Subfloor Insulation: To Replace or Not Replace

By · February 17, 2010 · Filed in Consumer Q & A's

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 psychrometric chart 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, or “breathe”, 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.

If you just want the bottom line, don’t read the following story that lead me to my conclusions. For the rest, I share this story, in addition to my anecdotal evidence which has shown that more crawlspaces with subfloor insulation have problems than those that don’t have subfloor insulation.

A realtor friend of mine had mold in his crawlspace, in addition to extreme condensation on his ductwork. It was raining off the ductwork about like the rain forest. Now, I must add, his wife liked to keep it cold enough to pack meat inside the house, so the ductwork had very cold air in the ducts. My hypothesis was that the cold air was leaking out of the seams in the ductwork, and thereby, disabling the ductwrap insulation. My proposal was to remove the subfloor insulation so we could do a complete job of mold remediation, remove the duct insulation and seal the seams in the ductwork with duct mastic, and reinsulate the ducts with the highest R value insulation available. He accepted the proposal and we did the work. I went back after a week, to inspect the work, and found that there was no condensation on the ductwork. I concluded I was a genius, because I figured it out.

At that meeting, he wanted us to give him a price on reinstalling the subfloor insulation because he was concerned about getting “gigged” by a home inspector for not having it. While Home Inspectors are not Codes Inspectors, I have gotten conflicting answers on the requirements of subfloor insulation from several conversations with Codes Inspectors. Anyway, we gave him a price, he accepted it, and we reinstalled the subfloor insulation. I went back after a week, and we found the ductwork to be sweating again. This project was done during warm, humid summer months, so the results were pretty immediate. I concluded that the subfloor insulation can trap the humidity in an “open” crawlspace and create a dewpoint situation on the ducts, and a subsequent mold situation on the wood subfloor.

If the crawlspace is conditioned, I don’t think the subfloor insulation makes much difference, because the humidity is much lower in the crawlspace. Sometimes, the absence of subfloor insulation can be the tipping point of preventing mold growth. Other crawlspaces need more aggressive solutions, such as the Extreme Makeover, the dehumidifier installation, or the Crawlspace Conditioner.

Please call 615-371-5355 to schedule a customized solution for your crawlspace.

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