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I have a question about my AC system/attic/laundry room.

This is a new house built in 2019. Been living here for a year. I Noticed the laundry room has always been humid and figured it was because of the dryer. After finding mildew around a bedroom register, I decided to investigate the other ones.

This is the "second grille" in the laundry room enter image description here As soon as you get near it you can feel warm, moist air coming down into the room. Taking off the grill reveals this, which is just a tube right into the attic. You can see part of a truss up there in the picture. Once I took the grille off, the warm air "pours" into the room even quicker and it is very humid air.

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This is what the vent looks like as it comes into the attic. I didn't take this picture.. I found it on the internet. It's the exact same though.

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And just to be sure, here is the actual AC vent in the same room, on the other side of the room. It has no mildew under it, but it does sweat insistently:

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So what I originally thought was just humidity from the dryer is FOR CERTAIN coming from the attic. If I leave the doors to the laundry room close, it gets pretty mucky in there, even if the dryer/washer has not been running for more than a day.

Shouldn't warm air be going UP into the attic? Where would I even begin looking to get to the bottom of why warm/moist air is coming into the house from this one single vent?

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  • Gas dryer? Might be a make-up air vent. If air's coming in it's because you have a negative pressure condition due to various exhaust fans running in the house.
    – isherwood
    Aug 26 '20 at 18:27
  • if you leave your laundry room door ajar while running the dryer, you can seal that off to prevent all those problems. Sheet magnets are a good non-invasive way to quickly and effectively seal off registers.
    – dandavis
    Aug 26 '20 at 18:28
  • @isherwood there IS a gas dryer in the room, so after looking it up it appears that is probably one of these make-up air vents. Even with the exhaust fans off it appears that hot air is still leaking in... Aug 26 '20 at 18:37
  • @dandavis If a gas dryer needs so much air to run, it seems silly to pull hot air from the ceiling. I've read that a really well sealed house can have issues if using a gas dryer with no make-up air vent though. I'll have to look more into using a sheet magnet. Thank you for the recommendation. Aug 26 '20 at 18:39
  • Sealing the room seems like treating a symptom rather than the problem. Do you have the opportunity to meet with your builder? It's a matter of your entire HVAC/ventilation system working well together.
    – isherwood
    Aug 26 '20 at 18:44

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