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Roxul vs cellulose for flash-and-fill in basement walkout and above

oldbungalow | Posted in Energy Efficiency and Durability on

The rear wall of our planned basement will be a framed (stick built) walkout at grade, the other 3 sides will be CMU/foundation.

For the upstairs we are currently planning flash/fill with 2″ closed-cell plus cellulose in 2×6 walls in Zone 4. We thought a lot about Zip+R and other exulation options but the costs were just too high. Flash/Fill or Flash/Batt are a compromise. Depending on the calculator used, the whole wall R-value including thermal bridging considerations ended up being about R17 with this approach.

So my questions are
– is cellulose (on top of 2″ foam) OK for the at-grade framed walkout wall? (not the foundation walls)
– would 3.5″ R14 Roxul comforbatts be better overall than cellulose? cellulose would seem to seal better against an uneven foamed surface, and also has some moisture slow-drying capacity. Roxul has a slightly higher R/inch.
– what is the final word on cellulose and fires? I can’t find a straight answer. I know the fiberglass and cellulose industries are oppositely biased. see (presumably) non-industry articles below.

Fire & Cellulose Insulation

Cellulose Insulation: Energy Saver or Fire Risk? Depends on Who You Ask

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  1. user-2310254 | | #1


    I will give your post a bump to see if anyone has an opinion.

  2. Expert Member
    Dana Dorsett | | #2

    The flash fill of 2" in 2x6 cavity is a very poor value. In zone 4 as little as a continuous half-inch of foil faced polyiso on the exterior would provide higher performance, and provide more than adequate dew point protection for the structural sheathing. Do the math:

    Any siding type can be long-nailed through half-inch foam board most will still be OK at 3/4". An R20 + R5 wall (3/4") would outperform a 2" closed cell flash & batt by a good margin, and at a lower up front cost.

    Using cellulose rather than rock wool ends up at modestly lower R value, but will provide a perfect fit, and the borate fire retardents are protective against wood boring insects. The cellulose also has substantially lower thermal diffusivity due to it's higher thermal mass, and may even outperform the higher-R rock wool during the shoulder seasons.

    The "final word" on cellulose and fires is that yes, it will burn if subjected to 1000 F flame fronts for several minutes (so would the closed cell foam) but at those temperatures the house is probably already gone. Fiberglass melts at about 1000 degrees too, though rock wool would survive. From a home safety point of view it it only makes a difference if there is major conflagration already in progress. Is this house in high-dry pine forest closedly surrounded by trees that might light off after a drought?

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