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The Challenge of Relative Humidity and Crawl Spaces

Thursday, August 9th, 2012 by Richard Fencil

Air is a very efficient way to move water.  Air, including humid air, moves easily in and out of spaces.  It’s all around us, and moves in vast quantities through the largest and smallest of spaces.  Air brings its moisture content with it wherever it goes.

When air is heated or cooled, its relative humidity changes.  When we say "relative humidity" we mean how full of water the air is relative to the maximum amount of water it can hold at a given temperature.

Warm air holds more moisture than cold air.  The relative humidity of air goes down by 2.2% for every degree we heat it and up by 2.2% for every degree we cool it.

open crawlspace ventCrawl spaces are cool, because the earth is about 55 degrees year round (ground surface temperatures can vary based on season and geography).  So when we bring warm humid air into a crawl space, the air is cooled, and the relative humidity goes up.  High relative humidity causes rot, mold, energy loss, and attracts pests.

So here’s the problem.  Many years ago when some guys were writing the building code, they rationalized that since we have a lot of moisture from the ground in our dirt crawl spaces, we need to do something about it.  They figured that if they vented a crawl space, the moisture would flow out through the vents.

Maybe it was too obvious.  Maybe they didn’t notice how it rains twice a week or so, and didn’t stop to think about what causes it to rain.  Maybe it didn’t occur to the authors of the building code that when it was a damp day, that they’d be venting the crawl space with damp air.  And when it was a cold day, they’d be venting the crawl space with cold air.  And when it was a hot day, they’d be venting the crawl space with hot air. And they certainly didn’t know about or consider that air flows upward in a house.

Venting a crawl space actually creates worse moisture problems
Sometimes the solution is worse than the problem.  And sometimes the solution is not a solution. That’s what venting a crawl space is.

My examples below are for a four-season climate, like much of the U.S. has.  However, if you are in the Southern U.S., then you know that my summer example is what you have (and worse) most of the year.  If you are in Maine, then you know that my summer examples are for two or three months out of the year, and the winter examples are for longer periods.

Venting on a hot summer day brings moisture into a crawl space
Let’s look at what happens on a hot summer day.  You have 84-degree air with 75% relative humidity entering your vents.  Your crawl space is 66 degrees, but the surface temperature of your walls, dirt floor and floor joists is 62 degrees.  What will happen when this air comes in?

We said that for every one degree we cool the air, the relative humidity goes up by 2.2%, because cool air holds less water than warm air.  So looking at our summertime situation, the difference between the outside air we let in at 84 degrees, and the crawl space at 62 degrees, is 22 degrees.  22 degrees multiplied by 2.2% is a 48.4% increase in relative humidity.

Our 84-degree air started out with 75% relative humidity; in other words at 84-degrees it was 75% full of water. We cooled it to 62 degrees so we have to add 48.4% to the relative humidity.  So that’s 123.4% relative humidity.  But wait a minute; we can’t have over 100% relative humidity.  Why not?  Because at 100% the air can not hold any more water and must give up its moisture.

What do we mean, “give up its moisture?”  We mean it will either rain, or it will come out on surfaces as condensation.  When the relative humidity reaches 100%, we call this the dewpoint – the point at which the air gives up its moisture.

When this warm humid air enters a crawl space, if the crawl space air was colder than the crawl space surfaces, it would rain in the crawl space.  But that is never the case.  The source of the cold is the earth, and the source of the warmth is the air coming in from the vents, so the surfaces in your crawl space are always colder than the air in a crawl space.

condensation on batt insulationSo on this summer day, we get condensation.  Which means our crawl space walls get wet.  Our dirt surface of the floor gets wet.  Our air ducts get wet, especially if we have the air conditioning on because the ducts are cold.  Our cold water pipes get wet.  These surfaces are the coldest.

When there is a big difference between the dewpoint of the outside air and the crawl space temperature, like in our summer day example, condensation will occur even on surfaces that are not in contact with the ground, but are cold just from being in the crawl space environment.  Our floor joists, girders, sill plates and insulation get wet with condensation.  As the insulation gets wet, it gets heavy and falls down to the crawl space floor.

Having high humidity in a crawl space also causes all porous material to soak up moisture from the air like a sponge.  There is a direct correlation between relative humidity and wood moisture content.  Wood in a damp environment will become damp itself – and damp wood rots, and mold grows on it.

All these wet surfaces in a crawl space will eventually have to dry to somewhere.  So let’s say we had a few hot summer days which caused condensation in our crawl space.  Then the next four or five days are cooler and mild.  Is the problem over?  No way.  After the hot days we are left with a crawl space with wet surfaces everywhere.  They dry into the crawl space air over the next weeks and months – and meanwhile mold and wood destroying fungi are having a party, eating your house.

Venting on a Spring or Fall Day
If we have a day that is 72 degrees outside, just room temperature, and it is a humid day such as 80%, then when we bring this air into our crawl space to make things better, it will cause condensation.  80% relative humidity (RH) air cooled ten degrees increases its RH by 22%, which is over 100%, which means we have condensation in our crawl space.  Is this an extremely hot day?  No, it’s a normal room-temperature day outside, and we still have a wet crawl space.  Let’s say it’s not so humid.  Let’s say it’s only 60% RH outside.  We cool our 72 degree 60% RH air when we bring it into the crawl space to 62 degrees and increase the RH by 22% to 82%.  That’s less than 100% so we are good right?  Wrong!  Mold and fungus and rot happen at over 70% RH, and some can thrive at less than that.  82% RH in our crawl space is way more than we want and very unhealthy.

So far, we have learned that venting doesn’t work because it doesn’t get rid of the dampness (and wetness) in our crawl space, but increases it instead.

Venting on a Cool or Winter Day
If the RH of air goes up when we cool it, it goes down when we heat it.  So if we vent our crawl spaces in the winter and bring in 35 degree air with 60% RH, and we warm that air in our 62 degree crawl space, the RH goes to 3%. With this dry air we can begin to dry our crawl space.  Of course the dry cold air mixes with the crawl space air and cools the crawl space, and we have water evaporating from the earth into the crawl space air, so we never achieve 3% RH in our crawl space, but materials dry out and there is no condensation.

Hey, we’re drying our crawl space with vents now!  This is great, right?  Well, the down side to this scenario is that you are now venting in the outside winter temeratures and having to pay to heat that.  if you like high energy bills, cold floors and cold drafts, then my next article will be for you!

About the author
Richard Fencil lives in Guilford, Connecticut with his wife and three sons. He works for Basement Systems in Seymour, CT that specializes in basement waterproofing, crawl space moisture control, foundation repair, and basement finishing.

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