Closed / Encapsulated Crawlspace with Dehumidifier

The concept of closing the vents of the crawlspace or building a crawlspace with no vents in new construction is relatively new in the construction industry and has only been an option in the last ten years or so. Building codes have been evolving to address these new crawlspace setups.

The concept is seal the crawlspace up with heavy duty plastic barrier both on the ground and up the side walls. The plastic barrier will suppress ground moisture and keep humid outside air from coming into the space. It is recognized that closing a crawlspace with no drying action is not sufficient as moisture will still be present in the crawlspace unless addressed. (Although that has been done by contractors as well, most agree that it leads to problems unless a drying tool is used.) In the closed crawlspace setup, the crawlspace is generally kept dry with the fullly sealed vapor barrier limiting moisture and through the use of a dehumidifier

While the theory is sound and it seems to be working in some homes, in reality, the applications are leading to issues and concerns. Here are some of the primary issues:

  • The closed crawlspace has no fresh air. Over time, some of these crawlspaces are beginning to have musty odors from beneath the plastic. As the entire crawlspace is sealed off with no outside ventilation, the odors have nowhere to go and are permeating up into the living space of the house.
  • The dehumidifiers that are primarily being used are often set on a low humidity level to insure that the crawlspace stays dry. Some of these dehumidifiers will run almost continuously particularly in colder temperatures when a dehumidifier is less effective. Running a dehumidifier so frequently can become very costly to operate based on its electricity consumption and also very energy inefficient. See more information on dehumidifiers.
  • The dehumidifiers are using the humidistat on the dehumidifiers which can be inconsistent. Additionally, the longevity of dehumidifiers that are running so hard all of the time is becoming an issue as many are starting to break down and needing replacement after only a few years. Dehumidifiers are worked even harder when the vapor barrier is not properly and completely sealed as the dehumidifier has even more moisture to overcome and remove.
  • Dehumidifiers also don’t reach all areas of a crawlspace evenly. Especially in larger crawlspaces, the wood in areas just around the dehumidifier is actually overdrying and sections of the crawlspace further away from the dehumidifier aren’t getting enough air circulation and moisture reduction. This can be mitigated with the use of dead space fans to improve air circulation with use of a dehumidifier. However, overdrying wood can be just as damaging as wood that is too wet and can sometimes have effects on hardwood floors cracking from being too dry.
  • There is a potential radon concern as any radon will be trapped in the crawlspace with no chance to escape.
  • Homes with any gas furnaces or other gas appliances located in the crawlspace are also of concern as any gas leaks or combustion issues will also be trapped inside the crawlspace.
  • Most homes with closed crawlspaces are starting to see issues about five years after installation of this setup. If you are experiencing this, see how the ATMOX System may be able to help in your situation to re-vent or partially open a closed crawlspace.
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