Subfloor Insulation: To Replace or Not Replace

Most of our Crawlspace Makeover projects will not include replacing subfloor insulation when it is removed for mold remediation purposes. There are several reasons for this. We have found it can create more problems than it purports to solve. In the Southeast, our climate is warm and humid most of the year. The only time subfloor insulation would be of any benefit would be in the winter, and even then, only a few days would make a noticeable difference in the temperature of the floor. The rest of the year, the insulation acts like a sponge to trap the humidity in the crawlspace. This creates several problems.

Have you ever seen or heard of sweaty ductwork in the crawlspace? The correct description would be that condensation is forming on the duct insulation due to dew point conditions being reached. Dew point is a function of temperature and humidity. See graph below.

The temperature is graphed along the bottom of the chart. The relative humidity lines are the ones which curve upward and their values go across the top and down the right side of the chart. The dew point values are indicated on the left of the chart curving upward. The illustration shows that at the intersection of 80?F and 50%RH, the dew point is 60?F. This presents several problems. When you set your air conditioner at 70?F, the air coming through your ducts is typically around 50?F. The duct insulation normally installed is not adequate enough to keep the exterior of the duct insulation much different than the air temperature inside the ducts, especially considering any leaks or gaps in the insulation. Therefore, water will probably condense on the ductwork in these conditions. Unfortunately, we usually have more extreme conditions. If you follow the 80? temperature line up to the curving RH line of 80%RH, you will see the dew point is a little above 70?F dew point, where it will most certainly create condensation, and possibly mold.

When the subfloor insulation is present in a crawlspace which has the foundation vents open, the humidity is trapped in the crawlspace. As the condensation drips, it eventually evaporates and raises the relative humidity even more. Continued high humidity is absorbed by the wood subfloor and reaches a moisture content level high enough to grow mold.

So what is the solution?

Increasing the R value of the duct insulation to a level high enough to prevent condensation is the most expensive route and does nothing to prevent the high humidity normally found in open crawlspaces from increasing the moisture content of the structural wood components in the crawlspace to a level which might support mold growth.

The least expensive way to prevent dew point conditions in the crawlspace is to leave the subfloor insulation out and let the humidity make its way through the subfloor to be dehumidified by the air conditioner or exhausted out of the attic.

The best way to solve the problem is to close all of the foundation vents except those utilized in our Crawlspace Conditioner, so that drier air is circulated in the crawlspace and more humid air, along with any earth gases, like radon, is exhausted from your crawlspace and never makes it into your home. Ask our sales representative to provide you with more information about the Crawlspace Conditioner if you are interested.