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Community and Q&A

Insulation delirium

adavid | Posted in GBA Pro Help on

I’ve been searching and searching and searching some more on the topic of insulation and have managed to find myself in a state of mental nausea trying to sort out what I think is best in my situation; so I finally decided to ask you all.

I am soon to begin construction on a cabin in northern Wisconsin. It is a 1 1/2 story 20’x32′ with cathedral ceilings and a loft above about 1/3 of the back of the building. The loft also has two shed dormers on both sides for extra head room.

I have chosen to go with 2×12 rafters with a ridge beam and am thinking about going unvented.

The usage of the cabin is intended to be 4 season but we will not be heating in the winter unless we are actually staying there so the rest of the time it will be frozen. Heating is going to be primarily a wood burning stove with baseboard heating as backup.

What I _was_ thinking about was a DIY 2-part closed cell foam on the underside of the roof decking followed up by blown cellulose as I read was a possibility in the “Insulate Cathedral Ceilings” article on this site. However, after reading about spray foam shrinking back and exposing the underside of the decking I’ve become terrified of thought of having to replace the roof if this happens as we are springing the extra $$ for a standing seam roof.

Currently I’m wondering if I can fill this space instead with rigid foam followed up by cellulose or if this won’t actually solve anything?

I suppose if someone hear really twists my arm I could be convinced to do the insulation above the decking, but I’m still concerned about the amount of work / cost of doing that will be especially as this is in Zone 6 and requires apparently R25, therefor making my eaves look really strange.

At this point my head is spinning worrying that every option I have is a stupid one..or even that its not worth it for a cabin that will only be heated several times a year in the fall / winter.

Help me come back to reality….

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  1. Expert Member
    Dana Dorsett | | #1

    Rigid foam can be the solution, but put it at least some of it above the roof deck, say 2" / R12 of roofing polyiso, which shouldn't look too weird at the eaves, but mind you 4.25" would make you fully code-legal, and isn't a tough build, and you could drop back to 2x8s on the rafters and still hit the R49 code-minimum. With a facia board covering the end of the foam at the soffit it would't look wierd at all. With the R exterior to the sheathing it would outperform R49 between I-joists since there would be an R25 thermal break over the rafters. I've seen "chainsaw retrofit" eave overhangs done with as much as 6" of exterior roof foam- it's possible to make the exterior appearance as conventional as you like.

    In a cut'n'cobble approach between rafters you can put up to 4" of unfaced EPS (R16) in there without making it too vapor tight, but you'd have to meticulously air-seal around the edges in each rafter bay with spray foam- it could take 2-4 of the 12 board foot FrothPak kits available at box stores.

    With either approach you can save a lot on material cost using reclaimed roofing foam. I have multiple vendors selling the stuff at 20-40% the price of virgin stock. will ship if you have a large enough order, but the shipping isn't cheap if they have to go very far. Check the local craigslist- or call some commercial roofers- the used stuff is around, if you dig for it, and it's usually dirt-cheap when you find it.

    Virgin stock roofing iso or EPS runs about 10cents per R per square foot, so at R25 you're looking about $2.50 per square foot, plus scrap rate (plus sealing foam if you take the less reliable cut'n'cobble between rafters approach.) Using reclaimed goods it's under a buck a square foot. Mounting the standing seam on purlins 24" o.c. through-screwed to the rafters with timber screws saves you some of the slip-surface issues and lowers the ice-damming potential to boot, but it takes longer screws than a secondary nailer-deck would.

  2. GBA Editor
    Martin Holladay | | #2

    You wrote, "I suppose if someone here really twists my arm I could be convinced to do the insulation above the decking." Well, it is evidently necessary for someone to twist your arm. I hope it happens soon, before construction begins.

    That's where your insulation belongs -- above your roof sheathing.

  3. adavid | | #3

    Ok..I hear what you are both saying regarding putting the insulation above the deck. Please allow me to ask a few follow up questions.

    If I were to put my insulation above the roof deck. For me it seems easiest to construct a roof 'sandwich' if you will. Like: so I use two layers of decking.

    Do I then skip putting any insulation in the rafter bays? Should I be covering the rafters at all (e.g. drywall?). If not shouldn't I just drop down to 2x10 or 2x8 rafters to try and recoup some of my cost?

