Is pouring concrete in a crawl space a good idea?

No. While a concrete slab may look cosmetically appealing and clean, pouring wet concrete into a crawl space actually worsens indoor air quality. It introduces massive amounts of moisture into an enclosed area, creating a prime breeding ground for mold that can easily migrate upward into your home's living spaces.

Why does a concrete crawl space floor cause mold?

A typical concrete mix contains over 350 pounds of water per cubic yard. When pumped into a tight crawl space, it completely saturates the environment with moisture. Additionally, concrete does not provide a thermal break. The temperature difference between the air and the concrete surface causes condensation to form, which leads directly to mold growth and wood rot.

Does a concrete crawl space floor create harmful dust?

Yes. Because crawl spaces are typically too tight for finishers to properly hard trowel the surface, the poured mass is usually just smoothed with a hand float. As it dries, it leaves a chalky finish that disperses silica dust into the air whenever it is scraped by boxes, contractors, or even children at play. Inhaling this silica dust can cause serious respiratory irritation and lung damage.

What happens if I install a vapor barrier before pouring the concrete?

Installing a vapor barrier under a concrete slab in a crawl space traps the concrete's water. Because the moisture cannot seep downward into the soil, it is forced to escape upward into the crawl space atmosphere. For a 1,000-square-foot crawl space, this means releasing over 500 gallons of trapped water directly into the floors and walls of your home as the slab cures.

What is the best alternative to pouring concrete in a crawl space?

Instead of using porous, wet concrete, the safest and most effective solution is professional crawl space encapsulation. This involves installing a heavy-duty, reinforced vapor barrier liner over the soil and walls. Encapsulation permanently seals out ground moisture, prevents mold, and provides a clean, dry storage environment without introducing thousands of pounds of water into your foundation.

Cosmetic Appeal of Poured Concrete Hides Serious Crawl Space Hazard

IAQ Press Release : February, 2009 – Las Vegas, NV

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At first glance, pumping concrete into a crawl space may seem logical to homeowners. After all, concrete slabs work well in the garage and basement, and it creates a nice finished look. But sealing and insulation experts say – look again. The cosmetic slab of concrete pumped into a crawl space after new home construction is different than for a basement poured during construction. It is actually worsening the indoor air quality problems it was meant to fix. The result may be the onset of “sick” house syndrome.

The number one reason a homeowner seals a crawl space is simple: it is a wet, musty breeding ground for mold. If left unchecked, mold will migrate upward into a home’s living space and create a costly health hazard. Nasal and sinus congestion, coughing, wheezing, sore throat, headaches, fatigue and eye irritation are among the common symptoms.
“Indoor air quality is something you can’t take lightly. There are over 16 million people in the United States who suffer from asthma alone. About 7 million of them are children,” says Tim Wilkerson, a crawl space expert and building science consultant.

The fact is, choosing wet, porous concrete to seal a crawl space is like hiring a wolf to guard the chicken coop. Why? A typical mix contains over 350 lbs. of water per cubic yard of concrete. A homeowner who agrees to have concrete pumped into a crawl space is unknowingly saturating the entire area with mold-friendly moisture. And don’t think the environment improves after the concrete dries. In a spacious basement, it is easy for finishers to hard trowel, or close, the concrete surface. Crawl spaces are too small and tight for that technique, so finishers merely top off the poured mass with a hand float. As a result, the slab dries with a chalky finish that disperses dust into the air when scraped by boxes or contractors – or children at play.

Unfortunately, the dust contains silica which can cause serious lung damage. So much for the “finished” look. Installing a vapor barrier – an absolute necessity – before pumping concrete also creates problems. Since the barrier prevents concrete water from seeping into the soil beneath the house, it can only escape upward. That is a lot of moisture when you consider that the concrete required for a 1,000 square foot crawl space with a 4-inch slab contains more than 500 gallons of water (which will be entrapped within the floors and walls of the home).

“I realized pumping concrete into the crawl space was actually a disservice to the homeowner. Cosmetically it may look great, but we all need to take a closer look.”
-Tim Wilkerson

Also, since concrete does not provide a thermal break, a temperature difference between the air and the concrete surface will cause condensation. And that leads to more mold.

Wilkerson knows the downside of pumping concrete. For years he too thought it was an acceptable solution for crawl space remediation. But when he discovered the moisture problems, he began experimenting with everything from water reducers to light weight concrete with the hope of developing a drier mix.

Although he succeeded in lowering the 350 lbs. per cubic yard requirement to 300 lbs., moisture and mold problems continued to persist.

“I realized pumping concrete into the crawl space was actually a disservice to the homeowner. Cosmetically it may look great, but we all need to take a closer look. A little education can save lives,” says Wilkerson.

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Frequently Asked Questions About Concrete in Crawl Spaces

No. While a concrete slab may look cosmetically appealing and clean, pouring wet concrete into a crawl space actually worsens indoor air quality. It introduces massive amounts of moisture into an enclosed area, creating a prime breeding ground for mold that can easily migrate upward into your home's living spaces.
A typical concrete mix contains over 350 pounds of water per cubic yard. When pumped into a tight crawl space, it completely saturates the environment with moisture. Additionally, concrete does not provide a thermal break. The temperature difference between the air and the concrete surface causes condensation to form, which leads directly to mold growth and wood rot.
Yes. Because crawl spaces are typically too tight for finishers to properly "hard trowel" the surface, the poured mass is usually just smoothed with a hand float. As it dries, it leaves a chalky finish that disperses silica dust into the air whenever it is scraped by boxes, contractors, or even children at play. Inhaling this silica dust can cause serious respiratory irritation and lung damage.
Installing a vapor barrier under a concrete slab in a crawl space traps the concrete's water. Because the moisture cannot seep downward into the soil, it is forced to escape upward into the crawl space atmosphere. For a 1,000-square-foot crawl space, this means releasing over 500 gallons of trapped water directly into the floors and walls of your home as the slab cures.
Instead of using porous, wet concrete, the safest and most effective solution is professional crawl space encapsulation. This involves installing a heavy-duty, reinforced vapor barrier liner over the soil and walls (like our Therma-Seal-System). Encapsulation permanently seals out ground moisture, prevents mold, and provides a clean, dry storage environment without introducing thousands of pounds of water into your foundation.