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Crawlspaces: Why They’re Moldy & What To Do

Friday, January 25th, 2013

By Jason Earle [Google+]

Being in the mold business, especially after Hurricane Sandy, we get an awful lot of calls from people concerned about mold in their crawlspace.

Mold growth in your home, even in the crawlspace, attic or other non-living space, is less than ideal. It can cause or aggravate a whole variety of illnesses, and stop a real estate transaction in its tracks.

It doesn’t take a flood, or water intrusion, to create a mold problem in a crawlspace. In fact, in most North American climates, they will all end up with an issue at some point because they are inherently flawed. The consensus among building scientists is that crawlspaces should be outlawed. They are considered a building defect in these circles.

Why?  You might notice that there are vents in the foundation around the perimeter of your house. The old school thinking was that this would allow ventilation, so as to prevent an accumulation of dampness, thus preventing a potential mold/moisture problem. At first glance, that seems logical, until one understands the way humidity is affected by temperature.  This is a rather dry subject, no pun intended, so I’ll do my best to get us through it without sounding like a tenth-grade science teacher.

Think of air as a sponge. Warm air is able to hold more moisture than cold air. When the temperature drops, it’s a bit like squeezing the sponge. If the sponge is already very wet, water will come out of it, right? So when we have warm, moist air, and the temperature drops, you will find water droplets appearing on cool surfaces. That’s condensation. Think about when it’s 90F outside and 90% relative humidity (RH), and the sun goes down. As the air cools, dew appears on the grass.

This is exactly what happens in a crawlspace during the warmer humid months. Warm moist air goes through the foundation vents into your crawlspace – a cool, dark place – and the humidity condenses into little water droplets, just like dew, all over the floor joists, insulation and debris, creating a perfect opportunity for stuff to grow. And it’s not just mold. This is an ideal place for all sorts of uninvited guests to get together and have a good time at your expense. It’s like a pool party in your crawlspace and everyone’s attending – bacteria, insects, rodents, you name it. Great fun.

If the conditions are right, mold can begin to grow very quickly – in a matter of days.  And once it begins to grow, it doesn’t go away. It will patiently wait for more moisture, and when it returns, the growth is exponential. One spore becomes a thousand becomes a million…

There’s also a misconception that a mold problem in a crawlspace isn’t a big deal since it’s so common, and also since it’s not in the house, so exposure is minimal. Nothing could be further from the truth. It’s estimated that 95% of homes with crawlspaces have ductwork running through them. All ducts leak, and return ducts leak inward, sucking air from the crawlspace into the ducts, and then redistributing whatever is picked up through the supply vents, into the home. This is also true for basements. There’s something else called stack effect, which I won’t go into, but for these two reasons, some studies show that as much as 75% of indoor air is derived from these moisture-prone spaces, especially in the winter months when we rarely open windows for natural ventilation. Not good.

There are lots of different products on the market that claim to prevent mold growth or even “kill” it once it’s already on its path of destruction. The bad news is that these products offer little to no value, especially in the long run. They may be able to stop or reduce the growth for a short while, but if the dampness persists even for a fairly brief period of time, the mold will grow. It’s a guarantee. It’s up there with death and taxes. I recently wrote another article on the subject of antimicrobials and mold. It debunks many of the myths about this stuff, including the one we hear most about how a bucket and some bleach is all you need. Not so.

Here’s a link to that piece, if you’re interested:


So, you have a crawlspace, I’ve got you worried, and now you’re thinking about putting a FOR SALE sign on the front lawn. Not so fast. The solution is not as complicated as you might think, and will cost you less than your realtor’s commission.

The way to fix this is something called a “closed crawlspace,” and there are various versions of it, but the idea is simple: First, the crawlspace is cleaned of debris, mold growth, wet insulation and the other byproducts of this defect, then sealed from the outdoors by modifying or blocking the foundation vents.  The purpose is to prevent warm moist air from coming in.

Now you have to prevent moisture accumulating from within. If you have a dirt crawlspace, you’ll either pour a slab or install an encapsulation system like the one offered through Western Pest (http://www.westernpest.com/home-maintenance/moisture-barriers.html). This kind of system involves installing plastic sheeting on the floor and foundation wall, like the photo below.

1-800-GOT-MOLD? testing, NYC, NJ

Then you would either install a dehumidifier that drains into a sump pump, or bring drier air from the living space above, into the crawlspace, using fans and ductwork, to dilute the damp air, effectively dehumidifying the crawlspace.

This is not a technical article, nor is it meant to be a how-to. If you want to know more about how this works and the science behind it, there’s been exhaustive research on the subject down in North Carolina. While the conditions in the Northeast are different, the dynamics are the same. Actually, NC has it much worse, so if they can make it work down there, it will most certainly work in less humid climates. There’s a wealth of information here, if you’d like to learn more: http://www.crawlspaces.org/

Unfortunately there is no one-size-fits-all solution. Every building has different nuances depending upon whether it’s in a flood zone, how it’s constructed, the type of heating and cooling systems, and even the sensitivity of the occupants. Like most things of this kind, it’s not really a weekend warrior kind of project, unless you’re into doing things twice.

When it comes to navigating these waters successfully, it’s usually a good idea to engage the help of an environmental consultant who has specialized experience in this arena, and who does NOT perform installations of these systems or provide mold remediation services. The consultant evaluates the problem and designs the solution, and an unrelated contractor does the work.

If this sounds like something you might like to learn more about, either give us a call at 1-800-GOT-MOLD? (1-800-468-6653) or visit us at www.1800GOTMOLD.com.

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