    If I don't need a covering or insulation for the rafter bays _and_ I should use the sandwich approach; is it then a stupid idea to use pine bead board as the sheathing directly in contact with the rafters?

    The reason I am asking is I would love the rustic look of the exposed board but I don't want to also be shooting my roof in the foot, so to speak. We originally planned on installing a T&G pine as the final ceiling covering.

    Lastly and this throws your entire rigid foam idea out the window..what about simply doing this: sheathing the roof as usual and fill the 2x12 rafters full??

    Thanks again I hope this delirium is at least understandable.


  4. GBA Editor
    Martin Holladay | | #4

    I suggest that you read this article: How to Build an Insulated Cathedral Ceiling.

    If you decide to install rigid foam insulation above your roof sheathing, you have two choices. Either all of the insulation can be installed above the roof sheathing, or only some (with the rest of the insulation installed between the rafters). All of the details governing these two choices are spelled out in my article.

    Pine beadboard without gypsum wallboard is not recommended as a finish ceiling if you plan to install an air-permeable insulation between your rafters. If you seal your roof sheathing carefully, and install all of your insulation above your roof sheathing, then you can just put up the beadboard. But if you put fiberglass or cellulose between the rafters, you had better install gypsum wallboard under the insulation as an air barrier.

  5. adavid | | #5

    Again thank you for the help. I did read your article, several times actually. Originally I thought this was going to be fairly simple after reading it. Spray two inches of closed cell foam then fill the rafters with more insulation, but after reading about spray foam shrinking away and exposing the roof deck I started to get concerned it was going to shake out as a huge waste of money.

    I guess I'm considering just doing 6" of rigid poly-iso, but finding a supplier up North has me worried. Shipping that much poly-iso can't be cheap.


  6. adavid | | #6

    Ok. Still sorting this out. I put together a spreadsheet comparing cost, r-value and materials. I can say for sure that it doesn't make sense for me to do 6" of rigid foam based on cost and r-value per dollar.

    So what I think I ultimately am going to go with is this setup for the roof. Either 2" of XPS or Polyiso on top of the roof deck and under the deck I am going to use Roxul Comfortbatt insulation. In either of these setups I can get an R40 or R43, respectively and at almost half the cost of doing it with 6" of polyiso. This option also leaves me the option of either decreasing the size of my rafters or having still enough room to add more Comfortbatt at 3.5" thickness for a total of R55 and R58.

    So the question I have now is in terms of layering and if there are any problems with this approach.

    From the outside not including the roofing material itself:

    2" of either XPS or Polyiso
    (does some other material go here)?
    5/8" Roof Decking
    7.5" of Roxul
    (airspace) - Can I put paneling here instead?

    Thanks again,

  7. GBA Editor
    Martin Holladay | | #7

    You are in northern Wisconsin (climate zone 7). If you want to build an insulated unvented roof assembly using a combination of rigid foam above the sheathing and air-permeable insulation below the sheathing, you have to follow certain rules designed to prevent moisture accumulation. These rules have been adopted by most building codes.

    In your climate, the minimum R-value of the foam layer is R-30. If you use XPS, that's 6 inches. If you use polyiso, that's 5 inches. That is the minimum thickness of the rigid foam layer.

    All of this, and more, is explained in this article: How to Build an Insulated Cathedral Ceiling.

  8. Expert Member
    Dana Dorsett | | #8

    Martin has it right- to meet the letter of the code you'd need at least R30 above the roof deck, if you have fiber insulation in contact with the underside of the roof deck. The only way you can cheat that is by adding closed cell spray foam on the interior side, as well as the rigid above the roof deck.

    Even without the exterior foam a couple inches of closed cell on the underside of the roof deck would be sufficiently protective, provided it was very air-tight between the fiber insulation and the roof deck. Otherwise, if you sandwich the roof deck between exterior rigid and interior spray foam it has to add up to R30. But that would also present the issue of creating a moisture trap at the roof deck, if there's more than 2" of closed cell on the interior.

    When all is said and done, you're almost always better off putting the 5" of polyiso above the roof deck, which keeps the roof deck warmer (= drier), and gives you great resilience against the incidental air leaks from the interior side (which you should still try to minimize, or avoid completely.)

