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How to (economically) condition the attic space?

7HLfQecNqY | Posted in Energy Efficiency and Durability on

I have a old 1,500 sq ft rancher with 5/12 pitch roof located in Virginia . The HVAC system is in the attic, and I am looking for an economical way to condition the attic space or to eliminate the wide temperature swings that occur in winter and summer.

I can use Icyene to put closed-cell foam in between the rafters, but I feel the cost is excessive and the payback would be too long.

I would like comments on the following idea. Use Owens Corning 2″ rigid board insulation (R-10) panels cut to rafter spacing and put in between rafters, flush with inside edge of rafter so that an air flow space is left to allow passive air flow from vented soffit to ridge vents. Then add Owens Corning 1/2″ rigid overlay on inside edge of rafters covering rafters and installed board. The ceiling insulation which is R-30 would be left as is.

Any comments?

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  1. GBA Editor
    Martin Holladay | | #1

    The work you describe is fussy and labor-intensive, and will result in only R-12 insulation, minus the thermal bridging through the rafters.

    My first reaction is, if you are willing to go to all that trouble, why not do it right and construct a real conditioned attic with code-minimum levels of insulation? In case you change your mind, here's more information on doing it right: Creating a Conditioned Attic.

    My second reaction is, why bother cutting the R-10 insulation into narrow strips and inserting the rigid between the rafters? You would achieve better thermal performance if you put all of the foam under the rafters and left the rafter bays empty. Ideally, of course, the rigid foam would be thick enough to meet minimum code requirements for ceiling insulation.

    My third reaction is, have you talked to your local building inspector about requirements for a thermal barrier? Code requirements vary, but many jurisdictions require rigid foam to be covered with drywall or a similar thermal barrier.

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