Crawl Space Moisture and Dew Point

PsychrometricChart.SeaLevel.SI.svgCrawl Space Moisture and dew point are critical to understanding what happens in some crawl spaces and the amazing issues it can create!

If you’ve never heard of a psychrometric chart, you’re about to!

From Wikipedia:

A psychrometric chart is a graph of the thermodynamic parameters of moist air at a constant pressure…

What on earth does that mean?

It’s a chart that plots temperature and humidity!

And for summer months, especially in our humid South-East USA, it’s sometimes a critical factor in understanding what issues are being created in crawl spaces.

Since I offer crawl space moisture inspection services in the Charlotte, NC region, I’ll focus on this area.

Understanding crawl space moisture and dew point issues.

First, consider the average temperature we have during the summer months here in Charlotte:

Charlotte NC daily_high_and_low_temperature_temperature_fAccording to the above, we average a high of 89f with a peak day of July 17th.

Now, consider the relative humidity for our region:

crawl space relative_humidity_percent_pctWhile I can’t read it directly, let’s assume we have a nice hot and humid series of days and hit 85% humidity.

(Relative humidity is the percentage of moisture in the air relative to the maximum the air could hold at that temperature.)

One more chart, not really critical but it will reinforce the psychrometric chart:

crawl space dew_point_temperatureNotice at July 20, the Dew Point is 71f, the same as the average low temperature from the first graph!  We all know this during spring and fall especially, when we have nice warm days, and there’s dew on the ground the next morning.

As the air cools, there’s too much moisture and it condenses out of the air onto the leaves and grass and our cars!

Well, consider that the same situation exists below our homes!  At least during those months, as the sun rises and temperature goes up, the air is able to absorb more moisture and “clears out” the crawl space.

But during summer months, it’s a different story.  If your Air Conditioning ducts are in the crawl space, they are cooling the air.  Not ‘leakage’ (although that can be a major issue) but just naturally the duct work gets cooler and cools the air.  Combine this with a crawl space that circulates poorly, and you have a situation where outside, 90f air at 85% humidity moves under the home and may cool to 70f.

THAT is what a psychrometric chart shows us!

PsychrometricChart.SeaLevel.SI.svg(sorry it’s in celsius – best I could find!)

90f is about 32c, so follow the bottom line to 32, and up to the average humidity assumed of 85% (the curvy lines) and you’ll see you’re WAY up at the top of the chart.

As the air cools, the AMOUNT of water in the air stays constant, but the ability of the air to hold it decreases.  Slide your finger horizontally to the left to say 75f (24c) and you’ll be off the chart!

Condensation has occured!


In certain areas of the country, and in certain crawl spaces, a combination of poor circulation, cool soil, cold floors above in the home, leaking hvac and the natural cooling from the duct work can lower the temperature in the crawl space such that the air can no longer hold the humidty, and the water begins to condense out of the air onto the joists, girders, insulation, floor, vapor barrier!

Frequently when this happens, the only true solution is a Sealed Crawl Space, as more circulation via fans, etc. may only push more humidity into the crawl space!

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