Lowering Crawl Space Moisture & Humidity- Explained
Relative Humidity (RH) is the byproduct of two conditions in the atmosphere, temperature and dew point (amount of crawl space moisture in the air). To control or alter the RH you must manipulate either dew point or temperature.”Dew point temperature is the absolute measure of how much water vapor is in the air”- Source. The dew point is always less than the air temperature. The closer the dew point temperature is to the air temperature the closer the RH gets to 100%. Example- Dew point temperature 70F and air temperature 70F = 100% RH
If the home has an encapsulated crawl space, then the dew point inside the house will be different than that of outside the house. Air temperature will actually dictate what the RH is at any given dew point temperature. Inside the home we keep the air temperature at a comfortable level, say 70 degrees (which I am assuming is this case). A crawl space that has been sealed and encapsulated will maintain an air temperature similar to inside the living area.
With that said, similar air temperature means about 5 degrees cooler than the living area. This is due to the consistent temperature of the earth under the vapor barrier, a geothermal effect. This is good in many ways for the home, because the air temperature will remain between 60F – 68F degrees most all of the time. Back to our RH problem, so the house (assuming ) is 70F degrees (air temperature) puts our crawl space at (again assuming but will be close) 65F degrees (air temperature). In this scenario I will need to know either the dew point or RH to figure out the rest of the equation. Once I know the dew point I can show you what is going on in the crawl space and why it’s happening.
House air temperature: 70F/49% RH
Crawl air temperature: 65F (house temperature minus 5 degrees)
Dew point temperature in the living area- 50F degrees
Using a dew point calculator to figure out RH in the crawl space with this information-
Crawl air temperature (assumption): 65F
Dew point temperature inside the home: 50F (also the dew point in the crawl)
Crawl Space RH is 60%
So you can see there is an 11% difference in RH levels between the home and the crawl space. The only difference is the air temperature between the two spaces. If you raise the air temperature in the crawl space to 70F, same as the living area, you will have the same RH 49%. Setting the crawl space dehumidifier at 49-51% means it is trying to lower the dew point temperature in order to achieve a lower RH.
To successfully lower the dew point temperature in the crawl space you must lower moisture for the entire house. This is quiet a task for any dehumidifier when you consider the traffic in and out of the house, the amount of showers taken, moisture from cooking, moisture in the wood inside the home and the natural moisture loss from each person living in the home. It will cause the dehumidifier to run more then 5 min an hour.
If you wish to accomplish a shorter run time for the crawl space dehumidifier, then you can raise the air temperature in the crawl space. This can be done by creating a conditioned crawl space with air from the home, helping to deliver heat to the coldest places- the crawl space. Of course when you raise the temperature to control RH, you make the RH level much more critical in the mold growth equation (explained better in the next paragraph). The other option is to set the controls on the dehumidifier to a higher setting, maybe 58-60% (based on 65F degree air temperature).
There are several factors in crawl space mold growth; dew point temperature is only one of them (amount of moisture in the crawl space air), the other being air temperature. RH is simplified as a mixture of dew point temperature and air temperature. This simplification does not allow for any absolute rules, for example the myth that crawl space mold will grow once RH reaches 55%.
As a convenience to avoid teaching in depth classes on dew point temperature and air temperature each time a crawl space dehumidifier is sized and installed it is widely expressed- ‘to avoid mold growth, a safe RH is below 55%’. But, once RH is really understood that statement does not hold enough information. Last week, here in Michigan, our outside RH was 85% but the temperature was 31F degrees. Mold growth is not imminent in this equation because the air temperature is far to cold, but under the understanding of “less than 55%” it would be widely considered a mold growth threat.
Just like RH is tweaked using air temperature, so is mold growth. In a crawl space that has an air temperature below 70F will not generally be threatened by mold growth until the RH rises past 67%. Keeping an encapsulated crawl space that has an air temperature of 67F and an RH of 58-60% will produce excellent results in the battle against wood rot, odor, mold and mildew growth and a high electric bill caused by the crawl space dehumidifier. Of course the owner if the house has to add the crawl space to the list of maintenance duties required by the home, bi-annual crawl space inspections are strongly recommend.