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Community and Q&A

How to insulate bonus room and use of air baffle

Regis Mizikar | Posted in Energy Efficiency and Durability on

I have read many articles on how to insulate a bonus room over a garage. I have a heat run in the attic space behind the knee wall. Therefore, I plan to include the bonus room and attic space in the conditioned space. I want to run 1” inch continuous air baffle from the soffit to the ridge vent and air seal it. I am working with
2×6 rafters and I want to get approximately R30 rating by using 5 ½ high density fiber glass R21 plus Styrofoam (extruded polystyrene insulation) Wallmate R7.5. These are 2×8 sheets slotted for a 1×3 firing strip. To increase the R value I would like to make my own air baffles from ½ polystyrene insulation. This would add R value of 2.5. I am concerned about trapping condensation inside the cavity. Do you know what the permeable rating is for ½ foam board and whether it can be used to create the an air baffle? I have checked with the various manufactures of the foam board, but I cannot get a clear and definitive answer from them.


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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Assuming that you will be using rigid foam without foil facing, the permeance of 1/2 inch of EPS is greater than 6 perms, and the permeance of 1/2 inch of XPS is greater than 1 perm.

    In any case, I wouldn't worry too much about the permeance of the materials used in this application. Airtightness at the ceiling plane is far more important than the permeance of the baffle material.

    Note that 1/2-inch foam may not be stiff enough for this application. I think it makes more sense to use 1-inch foam, which will be stiffer.

    For more information, see How to Build an Insulated Cathedral Ceiling.

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