Crawl Space Dehumidification

Having a Crawl Space Dehumidifier is also just smart!

A good, quality crawl space dehumidifier that is of the appropriate size for the crawl space it is installed in will make the difference between a unit that gets the job done efficiently and one that does not but makes you feel good… until you get your electric bill, that is.

Not all crawl space dehumidifiers are created to do the same job. Yes, they are all made to dehumidify, but their features and capabilities vary. So when someone has a unit designed to dehumidify a bedroom but they are using it to try and dehumidify an entire crawl space, they are unfortunately going to be very disappointed with the results. The quantity of moisture, pints per kilowatt hour (P/kWh) or liters per kilowatt hour (L/kWh), that the dehumidifier removes from a space is really the determining factor to indicate how efficient a unit is.

Essential to proper dehumidification is proper air flow and proper water removal, in that order. If the moist air is not getting to the crawl space dehumidifier, then it won’t matter how much water the unit is capable of removing. Placing the dehumidifier in the right location of the crawl space and implementing some fans will ensure the proper flow of air to the dehumidifier. Ensuring that the water is properly draining into a drain or outside of the crawl space should guarantee your crawl space is dehumidified properly.

Depending on your climate, your heating and cooling unit can control the moisture in your crawl space once you open the supply and return; this method is allowed by the building code (IRC R409.5.5 conditioned space). This method is better than only having a crawl space dehumidifier installed for several reasons:

1) You already own the equipment
2) This method will make the home more comfortable in the winter months
3) Your home gains some geothermal benefits from the naturally cooler temperatures in the crawl space during the summer months
4) Your home will be more energy efficient

Although, in order for this method to work in your favor on all points, the exterior foundation walls will need to be insulated according to your local energy code (usually 1? R-5 rigid in the south, and 2? R-10 in the north).

Just as not all basements need a dehumidifier because some have conditioned air, the same can be true that not all crawl spaces need dehumidifiers—though most do. Homes located in climates where the dew point is above 55 degrees will most likely need a dehumidifier in the crawl space. This is because if the temperature equals the dew point, that will result in 100% relative humidity, which is really not good. So this means that the higher the dew point, the greater the chance is of maintaining high humidity.

To provide an example, a contractor went to work in a crawl space just outside of Greensboro, NC, and inside this crawl space the temperature was 60°F with 83% humidity. Inside the home, the temperature was 72°F with 55% humidity. The home was in good shape, but the crawl space was not. With those numbers, a mold problem could develop in as little as 9 days. The crawl space was conditioned with air from the home via the A/C. The outside dew point was 67°F, and the A/C was able to keep that down to 55°F inside the home. The air in the home is the exact same air in the crawl space with the exception of the temperature. This is where things get tricky: when the air is cooled, it becomes more dense as it gets closer to the dew point. Once the temperature reaches the dew point, the moisture leaves the air and condenses, settling in the cooler parts of the home via 100% humidity. So if the humidity in the home were at 40%, the crawl space would be at 55% under this same scenario.

So as this shows, the A/C alone could not drive the moisture content down far enough to keep the threat of mold growth out of the crawl space. The outside dew point directly affects the impact of the A/C unit. Though at this same time, in the northern part of the country, in Michigan, for example, where the outside dew point is lower year-round, there is less of an impact on whether the A/C can effectively dehumidify the crawl space. With a house at 72°F, the humidity would be 38%. With the dew point outside being 44°F, instead of 67°F, the crawl space humidity would be 55% at the same 60°F temperature.

We apologetically understand how this may be confusing. There are two tools that can help to tell you what you will need to know, a hygrometer and a dew point calculator. The hygrometer can be used to monitor the humidity in the crawl space with its wireless remote. The dew point calculator can be used to determine how best to alter the crawl space environment with either dehumidification or a raise in temperature. In addition, we are of course always available and glad to help answer any questions.