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What suggestions do people have to solve a R-38 roof problem?

highcover | Posted in Building Code Questions on

We are building a single car garage over a studio apartment in the western North Carolina mountains, Zone 5A. The garage level will be accessed by an 18′ bridge. The project is being engineered to address the structural issues of both the bridge and a second level slab to support the car. At this point it appears that we will be building the lower level with Logix ICFs of varying dimensions up to 12″ and the second level slab will be supported by 10″ Insul-Deck forms. The slab will be 4″ thick.

The code problem comes in because the garage level is intended to uninsulated, bringing the code official to define the structure between the two floors as “roof”, required to be insulated to R-38. The Insul-Deck at the size we need is rated at R-25. Allowing R-0.7 per inch for the concrete, we arrive at almost R-28.

We have come up with several options for addressing this shortfall including (1) beefing up the Insul-Deck forms to the maximum 12-1/2″ for R-33 (according to the manufacturer). This seems likely to be an expensive approach in an already expensive project. (2) We have considered adding layers of blue board to the ceiling of the lower level, but that complicates installation of wall board and ceiling fixtures and lessens our desired room height. (3) The possibility of insulating the garage just to code using the least costly approach possible (which would be. . . ?) is another idea on the table.

How would you compare the options we have identified? Are there other ways we might address this issue? Are there “official” sources which might quantify the R-value of a properly constructed stick-built but uninsulated structure? Wouldn’t having an enclosed envelope over the “roof” have a positive impact on its effective R-value? Any advice, insights, or comments would be welcome.

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  1. GBA Editor
    Martin Holladay | | #1

    Increase the wall height (raise the ceiling) of the lower story to allow room for the needed ceiling insulation.

    Be careful of thermal bridging at the interface between the first-floor walls and the garage slab. Make sure that there is no concrete-to-concrete bridge leading from indoor conditioned space to outdoors, without a good layer of insulation.

  2. charlie_sullivan | | #2

    An uninsulated shell would be about R-1 to R-3, depending on details that aren't important because that's not really enough to help much. If Martin's suggestion is difficult for some reason, adding cellulose insulation to the garage walls and ceiling should be easy and cheap. You could get R13 nominal in 2x4 walls, and R as high as you want in the ceiling. Whether your code official would be willing to accept that is unclear, but you could propose it and see. And you'd have other benefits from that insulation--moderating the winter and summer temperatures.

  3. highcover | | #3

    Thank you, Martin and Charlie. It is reassuring to have your input. Depending on the final engineering, and of course, the building official, we will sort out which of these approaches makes the most sense. It has been crazy how much more involved our "simple" garage/studio building has been to accomplish than the house.

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