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Community and Q&A

Conditioned Attic – Roof Construction in ICF Home

user-7022224 | Posted in Green Building Techniques on

Hi,

We are building an ICF home in northern Indiana, climate zone 5A. It will be about a 3k sq ft ranch with a 1k sq ft unfinished basement. Exterior walls will be made from Build Block, using 4″ concrete forms above grade with thicker forms below. The builder tells me that they are effectively R-50 walls.

It looks like building standards say our overhead insulation should be at least R-38, but we’ve been talking about R-50. The bulk of the house will have a single sloping shed roof, with about 1/3 of the roof on engineered rafters over a cathedral ceiling and the remainder over trusses. We plan to have a sealed, conditioned attic. The HVAC contractor is working on calculating air exchanges for the attic space, to be completed by the combination ERV and whole house dehumidifier. Any suggestions on this (attic air exchanges) are welcome.

To make things more interesting, the interior climate will be controlled using hydronic/radiant ceiling panels from Messana.

My big question is with regard to roof construction. I had envisioned some sort of insulation within the joist cavity, OSB sheathing above, rigid insulation with staggered seams, Carlisle WIP 300 HT over that and sticking down onto the exterior ICF, covered by diagonally placed furring strips and a standing seam metal roof. The roof itself will be vented with something similar to Cor-a-vent behind the fascia, between the furring strips, and above the ice and water shield, which I am hoping should also serve as an air and vapor barrier.

Any suggestions on this construction method? How much rigid insulation do we need over the roof decking? I’m thinking long lag screws to fasten the furring strips….

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    User 702etc.,
    First of all, can you tell us your name?

    When your builder tells you that your BuildBlock ICFs have an "effective R-value" of R-50, he is repeating a falsehood spread by the manufacturer. There is no legal definition for "effective R-value." There is, however, a legal definition for R-value. And in the case of BuildBlock ICFs, the R-value is R-21, as even the manufacturer admits. (See this web page.)

    You didn't really ask a question, but I'll give you some pointers.

    1. If you include a vent channel, the vent channel has to be on the exterior side of the rigid foam layer.

    2. Here are links to two articles you might want to read:

    How to Install Rigid Foam On Top of Roof Sheathing

    Combining Exterior Rigid Foam With Fluffy Insulation

  2. Jon R | | #2

    > The roof itself will be vented ... above the ice and water shield

    What is the expected result of this vent?

  3. user-7022224 | | #3

    I'm the OP - Dustin. I appreciate the linked articles Martin, very helpful. I'll take another shot at my questions:

    Assuming we use a product like Carlisle WIP 300 HT as part of a conditioned attic/vented over-roof construction and we are NOT placing sheathing over top of the rigid foam, why does it need to be applied directly to the OSB sheathing and not instead over the rigid foam?

    How many air exchanges per hour should we plan for in the conditioned attic space?

    To answer Jon's question, it's a "vented-over roof" construction, designed to prevent ice dams.

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Dustin,
    I assume that Carlisle WIP 300 HT is a peel-and-stick product. Before you decide to install it on rigid foam, make sure that the manufacturer (Carlisle) allows that type of installation.

    One reason to place your peel-and-stick directly on the OSB is to make sure that the OSB is airtight, so that you don't get air circulation in the spaces between rigid foam sheets. Joe Lstiburek calls these gaps between foam sheets "complex three-dimensional airflow networks." The label is a little overblown, but you can read about his experience here: Complex Three Dimensional Airflow Networks.

    Another way to create an air barrier at the OSB layer is by taping the OSB seams with a high-quality tape.

  5. user-7022224 | | #5

    Martin,

    I forgot I had already read that article from Lstiburek and again thank you for bringing it to my attention.

    -Dustin

  6. Jon R | | #6

    Don't know what size you are planning, but Lstiburek suggests 2" or more for ice dams in ice dam regions.

    https://buildingscience.com/documents/insights/bsi-046-dam-ice-dam

  7. user-7022224 | | #7

    Jon,

    Snow loads within our area are 20 psf. Worried enough about ice dams to vent the metal roof, but not super worried. Do you think Lstiburek would still say 2" - in the article he is worried about 30 psf or more? Roof pitch will be 4:12, standing seam.

