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Vented and unvented ceiling, Zip System, metal roofing, underlayment question

user-6651407 | Posted in Energy Efficiency and Durability on

I am seeking some advice, and have been researching for quite a long time, but can’t really seem to find the best answer for our situation.

Currently building New Construction in Climate Zone 5. Using ZIP System for the walls and roof.

In the center of the home, we have Unvented Cathedral Ceiling. We decided to use Unvented design, and insulate with Closed Cell Spray Foam.

On the two sides of the home, we have Vented Ceiling, with unconditioned attic space, insulating with fiberglass batts.

We plan on installing a metal roof.

My question is, in trying to avoid moisture issues, is it preferable to use asphalt felt as the underlayment over the entire roof? I am reading it is preferable to use over Unvented Ceiling. Not sure if this is ideal over the entire roof assembly. I initially was looking into Grace I&W, but am now reading that might not be the best idea. Especially if the Unvented Cathedral needs to dry to the exterior, correct?

Any advice would be greatly appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    Whether or not your metal roofing will allow outward drying depends on the type of roofing you install. Standing-seam metal roofing will not allow outward drying (unless you include a ventilation channel above the roof sheathing, and install the metal roofing on 1x4 or 2x4 purlins).

    Some old-fashioned types of corrugated metal roofing will allow outward drying.

    Assuming that you are installing metal roofing that doesn't allow outward drying, you could (in theory) install Grace Ice & Water Shield over the entire roof. I don't know why you'd want to, but you could. (I've heard some people argue that the Ice & Water Shield will protect the house in case a hurricane removes the roofing. That's true, but in your case the taped Zip sheathing already performs that function.)

    Grace Ice & Water Shield is expensive. If your budget allows for expensive details, I think including a ventilation gap between the roof sheathing and the metal roofing is a better use of your money than installing Ice & Water Shield.

    If your roofer is skilled, there is no reason to expect roof leaks. Ordinary asphalt felt will work fine. You can also use one of the new synthetic roofing underlayments if you prefer, as long as the manufacturer of the underlayment permits their product to be installed over an unvented roof. Some do, and some don't.

  2. user-6651407 | | #2

    Thank you very much.

  3. Expert Member
    Peter Engle | | #3

    I'm looking at this issue right now for a client. I know that some manufacturers of synthetic underlayment do not allow use over unvented roof assemblies, but I am not sure I understand why. With ccSPF and a metal roof, nothing is drying in any direction anyhow, so why would it make a difference? Or is it just CYA?

  4. GBA Editor
    Martin Holladay | | #4

    Some types of roofing (for example, concrete tile roofing, cedar shingle roofing, and slate) allow outward drying, and if you are installing one of those types of roofing, you probably wouldn't want to inhibit outward drying. But if you are installing a type of roofing that doesn't allow outward drying, the vapor permeance of the roofing underlayment is irrelevant.

    I assume that the reason that some manufacturers of roofing underlayment don't allow their products to be installed on unvented roofs is that they want to limit their liability, and can't think of a good way to explain the circumstances under which their product is inappropriate.

  5. Jon_R | | #5

    > With ccSPF and a metal roof, nothing is drying in any direction anyhow

    The ccSPF could be ~1 perm (providing some moisture movement) and a standing seam metal roof is not nearly as airtight as some people think it is. It takes minuscule amounts of air movement to dry < 1 perm of diffusion wetting.

    I think the safer option is permeable underlayment (felt or synthetic). Even more conservative would be to also add foam backer rod and upper and lower openings.

    IMO, purlins would be wasted expense - there just isn't enough wetting to justify that much air flow.

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