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Lowered Humidity in Attic and Shrinking Foam Insulation

suect | Posted in General Questions on

We purchased a home in Texas last June.
After moving in we noticed mustiness and mold type odors that seemed to be increasing in the fall.
Last November we used stand alone dehumidifiers on the main level and attic.  The humidity was between 55-65%.
Over several months we lowered the humidity to 45%.  In the attic boards were beginning to crack and the foam began to shrink slightly.  The attic dehumidifier was off while away for one week only to return to an attic at 65%!
In the spring the fan speed on our A/C was decreased from high to medium low, which caused a drastic shrinkage change in the foam.
I read that 40% to 55% is the ideal and 40% being the goal for the winter months.
Now it is winter our humidity is in the high 40’s and more shrinking of the foam is occurring. One contractor mentioned it’s the low humidity and another states humidity should not matter, yet another states it should be at 55% as it was more than likely, never below this before.
Our home feels chilly, and I wonder if there has been shrinkage of wood that would create gaps.

Any thoughts on what occurred, and if this can be corrected?

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  1. Expert Member
    BILL WICHERS | | #2

    Wood DOES expand and contract with changes in humidity. This is one of the reasons you sometimes need humidity control in homes with hardwood floors. If the wood of the structure exapands/contracts too much, I suppose it could seperate from the foam, although I haven't seen that myself.

    Is this "foam" you mention spray foam or rigid foam? Spray foam DOES sometimes shrink after installation and pull away from the wood structure. This can be due to the spray foam mix being off during installation time ("off" as in not quite the right mix of resins), or if the wood is too wet during installation the spray foam won't adhere properly. Rigid foam expands and contracts with temperature changes, but shouldn't be significantly affected by changes in humidity levels.


    1. suect | | #3

      This is open cell on a home that is 9 years old.

      Only possible variables that may lead to this:
      1 - 6” open return was closed
      All duct connections were sealed with mastic
      Free standing dehumidifier was used prior to lowered fan speed.

      I sense the home had high humidity prior with the smells we experienced and may be the lowest it has been in its lifetime.

      I’m now trying to keep the humidity hovering around 50% and slightly below. (Possibly rehydrate?) Fresh air intake is closed for past 3 days to avoid humidity in the high 30s-low 40’s.

      We had dehumidifiers on last year to have humidity in the mid 40’s.

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