Conditioned Crawl Spaces – A Really Bad Idea
By Barry C. Westbrook
Converting a vented crawl space into a conditioned crawl space is like jumping out of the frying pan and into the fire!
The building construction industry is keenly aware that vented crawl spaces create problems with mold and moisture. In fact, many home builders have abandoned the construction of vented crawl spaces and adopted the installation of so‐called conditioned crawl spaces. This approach has a few pros and a lot of cons.
To many, the concept of a conditioned crawl space makes sense. There are several variations, but the basic idea is to seal the floor and side walls with plastic, seal off the foundation vents, and supply heated/cooled air to the crawl space from the home’s HVAC duct system. The goal is to convert the crawl space into a clean, dry environment much like any room in the living spaces. Let’s see what is wrong with this assumption.
1.) HVAC 101 – Always ensure that air delivered to a room through the supply air ducts can freely return to the air handler.
Conditioned crawl spaces are configured so that some of the air intended for the living spaces is hijacked and sent to the crawl space. In most cases there is no return for this air. It is supposed to somehow find its way back through cracks and penetrations in the floor until it reaches the return air plenum on the first floor. This violates the first law of HVAC design.
Many defenders of this approach claim that the amount of air stolen from the living spaces is not significant and the air can easily find its way home. That assumption invites the question, “If the volume of stolen air is insignificant, how do we know it is enough to keep the crawl space sufficiently conditioned”? And “Do I want cracks and penetrations in my floor that will allow air to freely flow back to the return air plenum”?
The truth is clear and fundamental. You always want to balance the supply and return air systems. The volume of supply air should equal the combined volume of the return air plus fresh air make up. Otherwise, your home will suck. And when it sucks, air comes into the home through the paths of least resistance: from the crawl space, through wall cavities, and from the attic. When this happens air quality and energy efficiency are compromised. (See the diagram to the right courtesy of Energy Vanguard.)
Last month I received a call from a homeowner in Cookeville, Tennessee. He had followed the advice of a local contractor and conditioned his formerly vented crawl space. He thought it would be a good idea to re‐test the radon levels in his home, although he had tested the levels before he purchased the property, and they had been below the EPA Action Level of 4.
When he tested this time, the levels exceeded 100! This is a similar lung cancer risk as smoking ten packs of cigarettes a day.
3.) Moisture and Mold
Ironically, conditioned crawl spaces are vulnerable to the one thing they are supposed to prevent – MOLD.
Here is why a conditioned crawl space cannot prevent mold. Hot, humid summertime air is the primary cause of mold in crawl spaces, not moisture from the ground. This has been well studied and documented. Covering the ground and walls with plastic does not hermetically seal the crawl space. Outside air can still flow into the crawl space whenever there is a suction. We call that condition: negative pressurization.
Conditioned crawl spaces rely on the heating and air conditioning system to keep the crawl space dry. That only happens when conditioned air is being delivered into the crawl space. Unfortunately, this air supply is determined by a thermostat that is located on the main floor, not in the crawl space. When the thermostat is satisfied, there will be no air delivered to the crawl space. During the cool night time and early morning, the outside air may be cool but it still holds a great deal of moisture.
Ironically, the same cool air that is delivered to the crawl space to keep it dry can cause severe condensation problem. Several years ago, I received a call from an individual who had just completed construction of his dream home, complete with a conditioned crawl space. He told me that mold was covering the floor joists and sub floor in his crawl space.
He also told me that he had constructed the crawl space just like the one we proposed to him almost a year earlier. He also said that the local codes inspector and his builder assured him that the crawl space was configured properly. Obviously not. I agreed to inspect the crawl space and diagnose the cause of the mold. When I entered the crawl space I observed mold covering the entire floor system just as the owner had reported. The white plastic liner in the crawl space was covered with condensation and the temperature was 62° F. I looked further and found a six inch diameter duct continuing to dump cold air into the crawl space.
I called the homeowner and told him what I had found. I suggested he close the duct and stop super cooling the crawl space. He said he had given up and planned to convert the crawl space to a vented crawl space. I explained that this was unnecessary and that he only needed to make a couple of changes. I don’t think he really understood my explanation since he later paid a contractor to cut foundation vents in a poured foundation wall.
How to Do It Right
If you insist on building a house with a crawl space, here is our best advice on how to do it:
- Seal the crawl space ground with a plastic liner with a minimum 10 mil thickness. Extend the liner up on the walls of the crawl space to within two inches of the sill plate.
- Caulk the sill plate and seal up foundation vents with foam insulation.Install a soil gas/radon exhaust system that will pull air from beneath the liner and discharge it to the outside. This will prevent odors and radon from entering the living spaces of the home.
- Install a dedicated electronic dehumidifier in the crawl space set to 55% relative humidity.
- Install a humidistat in the crawl space with an alarm to warn if the humidity exceeds 70%.
Crawl spaces are a convenient place to install duct work and plumbing systems, but they pose a number of problems, particularly with respect to moisture management and radon. The construction industry continues to struggle with them. Some are pumping air in, some are pumping air out, and some do nothing but wait until the complaints come in. The solution requires understanding all of the factors affecting the crawl space. Failing to consider the crawl space as an integral part of the entire building system can result in some very bad outcomes.