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Spokane, Washington  Est. May 19, 1883
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Ask the Builder: Finding answers to crawl space confusion

Tim Carter Tribune Content Agency

DEAR TIM: I have a 1,100 square-foot crawl space, and the slate floor above it is ice cold because there’s no insulation. My head is spinning from all of the conflicting information I’m seeing online about crawl space encapsulation. Should it be done, or should I just rely on the traditional vents I have in my foundation? What would you do if this were your home and why? – Richard J., Toledo, Ohio

DEAR RICHARD: I understand your frustration. While the Internet is an amazing resource, the barriers to entry with respect to publishing are lower than the bottom of Death Valley. If you can fog a mirror and type on a keyboard, you’re capable of posting your opinion about anything online.

To stop your head from spinning, let’s dive into the science of crawl spaces. A traditional crawl space under a home consists of soil or sand – essentially, what was the ground before construction started. Sometimes the topsoil is removed, but what you see is bare soil. I’ve been in crawlspaces where concrete or washed gravel was poured on top of the soil.

Unless you live in the Atacama Desert, the ground under and around your home contains moisture. This water wants to evaporate and get back up to the atmosphere. If you could see water vapor, you’d see a constant flow of this gas floating up into the air. When it’s warm, the flow rate is faster.

When water vapor meets wood it’s not a good thing. If enough water vapor collects in a crawl space before it can find its way to the outside atmosphere, it can condense and turn to liquid water again. This water fuels mold and fungi growth that you might call wood rot.

Old builders knew about this. The best they could do was to provide an escape path of the water vapor to the outside. That’s what the crawl space vents are for in your foundation. The trouble is they don’t work too well. I’ve been in crawl spaces when the wind was howling outdoors and barely a puff of air came into an open foundation vent.

Once plastics were gaining traction in the 1960s, thin sheets of vapor barrier were available. While not perfect, they did a magnificent job of blocking the movement of water vapor. Cross-laminated vapor barriers that meet or exceed the prevailing technical standard (ASTM E 1745) are some of the best products out there to block water vapor.

An entire industry has evolved in the past 15 years that specializes in encapsulating crawl spaces so the water vapor stays is the soil. When done correctly, encapsulation is an excellent way to arrest the movement of water vapor from the soil up into your home.

Realize that other gases can escape from the soil under your crawl space and cause any plastic or membrane to puff up like a balloon. Be sure to discuss this possibility with any contractor you’re getting bids from.

These same high-performance vapor barriers should be placed under poured concrete basement floors to stop water vapor movement. The same is true for any house that uses a slab-on-grade foundation. Water vapor can and does pass through poured concrete.

Once you’ve encapsulated the soil and stopped the water vapor from entering your crawl space, you should insulate the floor joists. Be sure there’s no mold on the wood. Clean it off using oxygen bleach if need be. Just mix it up, and spray it on the wood. Allow it to dry before you insulate. In fact, I’d do all this mold remediation before installing any encapsulation product or system in the crawl space.

Some crawl spaces are plagued with standing water or running water in periods of heavy rain. You can stop this water from entering a crawl space by installing a linear French drain around the outside of your home. My college course in hydrogeology taught me how to capture and divert water in the soil. Many homeowners like you have used my technique to dry out their wet basements and crawlspaces. Go to and type “linear French drain” to discover how to dry out a wet crawl space or basement.

Carter’s past columns are archived at You can also watch hundreds of videos, download Quick Start Guides and more, all for free.
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