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Dense-packed cellulose OK?

user-1084973 | Posted in Energy Efficiency and Durability on

We built a home 11 years ago with 5 inches of Icynene sprayed against the 1/2-inch plywood sheathing, no vent, in our cathedral ceiling.

12 inch I-beam rafters from soffit to ridge, no vapor barrier.

After energy audit it was recommended to fill remaining 7 inches with dense-packed cellulose. Heat is a wood stove and radiant flooring, minisplit for AC and auxiliary heat.

We are in southern Wisconsin Zone5A/6B.

What are your thoughts?

Thank you in advance,

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  1. user-945061 | | #1

    Is the spray-foam open cell or closed?

  2. GBA Editor
    Martin Holladay | | #2

    Assuming you're talking about open-cell Icynene, your 5 inches of insulation has a value of about R-18 or R-19. That doesn't meet code.

    Your insulation is vapor-permeable (that's not good) but air-impermeable (that's good). According to the building code, you need at least R-20 (in climate zone 5) or R-25 (in climate zone 6) of air-impermeable insulation before you can switch to air-permeable insulation (like cellulose). For more information on this topic, see How to Build an Insulated Cathedral Ceiling.

    So the Icynene doesn't quite get you to where you want to be, but it's darn close. The bottom line: You will probably be OK if you follow your plan, especially if the relative humidity during the winter on the interior of you house is kept in a reasonable range. If you want to sleep without any worries, though, you might want to install more spray foam instead of cellulose.

  3. user-1084973 | | #3

    #1. Open Cell
    #2 RH is held at 40% or below, without tearing out drywall (dense pack will be done thru 2" holes in each bay) this seems to be the reasonable option, are the concerns moisture forming in the cellulose?

  4. GBA Editor
    Martin Holladay | | #4

    Q. "Are the concerns moisture forming in the cellulose?"

    A. Since your spray foam layer is a little thin, the interior surface of the spray foam may be cold enough to allow moisture to condense or moisture to accumulate. Because the spray foam is vapor-permeable, the most likely location for the moisture to accumulate would be in your roof sheathing (the plywood). Keeping a low level of indoor humidity reduces the risk. You should never use a humidifier.

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