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Thermal Bridging of I-Joists

user-980774 | Posted in Energy Efficiency and Durability on

Building living space over a garage (zone 5-Colorado).
Floor will be 12″ I-Joists (3/8 web) with blown cellulose (R-42).
Roof will be 12″ I-Joists with blown Cellulose (R-42) plus 2′ foam (R14).

Given that the delta T (garage to Living) is only about 40 degrees, do I need to worry about thermal bridging with R-42?

Second question: Have seen a couple double wall projects where the inspectors required 3/4 plywood fire stop every ten feet. Wouldn’t this diminish the advantage of double wall?

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  1. GBA Editor
    Martin Holladay | | #1

    richard,
    Q. "Given that the delta T (garage to Living) is only about 40 degrees, do I need to worry about thermal bridging with R-42?"

    A. I would put rigid foam between the garage ceiling's drywall and the joists. But you don't have to if you don't want to -- the payback on the foam layer is probably a long time.

    Q. "Have seen a couple double wall projects where the inspectors required 3/4 plywood fire stop every ten feet. Wouldn't this diminish the advantage of double wall?"

    A. Yes, but by an insignificant amount. The ratio of one sheet of 3/4 inch plywood in ten feet is less than 1%. That means that less than 1% of the wall has an R-value of R-15, and 99.4% of the wall has an R-value of R-45. Not a problem.

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