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Rigid foam board

paisleydesyn | Posted in Energy Efficiency and Durability on

Hello. Remodeling attic that was converted around 35 years ago. Have had 1” of closed cell and 3.5” of open cell sprayed on all ceiling area, backed all knee walls and sprayed with 3.5” open cell, reflective felt on decking and cool shingles installed. Concerned about thermal bridging and considering interior rigid foam. I am being told this is unnecessary, but I don’t want to find out I need it after walls are complete. Thank you for your advice. Johnna

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  1. DCContrarian | | #1

    The cost-effectiveness of insulation depends on your climate and how well-insulated the rest of the house is. It would help us if you told us where you are located and what the rest of the house is like.

    Thermal bridging by itself isn't the issue, what you should care about is the total r-value of the wall or roof assembly, which thermal bridging will reduce. If you want a higher r-value you can either make the insulation thicker or take steps to mitigate thermal bridging.

  2. paisleydesyn | | #2

    The house is located in north Texas and has batt insulation downstairs. My concern is the thermal bridging upstairs. Thank you

  3. DCContrarian | | #3

    It looks like you are in DOE zone 4, which requires R-38 between ceiling and roof. One inch of closed cell foam= R6.5. Open cell foam is 3.5 per inch so 3.5 inches is 12.25 giving a total of 18.75. So it looks like you need at least another three inches of closed cell or polyiso or six inches of blown or batt insulation.

    Thermal bridging reduces the effective insulation by about 20%. You can get the same effect by either installing continuous layers that eliminate thermal bridging, or making the other insulation 20% thicker. It's really a question of which is more cost effective.

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