Best way to manage sealed attic humidity via /r/DIY

Best way to manage sealed attic humidity

I've got a fully sealed (spray foamed) attic, that has humidity problems when the weather isn't cold or hot enough to make my HVAC system run on a regular basis. From about March to May, and September/October, my attic humidity climbs to as high as about 75% at times. Been monitoring with a hygrometer for about a year to verify I've got a problem seasonally. When the HVAC is on consistently, everything stays in the ~40-45% range.

The only solution that makes sense to me is an attic dehumidifier. Just got one of these:

When I got to looking at the various ways to install these things, it occurred to me…would it make more sense to just have the humidifier running in the attic, or should I tie the inlet (sucking damp air) of the dehumidifier to my primary HVAC return and tie the outlet (blowing dry air) to my primary HVAC supply plenum? I'd originally planned to just have the thing circulating air up in the attic, but now I'm thinking that controlling the conditioned space better may head off the attic humidity before it becomes a problem (while also keeping the house more comfortable). I've seen it done both ways…wondering which is best practice, or if it just depends on the installer or house.

Since the attic is so well sealed to outside, I'm guessing the root cause is humidity leaching from the conditioned living area when my HVAC system can't run regularly. Any HVAC guys out there that can weigh in?

Submitted April 21, 2021 at 09:25PM by hotrodtaco
via reddit

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