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Crawlspace Mold and Summer Heat

~ By Jesse Waltz, Professional Engineer

A hot summer, and an unsealed crawl space with a dirt floor can create big problems for your home and health. As a result, this combination invites wood rot and crawlspace mold, while threatening the health and safety of your family.

In particular, it all starts with the air in your crawl space. Did you know that you breathe the air in your crawl space? It’sCrawlspace Mold and Summer Heat by Jesse Waltz, Professional Engineer true. The air is cycled through your home by the stack effect.

Here’s how it works: As you know, warm air rises. As the warmer air rises through your home, it carries the air that was previously in your crawl space and anything that is light enough to travel with it. This includes moisture, mold spores and even rodent fecal matter.

As this air rises in the home, replacement air enters from the lowest zone, the crawl space. Furthermore, this replacement air is made up with unconditioned outside air that enters through vents and other leaks.

As a result, whatever’s in your crawl space air ends up cycling through the living sections in your home. In fact, over 50% of the air you breathe on the first floor of your home comes from the crawl space.

“Over 50% of the air you breathe on the first floor of your home comes from the crawl space.”

If you have a dirt crawl space with open vents, then you have a never-ending source of moisture. Even if the dirt’s surface seems dry, damp earth is revealed only a few inches below. Furthermore, this moisture is constantly released into the crawl space.

As the temperature rises, so does the amount of moisture that’s released from the ground. As you can see, the hot and dry weather can create a problem when combined with your crawl space.


Crawlspace Mold and Summer HeatEncapsulating your crawl space is the best solution to moisture control. Some folks like to cut corners by laying down a thin, 6-mil, non-UV-resistant plastic liner.

This cheap “solution” has little-to-no effect since there’s still a lot of ways for air to enter. For example, crawl space doors, hatches, vents, leaking pipes, porous concrete walls, concrete block cracks, and other gaps or holes can all let moisture into your crawl space.

All of that extra moisture leads to mold growth, musty odors, and structural damage; not to mention, insects and critters love moist environments. Additionally, energy costs are higher. Moisture ruins houses by providing a hospitable environment for mold and other fungi, as well as insects that destroy wood framing.

Crawl space moisture, and the mold and mildew that thrive in this environment, affect the floor support system and the entire house. Three things destroy organic materials in general, and wood in particular: water (moisture or high relative humidity), heat, and ultra-violet radiation. Of these, water is by far the most damaging.

“Three things destroy wood: water, heat, and ultra-violet radiation. Of these, water is by far the most damaging.”

Crawlspace Mold: How to Stop Crawl Space Moisture

Crawlspace mold and Summer Heat: How to Stop Crawl Space Moisture

Properly sealing the crawl space and removing the moisture from the ground and air is part of the solution. This helps provide a mold-free and insect-free environment, which leads to a more energy-efficient and healthier home.

Furthermore, to avoid moisture’s negative affects, a crawl space should be completely sealed. This strategy is also known as encapsulation.

On the other hand, an effective method in lowering crawl space humidity is installing a crawl space vapor barrier system.

This encapsulation system, called CrawlSeal, is a 20-mil 7-ply sandwich of high and low-density polyethylene with polyester-cord reinforcement. It’s installed over the dirt floor and walls.

The extra-heavy, reinforced CrawlSeal crawl space liner is also treated with an antimicrobial finish that protects against mold and mildew. It warrantied for 20 years and is tough enough to crawl on and place storage items in your crawl space.

In some cases, a high performance crawl space dehumidifier is installed to ensure that humidity levels stay under 50% – below the level at which mold can grow. Before this is installed, also consult with your HVAC contractor to make sure venting is adequate.

Years ago, the theory was that crawl spaces would become dryer if they were vented. On the contrary, studies and experiences have proven the most overlooked space in the house needs to have its air and humidity conditioned.

Jesse Waltz

Jesse Waltz

Professional Engineer

Jesse Waltz, a Professional Engineer and a 1985 Virginia Military Institute graduate, is the founder of JES Foundation Repair. JES is a regional foundation repair, crawl space and basement waterproofing company with locations in Virginia Beach, Richmond and Northern Virginia.

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Request your free crawl space repair inspection and download your own copy of the e-book immediately, as our thanks for putting your trust in JES.

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