Very often a home has a crawlspace that for a variety of reasons does not manage humidity well and needs to be modified to close, encapsulate or seal the space and add a dehumidifier. The question of the advantages and disadvantages of each of these methods often comes up.

When an air ventilated, dirt crawlspace has limited or inadequate ventilation the temperature in the crawlspace can get lower during the hot summer months than the summer dew point humidity. Dew points in North Carolina during the hot and humid months can regularly get above 78 degrees and will average higher than 70 for extended periods. The dew point is the temperature in outside spaces where condensation will occur. To summarize the issue for crawlspaces, the cooling of the home from HVAC systems and HVAC ducts combined with poor ventilation and different construction add-ons (Decks, large front porch, additions, heavily wooded lot, etc…) can allow temperatures in the crawlspace to go below the average summer dew point and cause condensation and excessive moisture in the wood.

This can lead to heavy mold and fungal covering of the visible wood and moisture damage and even rot on some of the wood. Damp soils or water infiltration can cause similar issues and needs to be corrected for any solution but poor ventilation will create condensation and moisture issues even in crawlspaces with extremely dry soil and 100% coverage with a vapor barrier.

A great example is a glass of ice water outside during the summer. Condensation forms on the cold surface of the glass because it is colder than the dew point. Adding a sump pump next to the glass, putting a vapor barrier under the glass and grading the yard around the glass will not prevent this glass from having condensation. It is the exact same problem with many crawlspaces in North Carolina during the summer and fall. With this type of problem related to poor ventilation and cool temperatures, the solution often is to seal the foundation vents and convert the crawlspace to a closed crawlspace with a dry air supply (Dehumidifier or HVAC system) to prevent condensation and protect wood from water damage and future mold growth. There are multiple ways to achieve this.

The simplest and most cost effective way is to close or seal the foundation vents, ensure there is a good vapor barrier and install an appropriate dehumidifier in the space set up to run automatically with a humidity sensor and drain continuously either with a pump or a condensate drain.

The pro’s of this set up are:

  • potentially a lower cost than full encapsulation.
  • Good control of the relative humidity

The negatives to this set up are as follows:

  • A dehumidifier uses anywhere from 6 to 9 amps while running and will need to run for much of the day and night during the summer and fall months. This can cause a spike in the energy bill of around $30 to $60 per month or more. The dehumidifier outputs heat as high as 100+ degrees. With the added heat in the space during the hot summer months the HVAC systems may have to work harder to cool the home. This can also have a negative impact on the energy bill and an added load on the HVAC system. The dehumidifier introduces maintenance into the solution that many home owners may not be available to perform on a long term basis. The filter needs to be changed or cleaned, the condensate pump or drain needs to be cleaned and checked for backups in the drain. The system should be checked to ensure the coolant is not leaking and that the compressor is in fact working and dehumidifying properly. It is common for the compressor to fail but the dehumidifier still is blowing air (damp air) so no one notices the system has failed because air is coming out like normal. This introduces another issue, the risk of failure.
  • Risk of failure: If a crawlspace is permanently closed up to accommodate the dehumidifier and the dehumidifier malfunctions during times of excessive outdoor humidity, the crawlspace can get warm and muggy and mold can grow faster than the previous open vented crawlspace that stayed cool and wet. If the damp muggy crawlspace is allowed to persist then mold will return and remediation services may be needed again along with costly repairs or replacement of the dehumidifier. Many people may not mind monitoring the crawlspace humidity for the first year but it may not be realistic to expect a tenant or even busy homeowners to be diligent about monitoring the space and dehumidifier over the years after the initial installation.
  • Rodent and pest problems can persist in a closed crawlspace if extra care was not taken or possible to find and seal openings and gaps that allow the pest in. Removing the insulation is one of the only ways to find many of these openings that let pest in but also can let air infiltrate from the crawlspace into the home.
  • Finally and maybe most importantly is the concept of indoor air quality. A crawlspace is built with open foundation vents to allow for fresh air exchange. Many people are not aware that this is the main function of the foundation vents as many times that are really bad at humidity control. Dirt under a home is responsible for many air contaminates that can build up in a crawlspace. One of the better known soil born contaminates is radon gas. Homes with open foundation vents rarely have issues with radon and will pass the initial test done during the home inspection. When you permanently close the foundation vents the radon levels will go higher possibly now moving levels from lower than recommended standards to higher and can present a real health issue. It is recommended to re-test a home for radon after the vents have been closed or sealed for long periods of time but not many people actually do since the radon test is usually done at the time of a home purchase to check off a box and then not addressed again until the home is later up for sale. Radon may be the most well known issue but the soil also creates other gases like methane and ammonia from bacteria in damp soils, especially soils under a regular or even sealed vapor barrier that traps moisture in the soil. Even less known is the issue with many different molds and fungi that thrive in damp soil and exist under the vapor barrier and persist even with the use of a powerful dehumidifier. The dehumidifier cannot dry soils that are protected by a vapor barrier and unfortunately, the vapor barrier, no matter how well installed or even sealed, cannot prevent gases, odors, or mold spores from escaping into the air of the space. Crawlspace foundations and piers are constructed of concrete block or brick and mortar and these materials allow air, gas and mold spores to pass through no matter the material used. Now with the sealed foundation vents these air contaminates are trapped and get to levels that have a negative impact to indoor air quality. Recent studies show that closed crawlspaces that are extremely clean and very dry have 2 to 4 times higher levels of mold spores in the air than the ventilated crawlspace had prior to the project to seal it and prior to even cleaning and treating the mold. This data goes against the industry claims of better air quality with a dry clean but sealed or closed crawlspace.


  • The Pro’s of a properly sealed crawlspace using the “Healthy Home Method” with the SCV System are:

    • Lower energy bills (Documented by Advanced Energy and NC building code)
    • Better pest control and termite prevention
    • Ease of maintenance. Use of the existing HVAC system for the dry air source adds no additional maintenance and actually has been proven to lower energy use and the total load on the HVAC system.
    • Low risk of failure. The HVAC system is providing cool dry air into the space during the summer keeping the crawlspace dry and preventing conditions for mold and then the furnace or heat pump is providing clean filtered air for the space that is warm and dry in the winter keeping the space dry and preventing conditions for mold growth. If the HVAC system fails it is known immediately and the system is repaired before there will be an issue with returning mold into the space.
    • The most important aspect is indoor air quality. Closed crawlspaces in all forms have been shown to hurt indoor air quality the majority of the time due to the lack of fresh air exchange that was previously provided by the open foundation vents. Any conversion to a closed crawlspace should include an under-liner ventilation system to remove air contaminates from the source (mold and fungal spores, odors and soil gases from dirt under the liner) and allow for air exchange.

    The trick to understanding this is understanding that it is virtually impossible to install a sealed liner under a home that can effectively seal off contaminates like mold spores, odors or soil gases over time.

    Claims by most installers of the perfect thick crawlspace liner fail to take into account that the liner may be perfect but the quality of the poorly taped seams, the affect of people entering the space over time, and that the liner is installed onto porous concrete block foundations and CMU piers means the air contaminates will always find a way through. Concrete block or brick and mortar allow for moisture and air to pass through.

    So no matter how well you seal the liner to the foundation, the contaminates will find a way into the space. Studies show that over time these contaminates concentrate to unhealthful levels and the indoor air for the home will be affected. There are many more details to discuss on indoor air quality related to the crawlspace so please feel free to inquire directly for more information.

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