 inch centers?
    5 - 30 pound felt
    6 - standing seam metal

    This would allow me to leave the transition flashings in place. Do you think there is any merit to this approach?

  6. user-6883899 | | #6

    After cleanup....

  7. GBA Editor
    Martin Holladay | | #7

    Cut-and-cobble roof assemblies are risky, as I noted in my article, Cut-and-Cobble Insulation.

    Your roof assembly shows signs of moisture accumulation -- probably due to years of air leaks at the various seams between the framing and the rigid foam. The exfiltrating air carries moisture, and the moisture condensed on cold skip sheathing.

    Fixing the open section with more cut-and-cobble is only a temporary solution, since you'll probably end up with more air leaks as the rafters expand and contract due to changes in humidity and temperature.

    Eventually, you'll probably need to fix this problem on your entire roof. To really fix this issue, you'll need to strip all of the roofing off, install new OSB or plywood roof sheathing, some type of air barrier, and an adequate thickness of continuous rigid foam above the roof sheathing.

    If you want to do a better repair job than what you are proposing, you can rip out all of the rigid foam from above, exposing the ceiling boards, and insulate the rafter bays from above with closed-cell spray foam.

  8. user-6883899 | | #8

    Thanks. I had not thought of the spray foam approach. I will check into that. The rafters would still be exposed on the top and bottom. Would anything need to be done about that?

    Also, do you recommend DIY work on spraying closed cell foam? Another new area for me....

    I am finding your website very helpful and interesting. Thanks!

  9. GBA Editor
    Martin Holladay | | #9

    Q. "The rafters would still be exposed on the top and bottom."

    A. Could you describe your ceiling a little better? I can't quite visualize why the rafters are exposed on the bottom. Is your ceiling inserted between the rafters, just like the cut-and-cobbled rigid foam?

  10. user-6883899 | | #10

    I pulled out a bay of foam boards. The spaces are only 81 inches long. The material is Celotex Thermax, foil faced on both sides. There are two boards, 2 inches each.

    I'm thinking from a cost/benefit aspect, I should trim about two inches off of each board in both dimensions, leaving a 1 inch gap. Foam that gap all the way up the rafter with closed cell foam. That should be plenty of flex to take up any further movement in a 30 year old house. That should give me a complete air barrier. The top foam board will be replaced with 1 inch closed cell or foil faced, also, and I can tape over the rafters as a backup if any small cracks in the foam did occur. Then I go with the OSB and felt over that. For a small area fix, this seems like a reasonable compromise. For the main roof replacement, I will be able to do the full rigid foam cover and bring the roof up to code R value at that time. That will leave a small section with lower R value, but it is less than 140 square feet in total.

  11. Expert Member
    Dana Dorsett | | #11

    Cedar shingle roofs on skip sheathing are inherently ventilated to the exterior, and in most climates air leaks from the interior would just leak without leaving enough moisture in the skip sheathing to be a problem, but on the foggy-dew western foothills of the Cascades it's still not always adequate. (I know Enumclaw very well, having lived in lived in Black Diamond, then Buckley a few decades ago, with relatives still in Maple Valley.)

    Even so, it appears that the only moisture issues the skip sheathing appears to be around that (probably not adequately air sealed) vent, and at the bottom edge over the eaves, which implies that the inherent venting was "working mostly". The damage at the overhang was likely to be due to a combination of wind-driven leakage of the shingles combined with slower drying due to the colder temperature of the roof at the eaves where it isn't being heated from below, and scant winter/spring sun to warm it from above.

    Leaving the cut'n'cobble in place and putting R10 above the new roof deck still wouldn't quite meet IRC 2015 code-minimum, but R15 above the roof deck would meet code on a U-factor basis (not on an R-value basis) and would give you reasonable dew point margin. Using 3" of reclaimed roofing polyiso would be the cheapest (and greenest) way to get there:

  12. user-6883899 | | #12

    To Martin: I just meant that the rafters have no moisture seal at the bottom. The tongue and groove ceiling is continuous. Just wondering if moisture will still work up through the rafters themselves.
    To Dana: Thanks for the links and suggestions. I'm just not seeing how I can add 3 inches of thickness to this small section of roof without having to replace the two connecting 12/12 pitches, which I'm not quite ready to take on. I can see that the right way would be to add the rigid foam over the top, but I'm trying to find a reasonably efficient compromise which will stop the moisture problems in the future, even if not meeting full R value. The rest of the home has about 5000 to 6000 square feet of this roof construction. That's going to be my next challenge.

