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Humidity Moisture & Crawlspaces

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Crawl space damp problemsThe good and the ugly: In the top photo, moisture has compromised the walls and subfloor. An improved crawlspace (lower photo) features a covered subfloor, insulation and other features.

The first rule of the crawlspace: Never go in with out your cell phone. The latch is always on the outside, and you're one gust of wind away from being crapped inside. I've been in hundreds of crawlspaces and have seen only one with a string rigged to pull the latch open from inside. Most people don't give much thought to the short, dark, unconditioned area under dieir homes, but what happens down there matters.

The typical crawlspace is 3 feet tall with concrete block walls and plastic wall vents. The floor is dirt and the "ceiling" is a wood-frame floor with fiberglass insulation between the framing. That's typical, but in reality no two are alike. Crawlspaces can be short or tall, wet or dry, clean or dirt)'. I regularly find spider webs, empty beverage cans, lost tools, mouse poison and candy wrappers down there. My former sunglasses are lost in a crawlspace somewhere in New Orleans. And there are other, more disturbing things that should not be ignored.

Nobody likes to hear this, and I wish there were an alternative, but there isn't: If your house has a crawlspace, you have to go down there. You should go at least twice a year, including once in the summer and after any unusually heavy rains.

Once you're there, ask yourself: Does it feel humid? Do you sec mold or mildew? Is there standing water? Are the ducts wet? Are there any animals of any kind? If yes, you need to call one of the professionals listed in diis directory. While you're already dirty, verify that your downspouts are still attached to their underground connections. Watch the gutters during a steady rain to make sure they're draining into the downspout and that water flows at least 5 feet away from your house at the outlet end. This is cheaper, healthier and less scary than having a home inspector find mold when you're preparing to sell your house.

Water wars

Dampness is the biggest problem in crawlspaces. Once in a while, I discover a crawlspace that's been ignored for 60 years and is dry as a bone, but it's not common and I can't explain how it happens. "The principles of building science cannot reliably explain how a vented crawlspace located in Western North Carolina would fail to develop moisture problems. If you live here, you know these two things: Summers are humid, and humidity will form condensation on a cool glass of sweet tea."

Crawlspaces are like caves, with temperatures in the mid-50s year-round. When humid air comes in through the vents, it condenses on those cool surfaces just like it does on an cool glass. The tips of nails penetrating the floor above will rust. There could be condensation on top of the plastic groundcover, air conditioning ducts, pipes, block walls and wood framing. If it stays there long enough, a fuzzy green layer of mold will start to grow.

There are other ways water can get in. If you live on a hill, your downspouts probably empty into underground pipes that are supposed to carry water down the hill and away from your house. Maybe the downspout gets clogged, water backs up and the gutter overflows. Maybe the downspout disconnects from the underground pipe. Maybe the dirt around your house setdes and rainwater from your yard drains toward the house. Either way, you've got standing water against your crawlspace walls.

The water bleeds through the walls leaving white mineral deposits on the inside that are visible even after it dries. Water can wick up through the concrete wall and into the wood framing that holds up your floor, wicking along the length of the wood toward the center of your house. Mold loves water and wood, so you'll start to see green and black spots. The wet fiberglass insulation will smell like ammonia. Now, you've really got a problem crawlspace.

Crawlspace or menagerie?

Animals and crawlspaces don't mix. I know a builder who accidentally shut a cat in a crawlspace overnight. The cat was unharmed, but the crawlspace was less forninate. Imagine how inviting a warm, rain-free crawlspace must seem to wild animals. Wouldn't you want to climb in there and curl up against a nice warm floor with a thick, wooly blanket of insulation beneath you?

A missing crawlspace vent cover is a nine-alarm emergency — any animal smaller than a raccoon can climb right in. A friend of mine who works in crawlspaces once pulled back some insulation to see how thick it was and a 5 foot black snake fell down on top of him. Think about the smallest animal that you want living under your house and then make sure that you air-seal all holes larger than that in your crawlspace. This is actually a great way to approach air sealing your entire home.

Seal the deal

There is a better alternative, and its called a sealed crawlspace. Sealed crawlspaces are great options for new or existing homes but don't get too excited: You still have to go down there to check the dehumidifier and you still shouldn't use it for storage.

But your chances of a wildlife encounter or fungal situation are seriously reduced. There are many ways to seal a crawlspace, but a few features are common: permanently and tightly closed vents, a continuous plastic liner covering the floor and extending up the wall, a drain or sump pump to remove water from the lowest point and a humidity - control mechanism. The North Carolina building code lists five approved humidity-control mechanisms, but a dehumidifier is my favorite. You can install the other ones too, but you should install a dehumidifier first!

Some sealed crawlspaces have insulation on the walls instead of the floor above. This solves two problems: poor air sealing of the floor and fiberglass insulation falling down. If you move the insulation to the walls and you have air conditioning, you can supply conditioned air to the crawlspace (one of the five approved humidity control strategies). This is a great backup, but you should still install a dehumidifier because people in WNC don't run their air conditioning all year (and even if they do, it may not be enough). You want a system that senses humidity and reacts; you want a dehumidifier.

Sometimes it doesn't make sense to insulate the walls and condition it — if your crawlspace is 30-feet high, for instance. Instead, you can air seal the floor, insulate it and make a sealed unconditioned crawlspace. Obviously, you would not dump conditioned air outside the insulated boundary of your home, so you install a dehumidifier. Insulating the floor with spray-foam air seals the floor and is less likely to fall down, so it is a great option for insulating in a crawl space.

Who wouldn't want to make a twice-yearly visit to check out these nice clean sealed crawl spaces? Some crawlspace experiences are wonderful. On a 15 degree day when the wind is howling up on Balsam Mountain, nothing beats warming your hands on the work light of a friendly duct installation crew in a balmy 50 degree crawlspace. I can promise you this: a crawlspace is never boring.