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.

ANOBIID BEETLES:

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

 

“Difficulty” is not on their radar!

OK, this is the question of the day.

Why would “anyone” deliberately dump a 5 gallon bucket of sawdust in a crawl space?

Carpenter Ant Frass

Carpenter Ant Frass

I see all sorts of things dumped and stored in crawl spaces. I have even seen sawdust from when the home was built, or from floor refinishing that has filtered down through the cracks in the floor boards–making neat parallel lines on the black vapor barrier covering the ground.

But this stuff was just piled–more or less in one location–along the foundation. It was nowhere near the access to the crawl space, so one might think it was difficult to get it to where it was as well.

But the “someone” that dumped the sawdust, I am quite sure, never contemplates “difficulty.”

carpenter ant frass

This is especially true when you realize this stuff got there ONE BITE AT A TIME!

Carpenter Ants make amazing carpenters–although they seem much better at taking things apart than putting things together.

These carpenters have been working on this home for quite some time and have piled up their construction debris as prove of their industriousness and patience.

“Difficulty” and “impatience” are something that they obviously do not consider.

By Charles Buell, Real Estate Inspections in Seattle

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Beware the “Kleenex Effect”

While some would argue I am “nit picking,” the fact remains, sometimes it is very important to understand the terms we use.

kleenex-effect1I am sure there are interesting studies as to just how things become “generic.” Take Kleenex for example. Kleenex is a brand name that is commonly used to describe any kind of nose-wipe. While Kimberly-Clark Worldwide, Inc. may have an issue with the misuse of its brand name “Kleenex,” the fact remains when someone says they need a kleenex it really won’t matter too much who the tissue is made by as they all do pretty much the same job.

The term “Dry Rot” is similarly used to generically describe wood decay/rot in homes. The problem with using this term in a generic fashion is there is a huge difference between actual Dry Rot and other types of wood decay/rot and requires quite different protocols for elimination.

All types of rot require that the wood have sufficient “free water” to support growth (for this discussion we will assume oxygen, food and temperatures are suitable). Moisture levels to support Brown Cubical Rot, Soft Rot, and White Rot (the three most common types of rot) typically have to be above 30% for the organisms to be happy and prosper. With Dry Rot the wood moisture content will also have to have sufficient free water, but the wood gets to be at this level because the fungus is bringing the moisture to the wood as opposed to the wood being already wet.kleenex-effect2

This is a very important distinction because fixing a leaky roof or leaky toilet will be sufficient to stop the growth of most wood decay rot brought about by these conditions, but will not be sufficient to stop the growth of a fungus that is growing to the wood—bringing moisture to the wood from the ground.

Dry rot has the potential of attacking huge areas of a home’s structure without any real moisture issue already existing in the wood. Generally speaking brown rot or soft rot are more likely to be more localized. It can be more widespread if venting is inadequate or some other condition is present that elevates moisture levels in the woodwork above 30 percent, but generally speaking it will be more localized to the area of leaking—whether a plumbing leak or otherwise.

To date, there have been few to no documented cases of true “Dry Rot” in Washington State, but because of the Kleenex Effect—many people think it is common.

By Charles Buell, Real Estate Inspections in Seattle

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