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10″ Exterior Walls in 5-A

Rick Fetter | Posted in GBA Pro Help on

Since I can’t find an installer to dense pack my walls would Roxul give me the same R values?

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Replies

  1. Expert Member
    Michael Maines | | #1

    Rockwool (the new brand name for Roxul, matching the name of its European parent company) has a higher R-value than cellulose, about R-4/in vs. about R-3.6/in for dense-packed cellulose. Rockwool does not have the moisture-buffering capability of cellulose, though, so in thick walls it's even more important to use a thick rain screen, vapor-permeable sheathing and WRB, at least one excellent air barrier, and ideally a variable permeance membrane at the interior, as thick walls are prone to moisture accumulation.

  2. GBA Editor
    Peter Yost | | #2

    Hi Rick -

    If you do a dewpoint calculation for your climate (using local weather data--temps for the three coldest months of the year), you are likely to find the result indicating that your first condensing surface will be below the dewpoint temperature. But the question is: how wet will the wall get and for how long?

    With a good air barrier, we just don't find interstitial moisture problems related to even thick vapor-permeable walls in your climate zone, unless the building is operating at very high moisture content (of the interior air) during the three coldest months.

    But add a smart vapor retarder to the interior of the assembly and sleep easy those three coldest months of each year...

    Peter

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