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Crawl Space Ventilation Tips

Tips on Ventilating Crawl Spaces

Ventilation of residential homes for many years was ignored. I believe the reason for this was based in the fact that many homes were self ventilating. Older homes had little or no insulation. We didn't have vapor barriers. Weatherstripping was unheard of 40 to 50 years ago. Drafts were common.

Basically, as we made our homes more energy efficient over the past 20 years, we have created moisture problems we never really had before. This is also true of crawl space ventilation. 

Homeowners, builders and remodelers have known for years that crawl spaces, especially in moist climates, need to be ventilated. Those that were not ventilated suffered. It was not uncommon for the wood subflooring, floor joists and any other wood framing member to begin to rot. Excessive moisture would cause hardwood floors over these spaces to cup and warp. All kinds of problems were noticeable.

Foundation vents became the standard ventilation technique. These vents often had sliding mechanisms which allowed them to be opened or closed. But when should you do what? Should they be open in the summer, the winter or year-round?

Well, there is no one answer. It depends on your climate. Homes in colder climates would benefit by having the vents closed in winter. This would help keep the room over the crawl space warmer. Homes in warm climates could keep the vents open year-round.

With the evolution of the high-performance vapor barriers, these foundation vents may prove to be a thing of the past. Numerous room additions in Cincinnati are now built without foundation or crawl space ventilation. The building code permits it as long as the crawl space is tempered.Tempered crawl spaces are those that are slightly heated. In other words, they are similar to a basement. Think of it. In a regular house with a basement, you don't have vents. The basement may have one slightly opened heat duct. Or it may get its heat from the leaks in the heating duct trunk lines or the heat radiating from the boiler pipes.

In any event, a crawl space can be treated the same way. Eliminate the foundation vents, install a high performance vapor barrier, and then have the heating person dump a little bit of heat into the crawl space.

This can work to eliminate many problems. Often there are plumbing lines that travel through crawl spaces. People are constantly trying to keep these from freezing in colder climates. The crawl space can now be used as a realistic storage area. Dampness will be negligible. Temperature swings will be a thing of the past.

If you have an existing crawl space that is ventilated, you may consider switching to an unventilated one. There are distinct benefits.

In the event you wish to keep your crawl space ventilated, the more air movement within the crawl space the better. This is true if you currently have a low performance vapor barrier. Don't count on the vapor barrier to solve your problems.

Treat your crawl space like your attic area. The more air the merrier! You can't have too much air change in your crawl space, especially if you have a poor vapor barrier.

If you decide to install a high performance product, be sure to follow the instructions. The vapor barrier is only as good as the installation! Be patient and take your time.


Author's Notes:

I received this email from Michael P, Cincinnati, OH. Here's how he helped with his crawl space problem.

"I live in Turpin Hills and wanted to thank you for your advice for sealing a crawl space. This is our third winter in our home and the basement is always much colder than previous basements from other homes. I did as you suggested and put heavy duty, thick plastic on the gravel and put doors on the opening of the crawl space. It's been a few weeks and I can see water droplets forming under the plastic. I may put a second layer of plastic for good measure. The basement is warmer and so is the room above the crawl space. I don't smell the mustiness either. Thanks again for the tip."


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