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Insulation options to add R-value

52Dave | Posted in Energy Efficiency and Durability on

I had the opportunity to reroof this spring and add exterior rigid foam. I used foil faced polyiso 2.75″ thick. Due to time and money constraints I applied a single layer taped and foamed. Through fastened wth .5″ OSB cut into 3″ strips. A new OSB deck was installed and asphalt shingle roof on top of that.

Approximately 70% of my roof had an existing 3″ polyiso nail base secured to a 2″ t&g celling. This portion is framed with 4×10 timber beams running 24″ oc. At some point in the life of the house someone covered the interior face of this cathedral celling with sheetrock in order to add fiberglass batts. I am unsure of the exact thickness, i plan on dense packing this space when the funds become available. My question regards the rest of the roof.

The remaining 30% of the roof is 2×8 running 16 oc. On the interior it is a mix of cathedral ceiling and attic space. the cathedral ceiling is insulated with fiberglass batts and finished with sheetrock. The attic floor is 2×6 joist with fiberglass batts and a plywood deck. This was obviously far from code minimums and i made the decision to dense-packed all the bays creating a conditioned attic. The fiberglass batts in the cathedral ceilings were obviously left in place and compressed tightly when the ceiling was dense packed. With the current configuration I’m calculating an r value of r42. Thats with a r5 per inch for the polyiso and an r4 for the cellulose.

My question is, can i safely add any additional insulation on the inside face to add r value? I know that the exterior foam to fluffy interior ratio isn’t ideal. I am thinking the cellulose and its hygroscopic properties should prevent moisture issues with the plywood deck in the winter. I am in zone 5, northern CT about 20 min north of Hartford.

Comments and critique welcome.

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  1. GBA Editor
    Martin Holladay | | #1

    If you haven't read it yet, you should read this article: Combining Exterior Rigid Foam With Fluffy Insulation.

    The 2 3/4-inch-thick polyiso that you installed above your roof sheathing has an R-value of about R-14 or R-15 (depending on how much you want to de-rate its R-value for poor performance at cold temperatures). In Climate Zone 5, with this type of roof assembly, at least 41% of the total R-value of your roof assembly needs to come from the rigid foam layer. So you can have a maximum total R-value of your roof assembly of about R-34, with R-14 from the rigid foam and no more than R-20 from the fluffy insulation.

    So what you've already done is risky.

  2. 52Dave | | #2


    I realize the assembly is risky. Would adding a smart membrane on the interior face help in minimizing moisture collecting on the plywood? Interior humidity stays between 29-35% between November and March.

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