I'm in Climate Zone 4A. I am wondering how sealed crawl spaces work in this region. I am concerned about moisture build up.

Asked by Ron Myers
Smyrna, TN

I am an Energy Rater and have several customers asking about switching to sealed semi-conditioned crawls vs vented crawls with insulated floors. I'm all for it but want to make sure I direct them correctly with regards to moisture issues and potential mold growth if done incorrectly. I've suggested dehumidification exhaust fans. Are these necessary or what other options are there?


Danny Kelly

Answered by Danny Kelly

Charlotte, NC

Kelly McArdle Construction

September 8, 2010

On a vented crawl a dehumidifier or exhaust fans will not do any good.

  • Since the crawl space is open, the dehumidifier is trying to dehumidify the entire world - it will never work, especially at 80 pints a day.
  • Exhaust fans are also problematic; some wire them to a humidistat so they turn on when the humidity is high in the crawl.  Problem is when it is humid in the crawl it is also humid outdoors so you are pulling in more moist air.

A closed crawl is an excellent option to control moisture. According to research from Advanced Energy you will also save energy with this approach.  There are several ways to condition your crawl:

  • Balanced approach – install a supply and a return in your crawl space.
  • Supply only – install a supply duct into your crawl or install an inline fam from the upstairs into the crawl.
  • Exhaust only – install an exhaust fan in the crawl exhausting to the outdoors – 1CFM per 50 square feet of crawl area.

For new construction, all three options work fine. For a retrofit on an existing crawl that has a musty smell, I prefer the exhaust only option to be sure that the musty crawl air does not get into the home.

If you want to see the science behind it, Advanced Energy has a great website that explains everything in detail. Basically the concept is you are pulling conditioned air into your crawl which will have less moisture in it than the typical Spring and Summer air thus lowering the dewpoint in the crawl so you will be avoiding condensation in the crawl.

All closed crawls must have a full 100% vapor barrier over all ground cover and wrapping up all piers and the exterior foundation anywhere it is below grade to control the moisture. Even better tape all of the seams. Typically there is not any insulation in the floor system and the exterior walls of the foundation are insulated with rigid foam or closed cell foam.


For more information:

Check our basement moisture Q&A to see what other homeowners and contractors are saying about waterproofing, mold removal, etc.

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