Q&A / 

Crawl Space Vents

DEAR TIM: Should I keep my crawl space foundation vents open year round? Are they really necessary? I live in Maryland and get all four seasons and temperature conditions. Stephen King, Lexington Park , MD

DEAR STEPHEN: There are thousands of people who wonder what to do with crawl space vents. In fact, years ago when I installed them in room addition projects I often thought about the science behind these smallish vents that didn't seem to let in much air at all into the crawl spaces. I would actually be in the crawl spaces on windy days and could barely feel a puff of air come through the vents.

Here is the reasoning behind crawl space ventilation. Take a clear piece of plastic and place it over what appears to be very dry ground in your yard. Put some boards around the edges to weight down the plastic so no air gets under the large plastic sheet. If you can do this on a sunny day you will get almost instantaneous results.

This vent is open now.  But should it be closed in winter and open in summer or closed in summer and open in winter? PHOTO BY: Tim Carter

This vent is open now. But should it be closed in winter and open in summer or closed in summer and open in winter? PHOTO BY: Tim Carter

I'll bet within minutes you start to see a slight fog develop on the underside of the plastic. Wait longer and the fog will turn to water droplets. The soil around your house and inside of crawl spaces is constantly liberating water vapor. Sunlight, wind and natural evaporation pull this moisture from the ground.

The moisture content of the soil drives this water vapor engine. People who live in the extreme arid parts of the Southwest USA have little water vapor escaping from the soil for much of the year. But even they have a monsoon season and the soil does get wet at certain times of the year. People who live in the Northwest, Northeast, Midwest, and Southeast have water vapor streaming from the soil year round.

In the open parts of your yard, this water vapor readily escapes to the atmosphere. But under a crawl space, this water vapor can collect and begin to condense on the foundation walls and the subfloor structure. If the subfloor is wood, the water can create mildew in short order and eventually the wood will develop wood rot.

The theory for many years was to install the crawl space vents so that outside air could get into the crawl space and lower the relative humidity. But I am here to tell you that I have been in many crawl spaces with plenty of open vents and I felt as if I was in a damp cave. I just feel the vents do little good to circulate enough outside air into the confined crawl space.

The better idea, in my opinion, is to install a high-performance vapor retarder over the soil in the crawl space in addition to the building code-required foundation vents. This vapor retarder needs to lap up onto the sides of the foundation and be secured to the wall with treated lumber or rot-resistant wood strips. The best vapor retarders come with special tape that allows you to permanently seal any seams where the vapor retarder laps over itself or is cut around objects that stick up through the ground in the crawl space.

These vapor retarders block the water vapor and effectively cancel out the need for the crawl space vents. But be careful: My theory may not be synchronized with your local building code if you decide to build another room addition. They may make you install them. No matter what you do, always be sure to check with your local building department and obey the building code in your area.

Some local building departments have adopted code modifications that allow a variation of my theory. They permit installation of the vapor retarders as I describe and do not require any foundation vents. But if you do not install vents, you must insulate the side walls of the crawl space and the heating contractor must pipe in a small amount of conditioned air into the space. Furthermore, the foundation insulation must be fireproof or if it is not, it must be covered with a fireproof material.

The bottom line is if you install the high-performance vapor retarders correctly, you can forget about your vents. I don't care if you keep them open or closed, it will make little difference.

Crawl space ventilation is another prime example of how the building code can be argued to be a set of minimum specifications. The building code is indeed a wonderful set of regulations, but it by no means offers a builder or a homeowner the best possible way to do a task. There are countless examples of where builders can go beyond the code and do extra work or use better materials that will produce a home that will last hundreds of years instead of perhaps 40 to 70 years.

Water is a serious menace when it comes to residential construction. It can create havoc and misery when leaks develop or even simple water vapor condenses on crawl space floor joists, attic framing or even exterior walls. The best defense against water vapor problems in crawl spaces is to keep the water in the soil and don't let it enter the crawl space at all. The same is true for concrete slabs poured on grade. Always install a high-performance vapor retarder under slabs to stop water vapor from seeping through the concrete.

Author's Note: We've received other questions about similar problems. Here's one from Marilyn W. in Troy, MI.

"I read your answer concerning musty smells coming from crawlspace and slab foundations. The musty smell is in our cottage. Part is a slab and part is a modified crawlspace that we can not get under. The space is too small and there is no entry. If we remove the carpet and spray the the floor with the liquid water vapor barrier, would this take care of the smell? It is closed a good part of the year, and when we do open and use it, the smell permeates everything including our clothes, hair etc. Please help us make our cottage usable. Thanks!"


3 Responses to Crawl Space Vents

  1. During the rainy season we have a lake under our home in the crawlspace. My home has musty smells and high humidity year round. We are thinking of encapsulating the entire crawlspace, after first installing french drains and a sump pump. The people proposing to do the job (we consulted 3) all say we can then close off all but two of our 22 vents. This makes me so nervous...what if the pump fails when we are out of town...what kind of a moldy mess will we be creating down there with the vents closed? If we encapsulated but left more vents open...would that be a better safer idea?

    • we have installed many systems like yours; to be on the safe side have your contractor install a battery back up..

  2. I am curious of the logic of the foundation vents. Especially here in the South East. In the Summer the vents are open allowing very high humidity air to enter the crawlspace. If the humidity outside it nearing 80% it would seem that the air in the crawlspace would also become 80% or more. To check this theory I installed a wireless humidity gauge. During the summer with the vents open I would see wild humidity swings in the crawlspace. Anywhere from 80% to 90% humidity. I tried closing them all and the humidity levels were still in the low 80'& range, but stable.

    Can you explain?

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