I had a crawl space (top left) in my house which had a 12"x6" venting window. There is a room above the crawl space, so I decided to spray the walls of the space with closed-cell foam insulation which also closed off the venting window. The room above the crawl space is now warm in the winter so I'm happy with the insulation.

However, in the summer months, the crawl space (and, as a result, the entire basement) has a big moisture and mold problem. To alleviate the issue, I cut a small hole in the insulation where the venting window used to be and put a small 12V fan in which I leave on at all times. This helped the moisture problem a great deal - I would say that about 75% of the smell is gone.

My question is: what is the best approach to solving the problem completely? A larger fan? Spraying the floor of the crawl space? Something else?

Many thanks in advance!

enter image description here

4 Answers 4


When you bring a crawlspace into the building envelope by insulating it like this, you need to treat it like any other room and condition it.

Putting a fan in the wall that exhausts to the exterior defeats the whole point by circulating outside air through the crawlspace--which is now a part of the interior of the house. There's no point in insulating a space that has free air movement between the inside and the outside. The only reason why the fan makes a difference is because it's moving air, which is good. But it's also bringing in outside air which in the summer is humid; you're replacing very humid air with slightly-less-humid air. That's why the problem hasn't completely gone away. If you wouldn't open the windows to dehumidify the rest of your house, you shouldn't do it to your now inside-the-building-envelope crawlspace. Think of your crawlspace as now a room, not a crawlspace. Maybe it's a crawlroom.

Put HVAC supply and return vents in the crawlroom and the problem will disappear. During the summer, your AC unit will circulate air through it just like the fan is currently doing, but also dehumidify it, which is what you're currently missing. This will be much cheaper than running a separate dehumidifier. The very last thing you want to do is run a dehumidifier in a room with a ventilation fan like the one you have rigged up: you'd be trying to dehumidify the whole neighborhood.

Edit: if the floor doesn't have a vapor barrier of some sort, that also needs to be addressed. When you bring a crawlspace into the building envelope, you can't do a half job and forget about the floor. If the floor slab doesn't have insulation or a vapor barrier underneath it, then you need to add one on top of it, and seal it to the foamed walls with more foam.

  • resealing the hole in the insulation and leaving the access hatch open (or replacing it with a large grate) should be more than sufficient to get this crawlroom (great term!) inside the HVAC envelope. It doesn't have to be perfectly heated/cooled, just enough airflow to get the air here dried out. which it will do over time.
    – FreeMan
    Oct 2, 2023 at 17:18

The fan is the right idea, but it should probably be a stronger 120v unit. Look at bathroom fans and how they're rated for square footage. Some can be mounted in-wall and may fit your opening well.

  • Thanks. Why are you saying that the fan isn't circulating air? It's blowing air out the window throw a snug opening - that means that air is entering the crawl space through the door.
    – Wynne
    Sep 12, 2016 at 15:57
  • You didn't mention an open door now, did you? :)
    – isherwood
    Sep 12, 2016 at 16:00
  • I did not! But when I do close the doors, I'll make sure they are louvered somehow.
    – Wynne
    Sep 12, 2016 at 16:30
  • This is the wrong approach. With the new insulation, the crawlspace is now inside the house. You don't want to bring in a ton of humid outside air in if your problem is excessive humidity.
    – iLikeDirt
    Jan 15, 2017 at 23:12
  • Actually, if the humidity level in the crawl space is higher than outside, as it can be due to ground moisture, it is the right approach in the summer months.
    – isherwood
    Jan 16, 2017 at 0:58

use a dehumidifier.

The temperature in the crawl space is significantly lower than in the house above, right? This is a common problem with dirt floor cellars/stone foundations that have been insulated, and is easily alleviated by running a dehumidifier in the summer.

The low height of the ceiling makes for a fairly small volume, so it should work very efficiently and quickly. Make sure the fan you currently are using is turned off - with the dehumidified air it will be unnecessary to bring in fresh air from the exterior (and make it impossible to efficiently dehumidify your space!). it can drain to a condensate pump, which you can then plumb to an existing drain or the exterior.

  • Of course, that'll use quite a bit of power compared to even a robust fan. I've recently learned that it can cost as much as $20-30 per month or more.
    – isherwood
    Sep 12, 2016 at 17:06
  • It can, sure, but that depends on frequency of use. Again, based on the relatively small volume of the space and the fact that it is already partially sealed by foam, I don't think it will be near the $30/month. Also, that is only in summer months. Finally, the fan will not really work on getting moisture down unless you have a place for dry air to enter and wet air to leave. In the summer time, you don't really want hot+wet air to be circulated down into the crawlspace, because it will condense on the relatively cooler surfaces. The only alternative is to heat the crawlspace in the winter :)
    – aaron
    Sep 12, 2016 at 18:12
  • I hesitate to use a dehumidifier since there is no place to drain the water.
    – Wynne
    Sep 13, 2016 at 14:09
  • the water from the dehumidifier can drain to a condensate pump which can then be run anywhere convenient for you. the actual volume of water will be very small, so it can safely be dumped even close to your foundation. Alternatively, it could be plumbed to an existing sewage/drain line like that for a washing machine, etc. There are also some dehumidifiers with built-in condensate pumps, although the options are very limited.
    – aaron
    Sep 13, 2016 at 19:24
  • yes, I assumed this was obvious. I'll edit the post to clarify.
    – aaron
    Jan 16, 2017 at 14:16

I suspect your original approach was incorrect. the only thing that should have been insulated was the crawl-space ceiling under the room you wanted to insulate. The walls of the crawl space should have been left alone - including, of course, the vent.

Contrary to what one might think, a crawl space is best kept fully ventilated, just as is the case in some houses which are built a couple feet off the ground. This does not lead to massive heat loss due to the "magic" of convective air flow. Yes, insulate under the floor of the house, but nowhere else.

So, if you can, rip out the insulation and start again.

  • I wanted to store some things in the crawlspace like lumber and paint. That's why I wanted to bring it in inside the envelope.
    – Wynne
    Sep 12, 2016 at 16:29
  • What was incorrect was not insulating the walls, but rather adding the fan. Crawlspaces should be ventilated, or insulated and sealed, but not both.
    – iLikeDirt
    Jan 15, 2017 at 23:13
  • I disagree, @iLikeDirt. In my part of the world, summer and winter are very different scenarios. A fan certainly is appropriate during the summer, insulation or not.
    – isherwood
    Jan 16, 2017 at 14:25
  • @iLikeDirt If the entire crawlspace is going to be sealed, then you must install Prop-A-Vents or equivalent to allow airlflow from the soffits up to the remainder of the open space above the rooms. Jan 16, 2017 at 14:49
  • @isherwood summer ventilation to reduce humidity doesn't make sense. If the outside air was dry enough to reduce the interior humidity, there wouldn't be an interior humidity problem in the first place. The only reason why this helps at all is the air movement, which can be accomplished by adding HVAC vents into the sealed crawlspace.
    – iLikeDirt
    Jan 16, 2017 at 18:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.