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The Moisture Trap

How much quality time have you spent in the crawlspace or basement of your house? If you’re like most people, you avoid going there and only do so when there is a problem. They’re often poorly lit, if not totally dark. They are usually damp and this opens the door for major problems. It’s not unusual to find mold or mildew there, termites have been known to colonize the space, and you just know there are critters down there ready to crawl up your pants or shirt as soon as you show up.  Great place to hang out, right?

To address these problems, builders have been installing foundation vents, low and on the outside wall of a house. These vents are near ground level and the theory is that the moist air in the crawlspace or basement will exhaust out through these vents to be replaced with cool dry air. Unfortunately, that is rarely the case and the vents often make the situation worse.

Moist air is heavy and doesn’t like to move. We often see it hovering over rivers, creeks, and valleys where it creates early morning fog. When it is under a house it is quite content to just sit there and do nothing. If there are cool items in the area (such as water pipes or air conditioning ducts) the warm moist air will create condensation as the damp air goes from a gas to a liquid. If you have water leaking into the area through the foundation or poor drainage, the moist air is going to make sure that water doesn’t evaporate quickly.

The presence of this moisture can create some serious structural problems. More often than not, this moist air is in direct contact with wooden floor joists and the wooden subfloor. This often means that the moisture migrates into the wood floors and that’s when the annoyance of moisture in a crawlspace translates into a disaster. When wood takes on moisture it expands and moves, damaging your floors. You will see cupping almost immediately and in extreme cases, the floors begin to buckle. To say this is a nightmare is an understatement.



Options for Damp Basements & Crawl Spaces

So, let’s review the facts:

  • Crawl spaces and basements are often damp and cool.
  • Vents in foundation walls don’t mitigate the problem; they often make it worse by bringing in warm moist air.When the two mix, you often get condensation.
  • When moist air comes in contact with the wood joists and subfloors, it often migrates into the wood floor and creates additional problems.
  • If the treatment for the problem actually makes it worse, what can we do?

About thirty years ago, I began telling homeowners to put dehumidifiers in their basements and crawl spaces to address this issue.It was a little unorthodox but in my mind but I couldn’t see any other options.My own house was built around 1920 and I inherited a basement that leaked like a sieve.After spending a small fortune on waterproofing the basement, I knew there would continue to be moisture and I took additional precautions.I placed a simple dehumidifier in the basement and ran the drain hose to the sump pump and it has worked fine.

Recently, I was delighted to see an excellent article in the Journal of Light Construction that drew the same conclusion.The article was written by Dr. Allison Bailes of Energy Vanguard.Located in Decatur, Ga., Energy Vanguard is an energy consultant firm that helps the construction industry and homeowners make the best use of energy saving technology.Bailes goes through several options for dealing with the moisture issue and concludes encapsulating the crawl space (closing off the vents) and installing a dehumidifier offers the best solution.He also offers a laundry list of other energy efficient ideas and options builders and homeowners need to know about.Check out his blog entry 11 Building Science Secrets Every Home Builder & Remodeler Should Know for the complete list.

It is not unusual for building habits to change over time.Changing technology, the increased attention on energy conservation and building tight houses has forced many in the construction/remodeling industry to rethink their approach.The foundation vent has been dying a slow death.If you reside in the southeast or coastal areas and have them in your foundation, this is one issue you need to put on your radar.The cost and consequences of ignoring this could be extreme.

Michael Purser | © Rosebud Co. 2016

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