We have had moisture issues in our crawlspace this wet summer. We are thinking we will try a dehumidifier. Any suggestions?

Asked by Chris and Dara Gunnell
Pfafftown, NC

The insulation is very wet, and we didn't know to close our vents. We had the typical very expensive quotes, but are thinking we will try a dehumidifier to see if this method works, closing the vents, new vapor barrier paper, etc. will help. My question is since there was some mold on some of the joists, do we really need an $700.00ish fungicide treatment to treat whole area? We will do if necessary, but it seems a bit much and maybe not so home healthy? They also are saying we will need to remove all the insulation, and replace with new to do this, this is $800.00. Any suggestions are appreciated asap!


Danny Kelly

Answered by Danny Kelly

Charlotte, NC

Kelly McArdle Construction

September 23, 2013

It has been a very wet summer in NC this year and that is causing havoc in all the vented crawl spaces in the area.

  • Vented crawl spaced worked fine before we added air conditioning and insulation to our homes.
  • Now these same things keep our crawl spaces cooler and when the humid air from outside comes in through the vents it will condense on all the cool surfaces, ductwork, insulation, etc. which is what you are experiencing.

The best solution is to seal up the crawl vents and make a closed crawlspace system. This is typically the most expensive approach.

  • There are several ways to accomplish this and you will get varying opinions especially from the companies that specialize in this type of work.
  • Most of them operate on scare tactics (as do most mold companies) and talk people into spending much more than necessary on their crawl space. 

The other complicating factor is there are ways to correct this that are not very energy efficient (like running a dehumidifier) so finding a balance is important. You want to control the moisture but not waste a lot of energy to do it.

Is all of this necessary?

So back to your question - is all of this necessary? Well it depends on several things.

  • Is there currently mold growing on your floor joists? If so, it should be cleaned off but this can be done with a simple detergent solution, you do not have to use some expensive product.
  • It is better to remove the insulation but if it is not completely saturated you can leave it in place as long as it will dry out.
  • If you are not allergic to the mold, you actually do not need to do anything as it will eventually go dormant once the moisture is under control.

The minimal solution is a wait and see approach. It is important to monitor your crawl space. Check the humidity and more importantly check the moisture content of your floor joists. A good termite company can perform this test annually for you.

Making a closed crawlspace system

You would begin by removing all insulation, clean joists, treat the joists, install vapor barrier, insulate crawl walls, etc. 

Next step is close up the crawl space. Before sealing it up, you need to check your HVAC system if it is in the crawl space and be sure it is a closed combustion appliance that brings in its own combustion air. If it is, then seal up the vents and install a vapor barrier over 100% of the crawl floor and wrap up the crawl space wall wherever the crawl floor is below grade.

Now you need to control the moisture with some sort of mechanical ventilation. The code allows supply air, exhaust air or a dehumidifier.

  • I do not like the supply air option for an old musty existing crawl space as it will push the dirty air up into the house.
  • A dehumidifier will typically work but is a huge energy hog and you will end up spending much more in the long run. Also, the cheap dehumidifiers will not work well in a crawl space so you will need to purchase a high dollar one.
  • I would recommend the exhaust vent. You can purchase an Energy Star fan and run it continuously. It only needs to be 1 CFM for every 50 square feet of crawl space. This vent will exhaust all of the moisture (and musty moldy odors) and slowly draw a bit of dry conditioned air from the house above.

For more information:

Read "Are sealed crawl spaces a good idea?" a Q&A answered by Joshua Lloyd.

Also, check "What is the best way to encapsulate a crawlspace that seems wet all the time?" a Q&A answered by Sean Lintow.

Tagged In: basement moisture

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