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Roof insulation question

ethanlacy | Posted in Energy Efficiency and Durability on

So we are remodeling an old farmhouse in northern vermont. The upstairs was gutted 8 years ago and insulated (5″ rafters) with fiberglass batts before being re-sheetrocked. It’s a typical 1.5 story farmhouse with low kneewalls in the upstairs bedrooms. There is a small attic, not more than 3-4′ at the highest point. They put some loose-fill in there, and it’s vented.

We are going to replace with roof with a new standing seam metal roof. Seems like a good time to upgrade the R value of the roof. Initial thought was to strip it to the old sheathing boards, tar paper, two layers of 2″ XPS foam, furring strips, ply, ice and water, metal. Then maybe seal the attic space, making it essentially an unvented roof.

Thinking about how to simplify this, and reduce labor, I’m looking at SIPS, or “nailbase” like a SIP but with plywood on one side only. One question is, how critical is it to vent the sheathing here on the top side of the nailbase? If we use EPS SIPS, I know the foam is a bit permeable.

I realize I should add maybe a bit more thickness to the foam in this climate but I’m hesitant for cost and aesthetic reasons.

Anyway, any thoughts on these approaches would be welcome.

Also can anyone recommend a supplier/installer of SIPS/nailbase in northern new england?


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  1. user-2310254 | | #1

    Ethan. Northern Vermont is Zone 6, correct? Code would be R-49 total. If you combined rigid foam and air permeable foam, you would need R-25 (51 percent) on the outside with the balance on the interior. So does the small attic extend all the way across the second floor, or is that just the areas behind the knee walls?

    If cost is a concern, have you considered using reclaimed rigid foam?

  2. GBA Editor
    Martin Holladay | | #2

    Steve gave you good advice. To avoid moisture problems, you need to be sure that the rigid foam is thick enough. Here are links to two articles that will guide you:

    How to Install Rigid Foam On Top of Roof Sheathing

    Combining Exterior Rigid Foam With Fluffy Insulation

    Some brands of nailbase include built-in ventilation channels under the OSB facing. This ventilation channel isn't strictly necessary, but it may reduce the chance of ice dams.

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