Help! It's raining in my crawlspace, and it smells like my high school gym locker!

A few years ago, I received a phone call from a surgeon in a nearby town. He was the chief of something at a big hospital and had recently built a beautiful new home on some acreage.

The house had been built on a crawlspace instead of a slab on grade due to local soil conditions. This happens here in central Texas with our highly expansive, heavy clay soils. They had an engineer design a foundation plan. They put down deep piers to the bedrock and laid concrete beams across them to form a solid concrete floor/foundation on which to build. The crawlspace had more ventilation area than the code minimum and it was evenly distributed. Seems fine so far.

All was well until they noticed a musty odor in the house. Upon looking down into the crawlspace, they found that there was an inch of standing water down there! When they ventured down into the crawlspace, they found that the entire concrete deck forming the underside of their new home was covered in a light sheen of condensation!

The immediately contacted their builder who recommend that they consult with a mechanical engineer in the city. They had the consulting engineer visit the house and he said that the problem was too little air movement through the crawlspace and recommended that they install two ventilation vans each rated at 1,200 cubic feet of exhaust flow per minute. This would draw a total of 2,400 cubic feet of airflow per minute through the crawlspace on a continuous basis. They were assured that this would surly dry that damp crawlspace out.

They dutifully did what the consulting mechanical engineer had recommended and moved on with confidence. Then one day they noticed that the musty odors had not abated, but in fact had become more pronounced. They opened the scuttle door that allowed access to the crawlspace and were truly stunned at what they saw. The standing water in the crawlspace was now almost a foot deep! When they went down into the crawlspace (now wearing rubber wading boots), they felt what seemed to be a light, but steady rain on their heads. The sheen of condensation on the concrete ceiling of the space was now a solid cover of big water drops that were raining down and filling the crawlspace.

They talked to the engineer who was stumped to explain it. The builder had been to one of my building science and energy code workshops a few months before and thought that maybe I might be able to help. I went to the house and donned my rubber boots for a trip down into the crawlspace. Yep, it was full of standing water and the entire underside of the concrete foundation was covered by big drops of water like so much liquid bubble pack.

It was then that I heard a tell tale sound. The drone of a ventilation fan. I started to laugh. This earned me a quizzical look from the owner who didn't seem to find any humor in the situation. I asked him if they had installed a powered vent fan. He said yes they had done so recently. I responded by saying, "And it was shortly after that you noticed the water level in the crawlspace had greatly increased, and it had started to actually rain down here." He confirmed that this was correct. It was only then that he told me the full story including how the fans, "plural," had come to be installed. He said that he hadn't mentioned it because it didn't have anything to do with the problem.

I knew what the problem was and how we were going to fix it to make it go away forever. "Let's get out of here" I said. I had him cut the power to the fans and promise not to ever turn them on again. They wanted an explanation of why, so here goes:

The exposed soil in crawlspaces evaporates about 12.2 gallons of water per day for each 1,000 square feet of area. That's the rate if the soil is dry; it's a larger amount if the soil is wet like theirs was. This is why crawlspaces are almost always damp and musty. The code calls for ventilation with outside air to supposedly dry the crawlspace out. I explained to them that this code requirement was based not on science but on the experience of builders in the northern U.S. So, what does that mean they asked? Well, in the northern part of the country, the outside air most of the year is cold and cold air cannot hold much water, so it is very, very dry. It's so dry in fact that it is common in the north to have to humidify the air in your house to be comfortable. I reminded them that in south central Texas, our air is almost always humid, very humid. We only have a need to dehumidify and we do lots of that.

A basic fact of building science is that there is no such thing as a best building practice. The national codes have made this mistake for decades thinking that there is one best practice for all of America. This has lead to disasters. There is a best building practice for every climate, and you must know your climate and how to build in harmony with it.

The explosion of mold in the walls of homes across the south a decade ago can be mostly attributed to a building code requirement that all walls in the country had to have a one perm vapor barrier on the "warm in winter" side of the wall. That's a good building practice if you are building in Boston or Chicago, but it if you live in Tampa or Houston, it stops walls from drying and turns them into mold factories. It's all the fault of a little law of physics called the Second Law of Thermodynamics if you are interested. This law tells us which direction heat and moisture flow in building assemblies among other things.

Research has shown that in humid areas, the ventilation with outside air actually makes the crawlspace wetter on an annual basis, not drier! I told them that trying to dry things with our outside air was a lot like trying to dry yourself off with a towel that was just floating in the pool. It's too wet to do anything but make you wetter.

This left them a bit worried. If the ground has all of that water evaporating and the air is too humid to dry stuff, what can we do? I explained that we can unventilate the crawlspace. This didn't go over well at first; like me, they had been raised in Texas where all crawlspaces were ventilated . Even the local building official was at first totally in opposition to the idea. He said that the building codes required the installation of outside air ventilation.

I knew better since I had been the energy code trainer for the State of Texas since 2000. I showed him that since the 2000 building codes, there has been an option in the codes (R-408) allowing one of two ways to build a crawlspace. One is the old ventilated crawlspace and the second was a sealed, unventilated crawlspace. He hadn't noticed the change and was truly amazed that such a thing was in fact acceptable. I explained the two sources of moisture, the water evaporating from the soil and the humidity present in the outside air and how if we eliminated both of them, we would have a dry, healthy space. He agreed that this made sense and we moved forward.

The solution:

  • The exhaust fans were removed - I told the owner that the only good use I had found for exhaust fans once removed from either attics or crawlspaces was to bolt one down to a 2x8, attach a grinding wheel and make it a bench grinder.
  • All of the ventilation holes were sealed using 1 inch thick (R-5) blue Styrofoam from Dow and caulked at the edges for an airtight seal.
  • We installed a French drain around the perimeter to control rising ground water and sub-soil water flow from uphill.
  • The floor of the crawlspace was covered by a layer of 6 mil plastic, overlapped at the seams and sealed to the exterior wall and around all of the piers.
  • Since the floor of the house was concrete, we couldn't get supply a/c ducts down there, so we installed a 60 pint/day dehumidifier with a humidistat control set at 50% RH and a condensate hose to gravity drain into a pipe that lead to daylight outside.
  • We insulated the crawlspace walls with 1 inch thick Dow Thermax foil faced foam insulation which meets fire code and doesn't need to be covered to keep the space at a comfortable temperature and stop the pipes from freezing

That was over a decade ago and all has been odor free, dry, clean and healthy in that space since.

...Just another day in the life of a forensic building science geek.

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