Ask Captain Crawl Space – Crawl Space Moisture

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Ask Captain Crawl Space – Crawl Space Moisture

Hello Captain, 
I live in Shacklefords in King & Queen County. I recently had my crawl space looked at by my pest control company and he told me I had a lot of moisture. 
I immediately went online to look at ways to fix the problem myself and found everyone talking about condensation, dew point, and relative humidity. 
All I know is that I have moisture in my crawl- what the heck do these things have to do with moisture?
– Bill  
Hi Bill,
Thank you for your question, and by the way, I love going to Nick’s Spaghetti House in Shackleford’s at Christmas time to see all the fun decorations they have- I love their lasagna too! 
Your question is something that most people probably do not understand, and with the summer season starting, your timing could not be better.
Let’s start with some definitions:
Condensation is the physical change of a vapor/ gas into a liquid- it’s the reverse of evaporation. Condensation commonly occurs when a vapor is cooled or compressed to its saturation limit.
Relative Humidity:
Relative Humidity (RH) is the amount of moisture in the air compared to what the air can “hold” at that temperature. When the air can’t hold all the moisture then it “condenses” as “dew”.
So what does this have to do with your crawl space?

1) There is a scientific phenomenon called “The Stack Effect” that affects your house and your crawl space. Warm air rises through your home until it can find a hole to escape. When your HVAC is running, your house is “pressurized” with positive pressure on top and negative pressure on the bottom. This increases the upward air movement. As well, the upward movement feeds on itself – gradually and continuously it increases.

2) Your house always has the same number of molecules of air- every time a molecule of air escapes, another molecule is pulled in. The most likely culprit is moisture coming through your foundation/crawl space walls and open vents.
3) Vents were built in your foundation walls so that air could pass through from 1 side to the other and moisture would have a place to escape.
4) But that is not what happens- “The Stack Effect” starts pulling air upward when it comes into the crawl space instead of being able to pass through freely.
5) In the summer time, the outside air is humid and the Relative Humidity in your
 crawl space is high. When a molecule of humid air is pulled into the crawl space,
 it is cooled and it lets go of all the moisture
 it was carrying (condensation). This is known as the “dew point.”
6) When condensation happens, the moisture (or dew) gets all over everything in
 your crawl space making conditions ripe for mold and mildew growth, wood rot, and wood infesting insects.
So, I hope this answers your question, Bill. The reason we want to encapsulate your crawl space is because we want to control Relative Humidity and prevent moisture/ condensation from happening in your crawl space. We can’t control mother nature, so by encapsulating, we are bringing your crawl space inside- allowing us the capability of controlling Relative Humidity by conditioning with a dehumidifier.
Thanks again, and if you or anyone has a question for Captain Crawl Space, please send it to
Jun 17, 2016 | Posted by in Capt.'s Blog, Crawl Space | 0 comments

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