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Community and Q&A

Follow-up to insulation and condensation

Patrick Killelea | Posted in General Questions on

Sorry I cant get back to the thread that I started, I hope this gets where it needs to go:

thank you very much for your response; couple of follow-ups: so are you saying that an uninsulated stud cavity would be an issue in a basement, and the heated air wouldn’t dry out the cavity like in an above grade standard wood wall? How about if you just strapped the foundation and drywalled over it, with no insulation, is that a recipe for mold or not?
Also, if my rigid is semi-permeable, does the thickness matter? as long as the wall can dry to both sides? or is that assumption wrong?
Lastly, how about batt insulation in a partition wall that separates finished/unfinished portions of the basement? with no contact, is it still effective?

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  1. Expert Member
    Dana Dorsett | | #1

    Local climate matters. A basement in Tucson AZ would have little to no mold hazard with an uninsulated studwall for the finish interior. In Bismarck ND that would not be the case.

    Semi-permeable foam only allows drying in both directions when it's above grade. Below grade ground moisture drives are (almost) universally higher than conditioned space moisture drives- the water vapor flow only goes from the sub-grade wall, toward the basement, and never the opposite.

    Furring on the foundation isn't any better or worse from a mold hazard point of view than an uninsulated studwall up against the foundation.

    Insulated partitions only isolate a semi-conditioned space from a fully conditioned space. Under different heaiting/cooling load conditions it will impose a temperature difference, but whether that's good or bad depends. It also needs to be air-tight to be effective- if air moves freely from one side to the other through joist bays or whatever, it's just a convection engine. If the insulated partition makes part of the basement below freezing in winter that could be actively bad. Similarly, if it keeps the semi-conditioned part of the basement below the dew point of the outdoor air, it could create a mold farm. The "right" place for the insulation boundary is the pressure-boundary of the house- the exterior walls/floor/ceiling, since that's where the temperature differences are greater (so you're actually benefiting from the thermal resistance), and the moisture issues can be better controlled.

  2. Patrick Killelea | | #2

    OK, one more then I'll drop it: I have a friend who finished his basement in Leominster MA; he only furred out the wall with strapping and didn't insulate. I was going to warn him that he faces a dangerous mold situation because the heated air will condense on the cold foundation and create moisture. But then I thought, if the cavity stays warm, it will dry it out before it creates a problem. Which is right?
    thank you, sorry to be such a pain

  3. GBA Editor
    Martin Holladay | | #3

    There are so many variables that it is hard to answer your question. The wall may be fine, or it may be damp.

    Here are two factors that can affect what happens: when more of the basement wall is above grade, the wall will be colder than when more of the wall is below grade. So the percentage of the wall that is above grade affects the wall's temperature in winter. The other factor is the indoor relative humidity. If the indoor RH is high, the wall is more likely to have problems than if the indoor RH is low.

  4. Expert Member
    Dana Dorsett | | #4

    The paper facer on the back side of the gypsum will run at the same temperature as the concrete, and can be cold enough to take on mold-inducing levels of moisture during the winter, turning into an asthma-inducing mold spore bloom in the spring. (I've observed exactly that situation in a friend's home in Sterling, MA, just up the hill from YOUR friend.) Even the R0.5 of the wallboard itself is enough to make a real temperature difference from the paint to back-side facer when you only have 6-8" of concrete between the cavity and the great outdoors. Without the cavity vented to the outdoors wintertime drying is slow, but if there are any air leaks to the humid interior side moisture transfer can soar too.

    If he keeps both winter & summertime relative humidity sufficiently low- under 35% in winter, under 50% in summer, and uses only latex paint on the walls, it'll probably be fine.

  5. Patrick Killelea | | #5

    Thanks to all who supplied answers- this is a great resource.

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