  13. user-6883899 | | #13

    Latest thoughts after pricing closed cell spray foam. I propose spraying 1 inch of closed cell foam onto the 15 pound felt that is on the top surface of the tongue and groove. (would it be better to remove the felt and spray the foam directly on the tongue and groove?)

    Then I will re-install the foil faced Thermax and tape the top surface across the rafters. Essentially, I'm going to be treating the tongue and groove as the roof deck, and installing all of my insulation above that. Then I'll install the metal roof over 30 pound felt. That will give me R27, (in the cavity, anyways...) I'll also have to be sure to seal every penetration. (drain vent, bathroom fan, skylight). This seems like it will address the issues for this small space. Martin, thanks for the idea of using closed cell to make the seal!

  14. GBA Editor
    Martin Holladay | | #14

    Leaving the asphalt felt in place and spraying above the asphalt felt is OK.

    You're going to end up with a sub-optimal roof assembly with below-code R-values, but it sounds like you are accepting this compromise with your eyes open. Good luck.

  15. user-6883899 | | #15

    Thanks again for the help. Luck will be appreciated if it comes with the rest of the job. I'm proceeding with the belief that this small section will be sub-optimal in energy efficiency, but fully durable from a moisture standpoint. If I have additional questions about other aspects of the job as I proceed, do you prefer a new thread, or adding on to this one?

  16. GBA Editor
    Martin Holladay | | #16

    You can add a new question to this thread at any time.

  17. user-6883899 | | #17

    Update for today....

    I removed shakes from a section of the 12/12 in order to reframe the gable end of the 4/12 section from a hip roof to a shed style. This area is over the second bathroom in the upstairs, so there was a lot of potential for moisture. The foam is pieced in and not tight to the rafters, but the roofing felt on top of the tongue and groove is in pretty good shape. There is some discoloration of the skip sheathing, but everything is completely solid and appears to be in excellent shape. Whew!

    A small sample, but in a high moisture part of the house, so I'm feeling a little better.

    Tomorrow I will post several pictures of that area.

  18. user-6883899 | | #18

    Here are pictures of the project today. I did not remove shakes all the way down to the eaves, so I don't know the condition over the uninsulated overhang. There does appear to be more moisture damage on the highest part of the shakes that I have removed. I'm debating whether I should pull shakes off all the way from the eave to the peak to get a full sample..... Not feeling up to it today. Any thoughts?

  19. user-6883899 | | #19

    The pictures need a little explanation:
    The top one is sideways, showing this section after the two part closed cell foam.
    The second one is correctly oriented, showing the small vertical wall section that I foamed.
    The third one is an overview of the opened roof section.
    The fourth one is a closeup of the middle of the opened section, before foam. No rot in this section.
    The fifth one is the top of the open section. Notice the rot in the highest skip sheathing piece.
    The sixth one is the lower part of the open section. No rot in this section.

  20. GBA Editor
    Martin Holladay | | #20

    Your latest photos provide another puzzle piece. Now we know the condensing surface for the escaping moisture.

    The original roofer installed strips of #15 apshalt felt, interwoven with the cedar shingles. It looks like a strip of asphalt felt was applied to the upper half of each course of shingles, as the shingles were installed. In winter, this asphalt felt was cold. This upper layer of asphalt felt was the condensing surface for the escaping moisture.

    The moisture was piggybacking on the exfiltrating interior air that was escaping through cracks in the cut-and-cobble foam, and that moisture condensed on the underside of the roofing felt. The moisture was trapped between the asphalt felt and the skip sheathing. The result: sheathing rot.

  21. user-6883899 | | #21

    Thanks for the reply. That makes sense. Are the higher boards seeing more damage simply because the warm air and moisture rise?
    My optimism about being able to lay a layer of foam that was 1 to 1.5 inches thick, allowing reinstalling the 4 inches of thermax was not realized.... My range is more like 1/2 inch to 3 inches.... So that presents a problem. I'm back to thinking I should cut loose closed cell or foil faced pieces of 1 inch and foam the edges. That way I can have a controlled thickness.
    In the two bays on the 12/12 that I foamed, I am thinking of putting fiberglass batt in and then the final layer of 2 inch thermax. Otherwise I would have to figure out a way to trim the whole foam layer to an even 1 inch. Not obvious how to do that.... The fiberglass would be 1/2 inch to 3 inches thick. I would foam the outer board edges once installed, also. Winging it....

