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Community and Q&A

Unvented vaulted roof/ceiling

timjwilson | Posted in General Questions on

I’m retired and disabled and therefore have a tight budget to build an addition onto my
house, a small apartment to accommodate a caregiver for myself. Some friends are
building it for me and I hired a local guy. We built a stone and cement
foundation with forms and used 3×8 fir for joists, insulated in between using
chicken wire to hold up fiberglass bats and vapor barrier.

The floor is flush 2×6 t&g fir and I had some 8×8 larch timbers I’ve had for 15 years which
were cut from ancient fire killed snags. These are the uprights at 3 and 6 foot
centers (6 foot for windows).

We used 4×8 fir for a plate and 4×8 fir for roof rafters on 3 foot centers. The span is 14 feet, For both roof deck and ceiling we used 2×6 t&g spruce covered with 30 pound building paper (felt). It is a simple shed roof and I have duraclad 36″ (5 ribs) metal roofing to apply. 2×4 strapping has been attached and I have styrofoam insulation to fit snuggly between them and then on goes the roofing. The square footage is around 350.

My question is related to venting the roof. I have read that it is best to completely seal this configuration of vaulted roof rather than attempt to vent it. If we are going to seal it, should we spray foam insulation around the edges so as to seal the ribs and all in the metal roofing? Or are the ribs alone enough to be considered ventalation?

We do have an option to place a solid sheet of 6 mil vapor barrier over the felt and strapping prior to the styrofoam and metal. I’m not so sure it would be a good idea to seal the strapping like this. Alternatively we could put the vapor barrier only between the strapping over the felt.

The ceiling will be coated with verathane.


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  1. GBA Editor
    Martin Holladay | | #1

    There are several issues you need to address:

    1. You need a good air barrier for your roof assembly. The tongue-and groove roof sheathing (which forms your ceiling) is not an air barrier.

    2. You need a layer of roof insulation with an adequate R-value.

    3. You need purlins or strapping above your rigid foam so that you have something to attach your metal roofing to.

    To address air leaks, watch out for the intersection where the top of your wall meets your roof sheathing boards. If the roof sheathing extends to the exterior past the walls, there can be air leaks between each board. These leaks are tough to seal.

    To create an air barrier, you can either use a European membrane with taped seams from Four Seven Five or Small Planet Workshop, or you can try to seal the seams between your pieces of rigid foam so that the rigid foam is your air barrier. The easiest type of rigid foam to tape is foil-faced polyisocyanurate.

    Your rigid foam should be installed in an uninterrupted layer. You don't want to interrupt the foam with 2x4 purlins; the 2x4 purlins go on top of the last layer of foam.

    You'll probably want at least two layers of rigid foam with staggered seams. If you are aiming for R-38, you'll want about 6 inches of polyiso.

    Once you have installed all of the layers of rigid foam, and taped at least one layer -- taping every layer is even better -- you can install 2x4 purlins on top of the foam. The purlins are attached to the decking or rafters with long screws.

  2. timjwilson | | #2

    If you had read my description and questions adequately I think you might have answered.
    "3. You need purlins or strapping above your rigid foam so that you have something to attach your metal roofing to."
    as stated;
    2x4 strapping has been attached and I have styrofoam insulation to fit snuggly between them and then on goes the roofing.

    The one bit of good info you gave is to seal the poly boards so thank you for that unless using the 6 mil plastic is more efficient.

    If anyone could answer the questions I actually asked, I will be apreciative.

    BTW, I live in the Pacific Northwest where even the 2x6 t&tg on its own will suffice as insulation. I moved here from the mountains of British Columbia which gets a little colder than Vermont. I have seen quite a large number of old log homes and post & beam where 2x6 t&g serves as roof decking as well as ceiling.

    My question is in relation to how best to seal the roof as it is or will be OR can I presume that the ribs in the metal roofing if left unimpeded might provide sufficient venting?

  3. GBA Editor
    Martin Holladay | | #3

    It's too bad that the roof strapping is already installed, because it would have been better to have a continuous layer of rigid foam below the roof strapping. I'm sorry if I misread that part of your question.

    If you want to install rigid foam between the strapping -- a method called "cut and cobble" -- you can, although it's not the best approach. For one thing, it sounds like your rigid foam will be only 1.5 inch thick, which isn't much R-value. For another, the strapping interrupts the insulation, introducing air leaks and thermal bridging.

    If you do take the cut-and-cobble approach, I strongly advise you to install at least one more layer of rigid foam -- a continuous layer of foam, with no strapping -- above the first cut-and-cobble layer. Then you can install more strapping above the continuous layer of foam.

    You can use a layer of polyethylene if you want, but it is hard to make polyethylene airtight.

  4. timjwilson | | #4

    I have some constraining height issues which preclude adding anything more than what is there. It is associated with the interface with the existent building's roof. As stated, I am not concerned with the R-factor. I have lived comfortably in a tent with a stove at 30 below zero.

    If you can try to understand, my questions are only related to the venting or not venting the roof and whether 1/ We should use sprayed foam to seal up the ends of the ribs in the metal roofing OR are they sufficient to actually vent the roof (14 feet)
    2/ Should we put down 6 mil plastic over the 30 lb felt prior to the styro board and if so would it hurt to cover strapping and all or should it be cut to fit between each run of strapping?

  5. GBA Editor
    Martin Holladay | | #5

    You definitely don't want to install any spray foam to seal the ribs at the perimeter of the metal roofing, because the spray foam could interfere with the drainage of condensation that sometimes develops on the underside of the roofing, and would make replacement of the roofing difficult when the roofing finally rusts.

    If you are choosing to insulate with rigid foam, there is no need for a ventilation channel above your insulation. You are correct that some limited ventilation occurs through the ribs of the roofing.

    Your plan still requires attention to airtightness. Because the seams between the rigid foam and the 2x4 strapping are subject to the expansion and contraction of the 2x4s with humidity and temperature changes, it's hard to make those seams airtight. I strongly suggest that you install a European air barrier membrane under your rigid foam. These membranes are designed to be taped. (You can't tape asphalt felt.)

    If your budget is too tight for a European membrane, you can try to use polyethylene as an air barrier. There are several problems: making the seams airtight; making the fastener holes airtight; and avoiding damage during installation. The traditional method for installing polyethylene as an air barrier is to use Tremco acoustical sealant at all seams. Seams have to overlap over a framing member or solid sheathing.

    TuTuf is a brand of polyethylene that is reinforced and is less likely to be damaged during installation.

  6. timjwilson | | #6

    Thank you. That is very helpful. I think what we will do is use the 6 mill as a solid piece with no seams except at the perimeter of the roof.

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