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Is there any rationale for removing attic floor insulation when converting to an unvented attic with an insulated roof deck?

Dana Dorsett | Posted in Energy Efficiency and Durability on

With or without ducts in the attic, if the mean wintertime attic temps exceed the dew point of conditioned space air by some margin with the roof deck insulation is in place, moisture issues in the attic would be well-controlled, and in summertime even more so, with less access to humid outdoor air to condense on ducts. I’ve yet to hear/read any reasoning or analysis behind the practice (or of specific instances where leaving it in place caused a problem) yet it seems to be almost a standard with spray-foam installers, independent of climate zone or actual installed R-values.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Dana,
    I don't think it matters whether or not you leave the insulation on the attic floor or you remove it -- unless the insulation is contaminated by rodent feces.

  2. Expert Member
    Carl Seville | | #2

    One reason you may want to leave it in place is to reduce sound transmission between rooms. One of the downsides of an insulated roofline is that when you have no ceiling insulation, you can sometimes hear everything going on in adjacent rooms. Privacy has its virtues.

  3. Expert Member
    Dana Dorsett | | #3

    Thanks!

    That has been basically my take- even in homes where it alters the temp of the air surrounding ducts, the attic is now within the pressure boundary, and the attic dew point will track that of the conditioned space air, even if it's somewhat cooler than the fully conditioned space in winter, warmer in summer. As long as the attic doesn't dwell below the dew point of the conditioned space air for days/weeks it should be just fine.

    The additional R-value will still reduce the heating & cooling loads on the building, so (unless it reeks of rat) removing it would usually be a net negative. Yet multiple foam-guys in multiple situations seem to insist that leaving it in place is a problem, but are never able to explain it in theory or on empirical evidence, which is why I'm throwing on the wall here. Apparently there some mis- or dis-information going around in the spray foam insulation biz (like that's a first, eh? :-) )

  4. Torsten Hansen | | #4

    You can leave the insulation but make sure you don't have a poly vapor barrier installed between the drywall below and the attic floor joists. You want the attic to equalize with the rest of the house so a vapor barrier could be a problem.

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