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Community and Q&A

Interior foam insulation in Climate Zone 4 (marine)

seattlej | Posted in Energy Efficiency and Durability on

We are currently renovating our house and wanted to get some help on adding interior insulation. Our house is located in Seattle and is a single story rambler with 2′ eaves.

It has has brick facade, drainage gap, 2×4 framed walls, and no current insulation. The great room that we are currently renovating has a vented cathedral ceiling where the rafters start out with a 5″ depth and increases to 23″ depth as it gets closer to the ridge.

For the exterior walls:

brick facade > drainage gap > 2×4 framed walls > 3.5″ roxul batt > 1″ rigid foam board > 5/8″ drywall


a) fiber glass batts between rafter bays > 1″ continuous rigid foam board > 1×3 strapping perpendicular to joists > 3/4″ rigid foam in between strapping > drywall

b) fiber glass batts between rafter bays > 2×2 strapping perpendicular to joists > 1.5″ rigid foam between strapping > drywall

Would these insulation plans work? If I seal the seams with sicrall tape would the rigid foam provide enough of a vapor retarder or do I need to use an interior wall membrane like majrex?

Thanks for your help. Any advice is greatly appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

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

    You haven't provided many details about your ceiling insulation plans. In your climate zone, it's a good idea to aim for R-49 ceiling insulation. But you haven't told us the R-value of of the fiberglass batts you intend to install between your rafters.

    If you have only 5 inches of depth at your rafters near the eaves, you're in a pickle -- especially if you intend to have a vented roof assembly (which would be required if you use fiberglass batts). After allowing 1.5 inch for the ventilation channel, you have only 3.5 inches for fiberglass insulation, which is about R-13 if you are lucky. Adding 1.75 inch of interior rigid foam brings you up to a total of R-21 or R-22 -- not much at all.

    One possibility is to create an unvented roof assembly by installing 5 inches of closed-cell spray foam on the underside of your roof sheathing. That would give you about R-30 or R-32 of insulation -- still less R-value than you need, but better than what you are proposing. You could supplement the spray foam with additional fluffy insulation (using the flash-and-batt method) in the areas of your roof where the rafter depth is deeper.

    Your plan for your walls will work. For more information on your wall plan, see this article: Walls With Interior Rigid Foam.

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