  9. adavid | | #9

    Thank you both again. I hope I'm not coming off as flippant by asking more questions I am just trying to get it right without spending way over my current budget.

    If you could please help me understand how this build would change if I were to instead add the 2x4 ventilation above the roof as described in the "How to Build an Insulated Cathedral Ceiling" by Martin.

    Is there still an R30 minimum for the roof in this setup?

    Should the rigid insulation be above or below the 2x4 furring?

    Since this is a metal roof this shouldn't require more decking right?

    Lastly with the venting above is it acceptable or not to put Mineral Wool insulation between the rafters?

    Thanks again,

  10. GBA Editor
    Martin Holladay | | #10

    If you install vent channels above the roof sheathing, you are no longer building an unvented assembly. Instead, you are building a vented assembly.

    There are many more choices of insulation if you are building a vented assembly. If you have ventilation channels, you can use 100% air-permeable insulation (fiberglass, cellulose, or mineral wool) if you want. Or you can use any combination of rigid foam, spray foam, and air-permeable insulation.

    Most building codes call for a minimum of R-49 ceiling insulation in your climate zone. That's a good goal to aim for -- unless you can figure out how to install even more.

  11. wjrobinson | | #11

    Moist interior air trapped in a cold part of your roof before exiting your roof is bad and is why vented and or open vent channels exist. So exterior foam combined with interior permeable insulation sets up the need to have enough foam to keep the interior unvented insulation dry without venting it. Venting between layers of insulation isn't to be done since the outer insulation then is no longer separating the inside from the outside as well.

  12. adavid | | #12

    If I'm reading you correctly, from outside to inside:
    Metal roofing
    2x4 Furring
    2" of Polyiso or XPS
    15# felt
    5/8" decking
    11" of Roxul

    Do I need a vapor barrier or gypsum board under the roxul? Is this still a terrible situation for T&G boards for a finished ceiling? With this setup and just by adding furring strips on top of the insulation (not in between layers) I can still hit R-55 for XPS and R-58 for Polyiso.

  13. GBA Editor
    Martin Holladay | | #13

    Sorry, that still won't work. The ventilation channel has to be above the plywood or OSB sheathing. In this type of assembly, the point of the ventilation channel is to help the plywood dry out.

    Your assembly will work, too -- but only if the foam layer is R-30.

    If you put a vent channel on top of the plywood or OSB, you can put anything you want under the plywood or OSB.

  14. adavid | | #14

    But no matter what I need R-30 for rigid foam on the exterior, even if I put the 2x4 furring directly on top of the decking and the rigid foam on top of that is that correct?

  15. GBA Editor
    Martin Holladay | | #15

    You never want to have a vent channel between two layers of insulation, because the cold air in the vent channel defeats the purpose of the insulation.

    If you decide that you want to install a vent channel above your roof sheathing, then all of your insulation has to go below your roof sheathing.

  16. adavid | | #16

    Got it.
    Q1. Do I still put #15 or similar on top of the decking between the decking and the furring or is this now going to prevent the plywood from drying?

    Q2. Are there any problems putting in a full 11" of Roxul and a continuous 1-2" sheet of Rigid foam beneath on the ceiling side? This would get me to an R-Value of 55 again.

    Q3. If Q2 makes sense what rules apply after that regarding drywall or vapor barrier, etc..?

  17. GBA Editor
    Martin Holladay | | #17

    Q. "Do I still put #15 or similar on top of the decking between the decking and the furring?"

    A. Yes. Asphalt felt is a "smart" vapor retarder that allows some drying to the exterior.

    Q. "Are there any problems putting in a full 11 inches of Roxul and a continuous 1-2" sheet of rigid foam beneath on the ceiling side? This would get me to an R-Value of 55 again."

    A. No problems.

    Q. "What rules apply after that regarding drywall or vapor barrier, etc..?"

    A. The usual practice in Vermont is to install 1x3 or 1x4 strapping, 16 inches o.c., under the rigid foam. You screw the strapping through the foam to the rafters, and the strapping holds the foam in place. Be sure to tape the rigid foam seams; foil-faced polyiso is the easiest type of foam to tape. Seal all penetrations to prevent air leakage. Then screw your drywall to the strapping. Don't install any interior polyethylene.

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