    -Dustin

  8. User avatar GBA Editor
    Martin Holladay | | #8

    Dustin,
    If you are building a well-detailed (airtight) roof assembly in Indiana rated at R-50, I don't think you need to worry about ice dams.

    If the R-value comes from rigid foam above the roof sheathing, in conjunction with fluffy insulation under the roof sheathing, that means that you are detailing the roof as an unvented assembly. Make sure that you get the right ratio of rigid foam to fluffy insulation.

    If you want to include a 1 inch deep ventilation space under the roofing, or a 1.5 inch deep ventilation space, you are ending up with a roof that is much better than the average roof. So, no -- you don't need a 2 inch deep ventilation channel.

  9. Jon R | | #9

    > Do you think Lstiburek would still say 2"

    He says no vent in your case. Only you can say how far you want to take it past recommendations. Diagonal furring will create some reduction in airflow.

    While also optional, consider having the vent provide a little outward drying (via a breathable underlayment).

  10. user-7022224 | | #10

    So looking again at one of Lstiburek's figures for the vented over-roof assembly, I see that he has drawn both sheathing and a roofing membrane over top of the rigid foam (attached figure). I think that's were I originally came up with the idea of putting the WIP peel and stick over the foam. Well, that and Matt Risinger has some videos where he has constructed roofs in that manner.

    In the second figure, the membrane is attached to the OSB, just below the rigid foam.

    I'm not sure which makes more sense. I know the OSB will stay warm with the correct amount of rigid foam and I don't want it to get wet. I suppose it doesn't matter if some moisture gets into the foam. In some sense, using the WIP as an air and vapor barrier over the OSB (below the rigid foam) will keep any moisture from the attic that might otherwise scoot past the OSB from getting up into the foam (sorption), where it would be trapped if there was a peel and stick above the rigid foam instead.

    Also, if the peel and stick is on the OSB, under the rigid foam, and the foam does happen to get wet, it won't get to the OSB and the foam will be able to dry out towards the ventilation channels under the metal roof.

  11. User avatar GBA Editor
    Martin Holladay | | #11

    Dustin,
    The best location for the peel-and-stick membrane is directly on the lowest level of roof sheathing, under the rigid foam. That way it creates an air barrier that prevents warm interior air from flowing through the cracks between the foam sheets.

    Above the rigid foam, you'll need either a continuous layer of OSB or plywood sheathing, or purlins (usually 1x4s or 2x4s, 24 inches on center). Before you decide to use purlins, make sure that the manufacturer of your metal roofing approves of that approach -- some manufacturers insist on solid sheathing.

    Between the rigid foam and the roofing, you'll need roofing underlayment. Roofing underlayment is a code requirement.

  12. user-7022224 | | #12

    Update - In discussion with the architect, he is questioning the insulation below the sheathing, between TJIs in a large area of cathedral ceiling. He is saying code will require an air barrier on the undersurface in an unvented roof.

    Since we are insulating above the sheathing as well, and planning an air/weather/ice shield on the OSB roof deck sheathing, I don't see the rationale for an air barrier on the underside, and in fact I am concerned about moisture being trapped at the OSB if we do this.

    I would rather fill the joist spaces with "fluffy stuff," but need to make sure the inspector will agree this meets code or is not a problem.

    Thanks,
    Dustin

  13. User avatar GBA Editor
    Martin Holladay | | #13

    Dustin,
    If you want to combine rigid foam on the exterior side of the roof sheathing with fluffy insulation below the roof sheathing, installing two air barriers is prudent (but not code-required). These air barriers belong:

    1. At the sheathing layer (the lowest layer of sheathing). This is typically accomplished by taping the sheathing seams or in some cases by installing a peel-and-stick product above the roof sheathing. If you are installing Grace Ice & Water Shield or a similar product above the roof sheathing, you are all set there.

    2. Below the fluffy insulation, at the ceiling level. The usual air barrier at this location is drywall.

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