  22. user-6883899 | | #22

    This does not look good.... When I cut the Thermax to refit it into the bays, this is what I found. No signs of active insects, but something made a meal of this material. Is this a common problem with isocyanate, or am I special?

  23. GBA Editor
    Martin Holladay | | #23

    Q. "Are the higher boards seeing more damage simply because the warm air and moisture rise?"

    A. Yes. Roofs with sheathing rot due to exfiltrating air always have more rot toward the ridge, and less rot toward the eaves.

    Your discovery of the difficulties inherent in controlling the thickness of the spray foam layers, and your improvisation with different types of insulation sandwiches, are somewhat worrisome. I think that you may be realizing why hiring an experienced contractor is sometimes less expensive than the DIY approach -- especially if you have to learn on the job.

    Good luck.

  24. GBA Editor
    Martin Holladay | | #24

    Q. "Is this a common problem with polyiso, or am I special?"

    A. The problem is especially common when the rigid foam is damp. Ants like high humidity, so the ant tunnels are a clue that your roof assembly has been damp. When foam is very dry, it is much less likely to have ant tunnels.

    For more information on this issue, see If Ants Like Rigid Foam, Should We Stop Using It?

  25. user-6883899 | | #25

    If I go out for quotes on the whole roof, I will be a much smarter customer, at least! I'll be bringing in some trained professionals to give me help on the ant problem.
    "If Ants like rigid foam...." was a difficult one to read. Having a shake roof, there is easy access for the ants. The foam was not placed tightly, so the edges are easily accessible. Looks like the builders of this house created a long term problem. And I bought it!

  26. user-6883899 | | #26

    I've closed up the 12/12 sections after the foam, batt, and board approach in the two bays that were opened up. I'll be starting the full inspection and repair on the 4/12 section next. Based on what I've seen in the 12/12, it is looking like that whole roof is going to have to be replaced, so I'm going to make the effort on this 4/12 section so that it won't have to be reworked, meaning I'll give up on the suboptimal approach and just do it... :)

    I'm going to pull all of the insulation out to inspect it for ants. I'll reuse what I can and replace the rest with reclaimed iso.
    I guess I should foil tape all of the edges for future ant prevention, right?
    I'll be foaming the boards in place by leaving 1/2 to 1 inch at the edges and filling those gaps.
    Once the bays are filled, I'll cover the whole roof with 3 inch, taped, with the outer perimeters foil taped for ants.
    This means I'll cut into the two adjoining 12/12 pitches to raise the valley flashing. I experimented with this and it wasn't as bad as I thought.
    Then comes 1/2 inch plywood (already have it...).

    Questions remaining:
    Over the eaves, where there is presently no insulation, should I cover with the 3 inch foam board there, also? It is a two foot overhang at the eaves and one foot on the gable end.
    Should the unheated eave spaces have vents?
    After all the work sealing and insulating, Is there any need for venting below the standing seam metal roof I'll be adding?

    Thanks again for all of the ideas and guidance.

  27. user-6883899 | | #27

    I removed the valley flashings and inspected all of the thermax I could reach. None of the pieces in the 4/12 section had any insect damage. One adjoining piece in the 12/12 above the 4/12 does have damage, however. The pattern seems to be that the pieces that had open access for insects due to incomplete framing or flashing are the only ones showing infestation. I guess that is as expected. I have one more section to open up for a skylight installation in the 12/12 roof. That will give me another data point.
    I reinstalled the foam boards, sealing them all in place with the closed cell foam. (more cutting, more cobbling...)
    The 2x6 tongue and groove on top of beams creates the structural roof deck, according to the old drawings. So I framed a perimeter of the insulated section, using 2x4s on edge, attaching them with structural screws every 24 inches and where they intersected rafters. I filled that space with 3.5 inch rigid foam (iso), sealing all edges and gaps with the closed cell foam. I will also tape the seams, and tape the perimeter.

    So now this section has 7.5 inches of iso rigid foam and 1 inch of styrofoam. R40, roughly?
    I'll add some pictures when it looks good...

    Next will come the 1/2 inch plywood, fastened through to the rafters with 6 inch structural screws.

    And then I can actually start what I thought I was going to do at the beginning of all this....

    Let me know what I'm doing wrong so I can fix it if possible, and have the right plan for the re-roof of the 12/12 sections.

  28. user-6883899 | | #28

    Here is the roof just before the 1/2 inch plywood... Well insulated and sealed up tight!

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |