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Community and Q&A

Clarification on unvented cathedral ceilling

Maiter | Posted in Energy Efficiency and Durability on

Hello all, this is Annie. I needed some help with my roof assembly. I have tried to put this in the comment section under Mr. Martin Holladay’s article about unvented cathedral ceilling but the system blocked my post. So if it cleared out and anyone has seen repeated questions I posted, I apologize here first.

I am in central Alabama, Climate Zone 3, and in the middle of building a tiny house on wheels. My roof is a very low slope almost flat shed roof (1/2 to 12 pitch) and I have decided to finish it unvented.
Here are the layers of the assembly and I want to know if it’s correct. From top to bottom:
1: Exposed fasterner metal roof PBR panels rated for 1/2 to 12 pitch and above
2: cor-a-vent rainscreen strips
3: Mento 1000 membrane as underlayment
4: 1 inch polyiso and 0.5 inch polyiso staggered with seams taped with tyvek tape
5: layer of tyvek housewrape as air barrier
6: roof deck of 5/8″ plywood with seams taped with zip system tape
7: trusses filled with roxul mineral wool
8: painted 0.5 inch plywood as interior finish
PS: I have confirmed with my metal roofing manufacturer and they said it’s ok to attach the panels directly through the foam to the roof deck with no need to add another layer of plywood or purlins.

Q1: is this assembly correct? Any layers I need to add or not needed?
Q2: do I need to use latex paint on the interior finish plywood? Or should I use tongue and groove panels? Does the interior finish also need to include a vapor open air barrier?
Q3: the metal roof company suggested 3″ screws and I also purchased some 4″ ones to accommodate rib height on panel overlap seams. All the metal roof panels should have about 1″ to bite into roof deck but not the trusses, is this sufficient?
Q4: I planed to put 1.5 inch total thickness of polyiso on top of the roof deck. Current plan is to have two layers, first half inch and second one inch staggered. Is it better to have more layers, for example, three staggered layers of half inch polyiso foam boards?

Appreciate any information and feedback! Thank you.

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  1. Maiter | | #1

    I am sorry guys, I have a typo in the question description. I'm in zone 3, central Alabama.

  2. GBA Editor
    Martin Holladay | | #2

    One problem with a house on wheels is that where you end up parking the house matters more than where you build it. Will this house stay in Alabama, or will it ever be towed further north?

    Assuming that the house stays in Climate Zone 3 (or anywhere warmer), the stack-up will work.

    The Tyvek housewrap is unnecessary, because the taped plywood is your air barrier. (Not only that, but your polyiso and Solitex Mento layers sound like they will be air barriers, too.)

    I'm not sure how you plan to treat the seams of your plywood ceiling, but your ceiling won't be airtight. Considering all the other air barriers in your roof assembly, you can probably get away with air leaks between the plywood ceiling panels. Tongue-and-groove plywood would probably be preferable to square-edged plywood (as long as you can persuade the panels into place).

    Two layers of polyiso totalling 1.5 inches is enough, as long as you don't tow your trailer further north.

  3. Maiter | | #3

    Thank you Mr. Holladay for your quick response. The tiny house is parked on my family land and I intend on having it only parked within the warmer climate areas so I followed code recommended for zone 3 and warmer.
    As for the ceiling airtightness, would it be better then if I put the layer of tyvek on the bottom of the trusses inside and then cover it up with tongue and groove panels? Or maybe a layer of smart vapor control layer is better? If I do so, do I need to have special tapes where nails or screws penetrate? My original plan with ceilling is to fill all the seams with wood putty and paint it to create that dry wall look.

  4. GBA Editor
    Martin Holladay | | #4

    If you want the "drywall look," why not install drywall?

    My guess is that the best way to achieve the "drywall look" with plywood would be (a) to tape the plywood seams with paper drywall tape, and (b) to skim-coat the plywood with drywall compound. I've done this with small pieces of 1/2-inch plywood (patched into a drywall wall when I needed blocking to mount something), and I've never had a tape failure with the technique.

    That said, drywall panels would make the job even simpler.

    Wood putty won't work -- you'll see shrinkage lines.

  5. Maiter | | #5

    Thank you Mr. Holladay, I just learned a new technique! :)
    I don't want to use dry wall simply because they are heavy and potential cracking in any kind of transportation of the tiny house. Most tiny house I saw or visited have choosen ship lap or tongue and groove boards to finish interior walls and ceilling but I just simply don't like that look. Back to the question again, should I include some kind of layer in between the bottom of trusses and finish panels for better air sealing?

  6. ethan_TFGStudio | | #6

    Won't you have fire concerns with interior plywood panels?

  7. GBA Editor
    Martin Holladay | | #7

    If you tape the plywood seams with drywall tape, you will have an interior air barrier. You wouldn't need an interior membrane.

    And I assume that your mineral wool batts don't need an interior thermal barrier. I don't think that the plywood is a fire hazard -- after all, tongue-and-groove boards are a commonly used interior finish material -- but you should check with your local code authority if you have any doubts.

  8. Expert Member

    Under our code interior surfaces aren't required to be fire-rated, but they must have a flame-spread rating under a certain limit. Both plywood and 3/4" solid wood meet that criterion.

  9. Maiter | | #9

    Thank you Mr. Holladay for your informative answer and advice. I will go with the dry wall tape and I'm sure it'll look real nice with some paint. Once again I appreciate your time and effort.

    Hey Ethan, thank you for you concerning about the safety with plywood. I believe both Mr. Holladay and Mr. Taylor have answered this question very well. Also, I will make sure to equip proper monitors :)

    Thank you Mr. Taylor for explaining the code for us. I was not very clear about code requirements on interior finish materials, I just know and have seen wood as a common interior finish choice and made my decision. Thank you again for the explanation.

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