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Community and Q&A

Rigid foam board over Flash and Batt

jcrowrva | Posted in Energy Efficiency and Durability on

I am converting my attic from vented to unvented conditioned. My home is a 1940 brick structure with a slate roof, in Richmond Virginia (Zone 4a). The attic is a simple gable attic with 7.5″ rafters and exposed brick gable walls. My plan is to put 1.5″ closed cell-spray on the gable walls, and flash and fill in the rafters, with rigid foam board added to prevent thermal bridging, as in the chart below (thanks for all the great info here!)

My questions are

  r-vale per inch Thickness R-value
Closed Cell 6 3 18
Cellulose 3.5 4.5 15.75
Foam Board 6 1 6
Drywall 1 0.5 0.5
total   9 40.25
r-value share     44.7%

My questions are:

1. Will the rigid foam board create moisture issues in the cellulose insulation?

2. I am choosing cellulose for the environmental impact, but I am worried it may settle. Would a fiberglass batt be better?



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  1. Patrick_OSullivan | | #1

    Continuous rigid foam below the rafters is likely an installation nightmare unless you have a completely unused attic with zero irregularities, etc. If you want the thermal break, I would do the Bonfiglioli approach ( on the rafters, fill the final thickness bay with fluffy insulation, and drywall on top.

  2. jcrowrva | | #2

    Thanks, I wasn't aware of that method. I will look into that.

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