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Wet spray foam?

suect | Posted in General Questions on

In the past I have asked questions about humidity in a spray foam attic and appreciated all the replies.  I am confronted with 2 different thoughts by local contractors in the field of HVAC and spray foam.

I have had 20 pint dehumidifier to keep the RH at about 45% in the attic as well as 2 – 50 pint on the main floor.  My foam looked good and the dehumidifiers were doing their job at times emptying them every other day. I had a contractor check my HVAC  in hopes of controlling the humidity without dehumidifiers. It was suggested to have a medium low setting on the a/c from a high setting as the inside coil is 5 ton, outdoor condensing coil is 3.5, which was done.

It was also suggested to close one 6” open return (no supply duct present) in the attic as negative pressure would be created, which was done.

There is an AprilAire fresh air intake that was on @ 6 minutes per hour with no limits and was turned off stating it was the cause for the humidity.

About two weeks later the foam had changed in appearance along with being less in thickness.  The fan speed was reset to the original speed of high to see if this would halt the process.  So far it has not.

One contractor stated I over dried the home too quickly resulting in the change. The advice was do not dehumidify but humidify if needed in the winter to keep the RH at around 50%, turn on the fresh air but set to 10 minutes.

Currently the humidity without dehumidifiers in the attic, but main level dehumidifier is at 50%., is 48-55% on the south section of the home with temps reaching up to 78 on a 90 degree day.
On the north attic 49-65% swings and up to 82 degrees on a 90 day.
it feels humid on this side.  I have consulted with the foam installer who advised there are no leaks.

I have had a foam contractor  advise the changes may be due to the foam being wet.  He advised an energy auditor to evaluate the home which would include the HVAC system.  An appointment has been set up.

Any thoughts on what may have caused changes and best actions moving forward? 

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  1. Expert Member
    BILL WICHERS | | #1

    Do you have open cell or closed cell spray foam here? Open cell is more prone to moisture issues than closed cell, but open cell isn't always a problem, either.

    I would not humidify the attic, ever.

    Having an energy auditor come out and look at this is probably the best advice you got from your contractors. Third party people like the auditor are most likely to be able to find the actual cause of the problems you're seeing here.


    1. suect | | #3

      I have open cell.

  2. Expert Member
    Dana Dorsett | | #2

    >"One contractor stated I over dried the home too quickly resulting in the change. The advice was do not dehumidify but humidify if needed in the winter to keep the RH at around 50%."

    Maintaining an indoor RH of 50% in winter can drive structural rot in a wood sheathed wood framed house in IECC climate zones 5 and higher, unless the wall & roof assemblies of the house are specifically designed for humidity that high.

    Where are you located?

    A peak indoor humidity of 60% @ 80F in the attic is not a mold or rot hazard to either the attic or roof deck as long as the average roof deck temp is >> 65F (the dew point of 60%RH 80F air.) Typically with an open-cell foamed roof deck on sunny days the hot roof deck (MUCH hotter than 65F) drives moisture out of the wood during the day, which diffuses through the fairly vapor-open open cell foam, raising the attic's RH. When the roof deck cools off at night to below the attic air's dew point temperature it draws some of the attic moisture back into the roof deck, but with a dehumidifier removing moisture during the daily peaks the overall moisture content continues to fall over days/weeks of sunny weather.

    1. suect | | #4

      I’m located in Texas. Unfortunately I did allow humidity in the kitchen stove area which did create a higher humidity. Prior to that advice I always kept the humidity below 50%. We had the attic blast that the north gets so it added a risk factor. Humidity was an issue on that same side of the attic even with main level humidity at 45%. I’m thinking possibly from not venting enough?

      Could possibly setting up a fan similar to what is used in greenhouses on low, intermittent setting be of help? There is an opening around the fireplace piping that exits the roof. Could this be a possible exit?

      Asking because there is a pull down step and would need to do several visits up there to empty the dehumidifier which I am willing to do, or try to devise a drainage system.

      Thank you!

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