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Insulation

user-7362555 | Posted in Energy Efficiency and Durability on

Hello…I have a new construction residential home going up in zone 4a (Long Island, NY)…We plan on having a full walk up conditioned attic…Our plan for roof insulation is as follows…Working from inside to out… *7-1/4” Roxul Comfort Batt (R30) * 2” closed cell spray foam (R13) – This maintains the 30% ratio for       condensation control * 5/8” plywood * Henry Blue Skin VP100 * 4” Roxul Comfort Board 80 – as per Joe Lstiburek video ”Building Science Meets Mountain Climate” * 5/8” plywood with ridge vent *Da Vinci shingle system So my question is, am I maintaining the 30% ratio for condensation control with the exterior insulation of R16 (30%) to the interior insulation of R43 (7o%) or does it really matter how much exterior insulation we use since the condensation ratio has been satisfied on the interior? Thank you & hoping to hear back, as this project takes off the end of September of this year…Peter DeLorenzo

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  1. Expert Member
    Dana Dorsett | | #1

    >"...am I maintaining the 30% ratio for condensation control with the exterior insulation of R16 (30%) to the interior insulation of R43 (7o%) or does it really matter how much exterior insulation we use since the condensation ratio has been satisfied on the interior?"

    The first condensing surface is the interior side of the closed cell foam layer. As long as 30% of the total-R is exterior to the first condensing surface there won't be excessive moisture accumulation, as long as there is at least a Class III vapor retarder on the interior (such as interior latex paint on ceiling gypsum.) Of course more R reduces the risk even further.

    But rather than 2" of closed cell polyurethane under the roof deck and 4" of rigid rock wool on the exterior side (R29 total exterior to the first condensing surface), with R30 rock wool in 2x10 rafters, the roof deck would have better drying capacity and higher performance at lower overall cost with 5" (R30) polyiso above the roof deck, and no closed cell foam in the cavities with R30 in 2x8 rafters. With a 50% R-ratio with all of the foam exterior to the roof deck the deck would stay quite a bit warmer (= much drier), and even the interior side Class III vapor retarder becomes optional in a 4A climate.

    Alternatively, R38 fiberglass installed the 2x12 rafter cavities (=R37 when compressed to 11.25") with the R16 rock wool above the roof deck & the ridge vent would allow drying in both directions, while staying a hair above 30% of the total R on the exterior.

    The proposed stackup with the spray foam inhibits drying toward the interior, and the drying capacity toward the exterior is limited through highly air-retardent rigid rock wool, even if soffit & ridge vents are added. With soffit-to-ridge venting under the 5/8" nailer deck the drying toward the exterior through the rock wool would be much improved, but that adds complexity & expense to an assembly that is already unduly complex & expensive.

    1. Stephen Sheehy | | #2

      And the fiberglass/ rock wool alternative avoids foam entirely.

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