Two months after having my attic professionally air sealed and insulated, the basement floor became damp and the top of the foundation started to show signs of horizontal cracking. Humidity levels in the living space are also high. The house is a one story ranch with a brick veneer. A gas hot water heater and dryer vent were also added to basement.

  • All the humidity that used to escape through the roof has been trapped inside, therefore, indoor humidity has gone up to the point where it's condensing on your cool basement floor. It's hard to tell from the wording of your question - does the dryer now vent into the basement, or just through the basement wall somewhere? If it vents into the basement, that A) seems like a bad idea in general, and B) will pump additional warm, moist air into the house, exacerbating the problem.
    – FreeMan
    Jun 3, 2016 at 16:34
  • Why did you have your attic air sealed?
    – DMoore
    Jun 3, 2016 at 16:57
  • How long have you been in the house? This sounds like typical seasonal moisture to me, and I doubt the concrete cracks are related to the attic thing.
    – isherwood
    Jun 3, 2016 at 18:14
  • Dryer used to vent into the basement and it was dry down there then. We lived in the house for two years with none of these issues. After we converted to high efficiency gas furnace and ac and had the attic air sealed as recommended by s home energy audit, humidity in the home increased and the cracking foundation started. The humidity is high in the house year round now. Help anyone...
    – Miley
    Jun 3, 2016 at 18:41

1 Answer 1


Horizontal cracking? That's odd, and I think that's unrelated to your moisture problem (even with a picture, I'm not sure I'd comment on that).

Well, you've air sealed your house, which means that now you have to actually pay attention to the V in HVAC: Ventilation. In a hermetically sealed house, I strongly suggest seeking a better filter system.

I'll take a wild guess that you lack a suitably sized fresh air intake for the furnace, as well as a make-up air inlet for the house (because most houses don't have either).

But you probably just need to look into buying a dehumidifier. Assuming that none of your power-vented equipment is over powering any of your gas-fired natural draft equipment, or that any other item like a fireplace is doing so (hopefully your CO detector will notify you of that).

Your A/C is over-sized or improperly charged. Have a tech come out to diagnose the unit. Hire whoever can explain the difference between superheat and subcooling to you, and what a wet-bulb temperature is.

And explain to them that, you need the A/C unit tweaked to run longer, whether it be from reducing the fan speed, or monkeying with the pressure to make it less efficient, with the goal being to drop out excess moisture from the air.

Central AC system not reducing the humidity in the air?dslreports.com

a standard AC unit will drop out more humidity the longer it runs - if it is oversized you will be too cold before it has a chance to do the job - the too large coil will make the air cold too quickly and the unit will cut off before it has time to drop out the moisture. A slower fan speed would help some because it gives the air more time at the evaporator coil and will let the unit run longer

The thread goes on about relative vs. absolute humidity for a while...

Going back to the OP, if the AC is cooling down the house too quickly, then open up some window shades [or room doors] and give the house some heat to force the AC to run longer. This will give more time for the AC to pull moisture out of the air and lower the relative humidity. Having a proper moisture barrier is important though. Never open a window or turn on an exhaust fan. By doing so, you are pulling humidity into your home and as it hits your cool air, it will condense and feed mold.

However if it's not all that hot outside but it's really humid, "Agreed with others in that you are sort of in the 'screwed zone'."

  • Does anyone use their A/C somehow as a dehumidifier, or is that less efficient than a little stand-up guy in the basement?
    – Mazura
    Jun 4, 2016 at 0:06
  • running a dehumidifier in the basement already. You were correct on the lack of make up air inlet. But wouldn't that just add more moisture to the house it is typically a humid climate here. Spoke with an engineer who thought horizontal cracking at the top of foundation could be coming from either air being pulled thru what was small openings that have now opened up since the house can't pull in air thru the attic anymore after being sealed. Can't I just remove the air sealing from the attic and return things to there original state.
    – Miley
    Jun 4, 2016 at 1:44
  • @Miley - Well, I still don't know about your foundation, but I think your A/C isn't running right and you need someone who really knows what they're doing to take a look at it. Please see the edit.
    – Mazura
    Jun 4, 2016 at 2:57
  • 1
    Well hvac company adjusted airflow lower on unit. Thanks for your answers. Hope this helps the situation. Now onto the foundation mystery
    – Miley
    Jun 9, 2016 at 12:25

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