Improving the use of crawl space dehumidifiers

Crawl Space Dehumidifiers and Humidity Levels

Let’s explore a commonly used tool in the crawl space – a dehumidifier. And, let’s look at how we can improve the use of crawl space dehumidifiers.

Target Humidity Levels

In a previous blog post, we answered the question of “what should the relative humidity in the crawl space be?” As we stated there, the goal for healthy wood is to have humidity levels that remain mostly in the 55% to 75% range. So, if that’s the range, why are so many companies quoting 55% as a target maximum? And, why are they setting the dehumidifiers at that low humidity activation level?

There could be many reasons why 55% is being quoted or used. Some of it could be because of confusion between ideal indoor humidity inside a heated and cooled home versus a crawl space. Some could be just a desire to be aggressive on moisture. This would be okay if the dehumidifier didn’t use so much electricity and have a nearly impossible time achieving that humidity level in low temperatures. The main reason is likely due to a limitation of a dehumidifier in a larger space.

Dehumidifiers are sized by capacity to extract water from the air usually measured in pints per day. As this capacity increases, the dehumidifier can generally dry a larger area. The dehumidifier range is often driven not just by the extraction amount but on the size and power of the fan within the unit.

Drying Near Dehumidifier

In a typical setup, the installer places the dehumidifier somewhere in the middle of the crawl space. Look at the illustration of the crawl space below. The dehumidifier will continue to cycle air closest to itself first. Over time, the dehumidifier will reach areas further away from the unit and dry that air as well. However, the air closest to the dehumidifier will generally be drier than areas further away. When a crawl space is extremely low, this is even more prevalent as the air has a harder time circulating.

Drying Range of Dehumidifier

Now, go back to the target humidity range. If a dehumidifier is set at 55%, the air near the dehumidifier will certainly achieve this target. At some point the sensor in the unit will also read the 55% range. However, if we had a sensor in a far corner of the crawl space away from the dehumidifier, the humidity reading is going to be higher. In some cases, it can be significantly different. Some installers recognize this variation, and therefore use a 55% humidity set point on the dehumidifier to actually keep those far corners in the ideal range of under 75%.

To some extent, the overdrying in one area to achieve lower drying in another area will work as long as the area near the dehumidifier isn’t over drying the wood. However, this is also incredibly inefficient. As mentioned before, the dehumidifier may need to be more powerful and expensive than really needed. Additionally, it will certainly use more energy as it has to work longer and harder.

The solution is to balance the air flow within the crawl space to assist the ventilation to and from the dehumidifier. The limitation of most dehumidifiers in a crawl space is not that they are maxing out the number of pints that can be extracted each day, but rather it is that the internal fan cannot effectively get air to areas further away from the unit.

Dehumidifiers with Ducting

Some dehumidifier models have the option to add ducting kits on one or both sides of a unit. This can extend range in another direction. Here is an example with a dehumidifier with both a supply and return duct kit on it. As you can see in this example illustration, there are still disparities across the crawl space.

Ideally, we want the entire crawl space to have even and balanced distribution of air. The goal would be to avoid under drying and over drying within the space. The other goal is to avoid overuse of the dehumidifier. There is an easy way to improve the use of a crawl space dehumidifier: add joist-mounted fans to circulate air to and from the dehumidifier. This will allow for a lower humidity setting as we no longer have to go extra low to really stay within the 55%-75% range.

Dehumidifiers with ATMOX Circulation Fans

ATMOX joist-mounted fans work extremely well at circulating the air efficiently and quietly. With fans in the corners or further away from the dehumidifier, the reach expands and balances the humidity levels and improves the use of a crawl space dehumdifier.