Vented Crawl Spaces in the Northwest

I wish vented crawl spaces would just go away.

Wood Decay/Fungal Rot

Wood Decay/Fungal Rot

We could then turn the entire conversation into a discussion about conditioned crawl spaces. It is still the norm to have vented crawl spaces in the Northwest.  New vented crawl spaces are being built every day, in numbers much greater than conditioned ones.

It is important to understand how crawl spaces work because we are going to continue seeing them in older construction, as well as in newer construction.

If you live, or have lived, in other parts of the country, some of this may seem counter intuitive.  One could create problems in the crawl spaces of Minnesota or Georgia if one did some of the things we have to do in the Northwest. 

The numbers one puts in the equations are different, but the science is the same.

Getting just a few things right will allow a crawl space to behave itself and manage moisture conditions adequately.

The crawl space should be constructed such that vents can be installed on at least 3 sides.  A good vapor retarder on the crawl space floor is essential to the success of most crawl spaces. 

It would be nice if all the seams of the plastic were welded together and all the edges were caulked/sealed to the foundation.  However this degree of tightness is not necessary.  I have seen crawl spaces with floating vapor barriers and normal wood moisture content levels.  Adequate passive ventilation will remove the amount of vapor that finds its way around the seams of the vapor retarder. The building codes dictate how much ventilation is to be installed around the perimeter of the crawl space.

Crawl spaces that only allow for the installation of vents on one side, or two sides, may require a means of moving air through areas of poor circulation.  This can be accomplished with vents at the open side that are ducted to the poorly vented areas.  We then install a power vent fan in the duct to move the air mechanically.

Power vent

Crawl Space Power Vent

It is unusual to see crawl spaces that need mechanical ventilation. It may be warranted in instances where enough passive ventilation cannot be installed. You certainly cannot fix a moisture issue in a normally vented crawl space with power ventilation.  You first  must address the moisture issue.

The purpose of crawl space vents is not to lower moisture levels created by flooding and plumbing leaks. 

The purpose of venting is to deal with minor amounts of soil moisture vapor and to lower humidity that builds up seasonally.

We must understand the science of Northwest crawl space moisture.

On a recent inspection there was a power vent installed.  Operation of the fan was based on crawl space temperature.  It was set to run at 50°F.  It was running at the time of inspection. The unit’s built-in sensor shuts the unit off at 40°F.  This particular system operated under the assumption that the higher the temperature, the more the fan needed to run. 

This is exactly opposite the science.

In the summer we have moderate relative humidity and higher temperature than we do in winter.  In winter, we have very high humidity with lower temperature. 

70°F at 65% humidity would be normal in summer, while 37°F at 80% humidity would be normal in winter. 

While humidity in summer is lower than in winter, the “actual amount” of moisture in the air is much higher in summer.  Warm air can contain more moisture.

In summer, when that warm moist air enters the crawl space, it mixes with the warm wet crawl space air to effectively raise humidity levels in the space as it passes through.  This elevated humidity in turn raises wood moisture content of the crawl space framing.

In winter, the air outside the crawl space has very high humidity but at low temperature.  When we bring that wet cold air into the crawl space, it mixes with the warm moist crawl space air and effectively lowers the humidity as the air passes out of the space. Wood moisture content of the crawl space framing lowers as humidity drops.

Crawl space wood moisture content goes up and down with the seasons.

If we do not recognize this, plan for it and build for it–bad things are likely to happen. A few of those “bad things” might be mold, rot and wood destroying insects.


When we do not control moisture levels in the crawl space, it becomes vulnerable to wood boring Anobiid Beetles. 

This is true even if there are no other uncontrolled moisture sources.  Anobiid Beetles prefer wood moisture content between 13% and 18% so it is important to keep wood moisture content below 13% in the summer.  A properly vented crawl space can do that.

When I find moisture levels around 13% in summer, I generally do not worry about it as much because I know moisture levels will drop below that in winter.

If I find moisture levels at 13% in winter, it is more of a concern because wood moisture content will be higher in  summer.  This is when the crawl space is vulnerable to infestation by Anobiid Beetles.  Moisture levels must be brought under control.

Anobiid Beetle exit holes

Some climates that are dry in the winter close their crawl space vents in the winter. Closing vents in winter in the Northwest would result in increasing moisture levels in the space year round.  

Wood moisture content will increase if the power vent is allowed to run all summer.  In the winter, when it is colder, the fan will shut down.  We will not lower the levels that built up in summer–levels that increased more than normal because of the fan. If you have a situation where power venting is necessary, the fan should run in winter and not in summer. 

A good vapor retarder on the crawl space floor and repairs to all bulk water issues is assumed.

There are vendors in the Northwest that would have consumers believe that no crawl space can be successfully vented.  This likely has more to support product sales than science.

Charles Buell, Real Estate Inspections in Seattle and the Great Northwest


The Cat House & Home Inspection Protocol

 Just walking around a home–looking at it from a distance–I can often tell what might be in store for me on the interior.  Home inspectors will usually take this “macro-view” of the home prior to getting up close and personal with the “micro-view.”  It is a very important part of the home inspection protocol to utilize both of these views of the home–and all the spaces in between.

Crawl spaces

At a recent inspection, as I headed around the back side of the home, I took the following picture.

Cats in the crawl space

I suspected the home probably had a crawl space even though I was told it did not–supposedly built on a slab.

As I saw Kitty-Kitty sticking his head out of the crawl space hatch I knew what this meant–there was indeed a crawl space as I suspected.

Well it could have also meant that this was just the access well and that there was a door behind Kitty-Kitty and he was just hanging out in the access well.

But I knew what this meant.  There was in  fact no cover at all and the crawl space was where Kitty-Kitty lived.  The best I could hope for was that Kitty-Kitty had not turned the crawl space into a latrine and that he would be amenable to my checking out his digs.

As it turned out, not only was he amendable–so was his partner.

cats in the crawl space

Neither one even minded me snapping a few pictures–in fact one never even seemed to wake up.

I was also grateful that their digs was still a crawl space–instead of a litter-box.

Crawl spaces are perfect environments for cats.  They are dark, there is a high likelihood of mobile food, and they can do what cats do.

By Charles Buell, Real Estate Inspections in Seattle

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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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