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The Science Behind a Wet Crawlspace

Friday, April 14th, 2017 by Braden Cook


You may know that your crawlspace gets wet, but you may not know how or why it gets that way. This blog post is going to go into a bit more detail on the causes of a wet and potentially moldy basement.

Hopefully through this blog, you will learn this, but the picture below shows a cold pipe that, when exposed to the warmer air from outside, condensation begins to form on the pipe.

The Science Behind a Wet Crawlspace - Image 1                                                                                                                                                                                       How does water get into my crawlspace?

While this topic has been covered in other blog posts, we will reiterate and go into more detail about how water can enter your basement. I'll give you one hint, it is not as obvious as you may think.

There are the obvious ways like an exploded/breached water heater, a leaky pipe, or even some kind of running water that has made its way under your home. While these are more extreme and less frequent examples, they are not out of the question at all. However, the real killer is not necessarily standing water, but water vapor. The vapor is what makes its way onto and eventually into your floor joists, ductwork, insulation, and everything else in your crawlspace.

Water vapor and how it forms is a much more complicated issue and topic. One of the biggest contributors of water vapor comes from the earth itself. The earth, in its normal state, is damp. Of course there are exceptions to this in areas like tundras, deserts, and places in drought, but all in all, the earth is damp. This damp mud, dirt, or soil slowly dries out under the home. When the soil dries out, the moisture only goes up....onto and into everything under your home including wood and insulation.

This is where things get much more interesting. Things so far have probably made a lot of sense and are very simple concepts. Now we get into a little bit of what we call "crawlspace science". Everyone knows what humidity is, especially if you live in South Carolina. The air has some kind of strange wetness to it, well that humidity, under the correct conditions, can be "released". We call this humidity, "relative" humidity and is basically how much water is in the air. This water is released when the relative humidity of air is changed, this can go both ways and depends on if the air has been heated or cooled air. The warmer the air, the more moisture it can hold. Some really smart scientists and researchers have found that for every degree that the temperature rises, the relative humidity goes down by 2.2% and for every degree cooler, it goes up by 2.2%.

Well that's interesting but who cares?

You should care. Anyone with a crawlspace that has vents or a dirt floor should care. A long time ago in a galaxy not so far, far away...the authors of the building code weren't exactly sure how to deal with this water and water vapor that came up from the ground. So they decided on vents. Their thinking was that if there were vents under the home, any water vapor would makes its way out of the vents! These people had to be really intelligent and knew the proper solution, right? WRONG. Unfortunately for millions of homeowners, these guys didn't really have a clue. It made sense to them, and everyone else around, so it just stuck. Could there have been better solutions? Definitely. Could there have been worse solutions? Ehhh, it would have to be a pretty wacky idea. But you're probably thinking that it makes sense, and so did I. So why is this such a bad idea? Relative humidity. It all goes back to what we talked about in the first section. Let's give an example to make things easier to understand.

It's a beautiful summer day, with 84 degree air and a relative humidity of 75%. You have vents under your home, so this air is rushing in. Your crawl space is 66 degrees, but the surface temperature of your walls, floor joists, and dirt floor is 62 degrees. So, between the two, we have a 22 degree difference (84-62). We already know that the RH(relative humidity) increase by 2.2% for every degree. This gives us an increase in 48.4% increase in RH. So if we add the beginning 75% RH with the increase of 48.4%, we get 123.4% relative humidity.

So how does that work? I guess the extra humidity over the 100% just disappears. Wrong again. This extra 23.4% humidity has to go somewhere, and that somewhere is just where you would expect; walls, pipes, insulation, floor joists, and anything else you can think of! This is the killer. Our building code writers weren't really sure what to do, so they proposed an option that made logical sense, but not a lot of scientific sense.

As always, thank you for taking the time to read and if you have any questions about your crawlspace please call us at 1-864-984-2308 or fill out a contact form for a free estimate by clicking here.

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