Are you NUTS? Why would I want to vent my crawl space in the winter?

I have been around and around with clients enough to know that there are many people that seem to think that venting crawl spaces in the winter is a bad idea–especially in the wet NW where outdoor winter humidity can be close to 100% for much of the time.

floodedcrawl3Why would anyone in their right mind want to draw all that wet air into the crawl space?  Sounds like a problem waiting to happen, doesn’t it?

To get to the answer however, we have to actually look at the science of humidity and how important to our homes (as well as to our answering the question) that we understand that humidity has a critical relationship with “temperature.”

The truth, as counter-intuitive as it may seem, is that we can actually “lower” the moisture in a wet crawl space by bringing in this 100% moist air.

Some of you will be absolutely certain that I am celebrating WA State’s new Mary Jane law–but I assure you that, not only do I not inhale, but that I am not smoking anything funny at all.

Again the key is “temperature.”

Air at 40 degrees F cannot hold as much moisture as 100 degree F air.

Because of this simple fact, 40 degree air will reach saturation (100% humidity) much quicker.  So now let’s drag that 40 degree, 100% humidity air into the crawl space that is 50 degrees at 75% humidity.  Not only can it easily handle the moisture coming in, as the air is warmed to 50 degrees its humidity drops and results in the 75% air dropping to less than 75%.  There are a lot of factors, such as air flow, and how much the air flow will reduce crawl space temperature that will effect just how much the humidity will be lowered, but the point remains that the crawl space humidity, overall, will be lowered–not increased.

When you do not control crawl space moisture, bad things can happen

When you do not control crawl space moisture, bad things can happen

So now let’s keep the crawl space temperature at 50 degrees and drop the humidity to a more normal 50%.  The ability of that 50 degree air to absorb moisture in the colder air improves even more.

Interestingly enough, very few crawl spaces ever drop much below 50 degrees except perhaps near the perimeter where the cold of Mother Nature occasionally “takes-out” an outside faucet–at least in the mild Northwest.  Your Mother Nature may treat you differently where you live–it is wise to pay attention to Mother Nature and understand her idiosyncrasies in your area.

Generally speaking, wood moisture content (as measured by a moisture meter) tends to be slightly higher in summer than in winter when outside air is at higher temperature and at lower humidity and then the reverse happens.  75 degree air at 50% humidity will work to raise the humidity of the air in a crawl space with 50% humidity at 60 degrees.  The cooler air in the summer cannot hold as much moisture as the warmer outdoor air and the crawl space air reaches saturation quicker resulting in higher crawl space humidity and thus raising moisture levels in the woodwork.

But before it can become a problem, the seasons change and moisture levels naturally start to go the other way.

Different climate zones are affected by these same principles in different ways–that is why they are called “different climate zones.”  It is important for the builder to understand the climate the home is built in to understand how to allow for these principles to work in ways that don’t destroy the house or provide an environment conducive to wood destroying organisms and mold.

In the South, where humidity can be high year round, and temperatures are near saturation year round, venting crawl spaces at all is especially problematic.

Of course all of this is relative only to “vented” crawl spaces.  Since we have millions of homes with crawl spaces (and more are built every day) any conversation I might have about how they should not be allowed does nothing to deal with the ones that we do have already.

It is more important to understand how to maintain good humidity levels in crawl spaces so that problems do not arise.

By Charles Buell, Real Estate Inspections in Seattle

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  1. It is important to control the humidity in your crawlspace as it will prevent various issues. There are dehumidifiers for crawl spaces that can be used. Great information on vented crawl spaces, thanks for sharing!

  2. Thank you for the information. I have a mid century home in the Berkeley, California Area and have been monitoring the humidity in a crawl space under my driveway where I have a leak due to an aged membrane between the concrete driveway and subfloor. It is vented. Would a 75% humidity at 55 degrees be considered problematic for this area? This is in early January. Thanks!

    • Charles Buell says

      Where is the humidity 75%? If this is the humidity inside the bunker and it stays at that level seasonally, I suspect the dew point would never likely be reached—but I may be missing lots of variables.

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