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Cathedral roof, unvented, rigid foam

airfix | Posted in Green Building Techniques on

Hey GBA,

I’ve been reading several articles on the merits of Vented/Unvented cathedral ceilings. I’m about to build an energy efficient home in a mountainous zone 6 with lots of potential snow load. I’m trying to get R-30 on the walls and R-60 on the roof. Our climate is quite dry and I assume most of the vapor pressure will be from inside to outside in winter.

As such I’m planning my walls with exterior rigid EPS foam and blown fiberglass or cellulose in the wall cavity. I think the plan is for this wall to dry to the outside. I was planning my only vapor retarder to be latex interior paint on the walls.

I’m confused about the drying of the roof. It sounds like the best approach for an unvented roof is to have exterior rigid foam over the roof sheathing at least R-25 per 2015 IRC Section R806.5. Then I’m confused about the insulation installed on the inside of the sheathing. I’ve seen recommendations that say you can’t go wrong spraying closed cell foam on the inside of the sheathing which in my mind means the sheathing can’t dry to the outside or the inside. However this is exactly what Section R806.5 recommends

However a article (RR-0108 – Figure 5) seems to recommend vapor permeable cavity insulation inside the rafter cavity and no vapor barrier.

Also is there a conflict when your walls dry to the outside but your roof dries to the inside?



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  1. GBA Editor
    Martin Holladay | | #1

    If your layer of exterior rigid foam is thick enough to be useful, it will be a vapor retarder or vapor barrier. You won't get any drying through the rigid foam.

    If we're talking about walls, it's important to note that a wall with exterior rigid foam doesn't dry in one direction. It dries in two directions. Everything on the interior side of the rigid foam -- the wall sheathing, the studs, the fluffy insulation, and the drywall -- dries toward the interior. Everything on the exterior side of the rigid foam -- the furring strips and the siding -- dries toward the exterior.

    In theory, the same drying paths may be true for a roof assembly, although in practice, most types of roofing prevent exterior drying. So exterior drying of a roof assembly only happens with some types of roofing (cedar shingles, slate, or concrete tile). But roof assemblies with rigid foam perform very well, even when the roofing isn't vapor-permeable.

    If you want to install a continuous layer of rigid foam above your roof sheathing, it's best if the insulation between your rafters is vapor-permeable. So on the interior side of the roof sheathing, choose cellulose, mineral wool, fiberglass, or open-cell spray foam.

    Here is a link to an article with more information: How to Install Rigid Foam On Top of Roof Sheathing.

  2. airfix | | #2


    Thanks for the response. I haven't done my vapor analysis yet but I was assuming with EPS having 2.0 to 5.8 perms (Class 3 vapor retarder territory) that with something between 2 and 4 inches of EPS on the walls I'd thought (before you corrected me) that I'd still get some drying through the foam.

    Great article you linked to. However there is a statement in the article that is part of my confusion and seems to contradict R806.5. In my case I plan to do part exterior rigid foam and part of the insulation between the rafters obviously in the right proportions to keep my sheathing above the dew point and shooting for an overall R-60.

    Here is the confusion: The linked article states:
    "If you follow Option 2, a wide variety of insulation materials can be installed under the roof sheathing. Among the possibilities: fiberglass batts, mineral wool, cellulose, or open-cell spray foam. (Closed-cell spray foam is not recommended for this purpose, since closed-cell spray foam prevents the roof sheathing from drying toward the interior if it ever gets damp. For the same reason, this type of roof assembly should never include an interior polyethylene vapor barrier.)"

    However R806.5 seems to allow air-impermeable insulation:
    "5.1.1. Where air-impermeable insulation is provided, it shall be applied in direct contact with the underside of the structural roof sheathing."


    "5.1.3 Where both air-impermeable and air-permeable insulation are provided, the air-permeable insulation shall be applied in direct contact with the underside of the structural rood sheathing in accordance with 5.1.1 and shall be in accordance with the R-Values in Table R806.5 for condensation control. The air-permeable insulation shall be installed directly under the air impermeable insulation."
    So the code seems to allow vapor-impermeable insulation in the place where your linked article says closed cell foam is not recommended. Does the code option of vapor-impermeable not just keep the sheathing wet with nowhere for the moisture to go, sandwiched between rigid foam on the exterior and close cell spray on the interior? How can the code allow this?
    I'm trying to get to the bottom of this because if I use close cell foam on the underside of my sheathing then that also helps with air sealing as I'm not sure how effective taping OSB sheathing will be. If I go with cellulose between the rafters then I guess I make my ceiling sheet rock the secondary air barrier (taped OSB being the primary) and it seems to me closed cell spray foam is a better air barrier than sheet rock which will have electrical boxes installed in it (but not canned lights obviously).
    Thanks for any clarify you can provide.


  3. airfix | | #3

    I just did a quick calc trying to get to R-60 in my roof to see what I needed for insulation to get my sheathing temperatures up. The average temperature for the 3 coldest months (Using Hayden CO data) is about 20F. Using 68F inside temperature with 6" of exterior rigid XPS and 8" of cellulose I get an R value of 63 and my sheathing temperature is 43F which should be right around the dew point of indoor air at 68F and 35% relative humidity.

    It seems 6" of exterior foam will be a pain to install. Maybe I need to target a lower R value. 4" exterior xps and 5"of cellulose gives R-40 and sheathing temps of 43F. R-40 doesn't seem enough for a roof though when I'm planning my walls to be R-30.

    What are your thoughts on those numbers?


  4. GBA Editor
    Martin Holladay | | #4

    Here is a link to another article to guide you: Combining Exterior Rigid Foam With Fluffy Insulation.

    If you are building an unvented roof assembly combining exterior rigid foam and fluffy insulation between the rafters in Climate Zone 6, at least 51% of the R-value of the roof assembly must come from the rigid foam layer.

    You're right that the code permits the use of closed-cell spray foam applied to the underside of the roof deck. If you go that route, you don't need any rigid foam above the roof sheathing. For more information on that approach, see Flash-and-Batt Insulation.

  5. Jon_R | | #5

    Any amount of external wall foam will reduce condensation at the sheathing (and thermal bridging). The question is if the reduction in permeability (drying) more than offsets the reduction in condensation (wetting). The answer to this depends on the foam used (eg, plain EPS vs foil faced), wetting rate (the interior side perms, air sealing, etc), climate, etc. The simple solution is to a) use the recommended R value of external foam or b) use something else to reduce thermal bridging (eg, interior rigid foam, external mineral wool or double studs).

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