Basement and Crawlspace FAQ: Dampness / Condensation

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Condensation on the Foundation

I have a home in the mountains of Arizona, and I find condensation collects on the beams bolted to the foundation. Vents have to be closed in winter to keep the pipes under the house from freezing. There is 2 inches of foam insulation on the inside of the foundation wall. What can I do to keep the water from accumulating on the 2 x 8 bolted to the foundation?

Moisture buildup in a crawlspace—assuming it’s not the result of water seepage—is normally caused by subsurface water. Even when the ground in the crawl- space seems dry and dusty, moisture can accumulate in the area as a result of capillary action. Capillary rise occurs in nearly all crawlspaces built in areas where the soil is clay or silt. According to the University of Illinois Small Homes Council, as much as 18 gallons of water per day can evaporate into a craw under a 1000-square-foot house.

If this is the source of moisture, then it can be con trolled by covering the ground with a vapor barrier (4-mil polyethylene). The interior joints should be overlapped by a minimum of 6 inches, and the perimeter edges should be turned up onto the walls of the crawlspace.

Another possible source of moisture into a cold crawlspace is from the warm house above. Excessive moisture inside the house (high water vapor pressure) can travel downward through the floor into the cold crawlspace (low water vapor pressure) and condense on the cold surface. To prevent the downward flow of moisture vapor, you can install a vapor barrier between the overhead floor joists in the crawlspace.

Dehumidifying a Damp Basement

We have a moldy, musty odor in our basement that seems more pungent in the spring and fall. We have tried using mesh sacks filled with calcium chloride to reduce the odor, but they’re ineffective. Occasionally, the sacks drip water. How can we decrease the odor?

The sacks hanging in your basement absorb moisture that can lead to mold and mildew growth, but they are not as effective as a mechanical dehumidifier. You need an electric dehumidifier. These appliances have a humidity control and will run continuously until the humidity setting is reached. The water that they con dense out of the air, known as condensate, collects in a tank. You have to manually empty the tank, pump the condensate to a disposal drain, or drain it with a hose to a floor drain.


above: Great for keeping a basment dry: Frigidaire 70-Pint Dehumidifier (Wal-Mart product link)

Moisture problems such as yours are more apparent at times of high humidity. During warm weather, the basement is cooler than the outdoors. Warm air can hold more water vapor than cool air. When warm out side air works its way into the basement, its temperature drops and causes the relative humidity in the basement to increase. Aside from contributing to mold and mildew growth, this makes the basement air feel clammy and uncomfortable during the summer.

Humid Basement

Our basement is quite humid most of the time, although water does not seep into it. Is it helpful to coat the foundation wall using a cement based paint, or will a good quality water based paint work as well?

It’s not unusual for a basement to become humid, especially during the warmer months. A basement, by its very nature of being below grade, will be cool. Cool air can't hold as much moisture as warm air, and , as a result, the humidity level increases when the warm outside air works its way into the basement.

I doubt that the humidity problem in your basement stems from water vapor permeating the foundation wall. You can check this by taping a 6- x 6-inch piece of aluminum foil to the wall. Leave it there for 24 hours and then remove it. If the side of the aluminum facing the wall is damp or has beads of condensation on it, then your assumption is correct.

However, coating the foundation walls with a cement based paint won’t solve the problem. Cement based paint is formulated to stop water seepage through the foundation, but because it “breathes” it won’t stop water vapor. Water based paint won’t stop water vapor either, and an oil based paint will form a film on the wall that will eventually blister and peel.

To prevent water vapor from coming through the foundation wall, you will have to cover the foundation on the exterior side with non-permeable membrane, or with a tar or asphalt coating. Another option is to remove the water vapor from the air by using one or more dehumidifiers. However, you must let the appliance run until its hygrometer setting shuts it off. Over the years, I’ve had clients call me and say that their dehumidifier was not doing its job even though they let it run for several hours each day. Depending on the capacity of the dehumidifier and the size of the basement, the dehumidifier may need to run continuously for many hours.

Hot TV Room

I have a TV room in my basement. The room is hot during the summer months. We have a dehumidifier in the room, so we thought the room should be cool. Could you please tell me why it’s hot down there?

The room is hot because of the dehumidifier. A dehumidifier is basically a small self-contained air conditioner. An air conditioner discharges the heat removed from the circulating air and from its compressor to the outside, but a dehumidifier dumps that heat into the room.

If the TV room is small and the dehumidifier runs continuously, it discharges enough warm air to heat the room. A dehumidifier is not used to cool a room. It makes a room more comfortable by lowering the relative humidity.

You would be better off with a small wall-mounted air conditioner. This unit also removes excess humidity and cools the room. Even though the TV room is in the basement, a section of the foundation wall is generally above grade. If it’s a concrete block wall, an opening can be cut in the foundation for an air-conditioning sleeve, or an air conditioner can be installed in a basement window.

Damp Basement Closet

I hope you can help me with a very aggravating problem. About a year ago, I built a closet in the basement to store our clothes in the off-season. Our summer clothes were stored last winter and they were fine this spring when I took them out. However, when I retrieved our winter clothes, which had been stored for the summer, I found them full of mold and mildew. How do I solve the problem?

Basements, because they are below grade, are cooler in the summer than the rest of the house. As a result, the moisture in the humid summer air tends to con dense in the basement, making that area quite damp. This, in turn, promotes the growth of mold and mildew. Opening the windows and using a fan to circulate the air will only work when the humidity of the outside air is not as high as it usually is in the summer.

And, the problem is compounded in a closet because of the confined space and stagnant air.

The best approach is to install a dehumidifier in the basement. While it’s not practical to place it in the closet, you can install vent openings in the top and bottom of the closet door to help circulate the basement air.

Another approach is to use chemicals that absorb moisture such as silica gel and activated alumina. These have the capacity to absorb half their weight in water. They can be placed in the closet in a bucket or cloth bags hung from the closet pole. After they’ve become saturated, the water can be drawn off by heating and the chemicals can be reused.

Damp Basement Room

I am interested in making a below-grade finished basement into a comfortable office. There is no water seepage, but even so, the basement is cold and clammy in the winter and humid and musty in the summer, I’m using a dehumidifier and a portable heater, but can't overcome the discomfort. Any suggestions?

You have the ingredients for making the finished basement into a comfortable office all year long—a dehumidifier for the summer and a space heater for the winter. However, either you need additional units or you are not using the units that you have for a sufficient time period.

If you have two or more rooms in your basement, a single dehumidifier in one room won’t effectively reduce the humidity in the other room, especially if the door between the rooms is closed. If the door is open and you want to use a single unit you will need a fan to provide sufficient air circulation between the rooms.

Most humidifiers are equipped with an automatic control switch, which will shut off the unit when a pre determined humidity setting is reached. In order for the humidifier to wring out a sufficient amount of moisture from the air to reach a comfortable level, you must let the unit operate until it shuts itself off. During hot, muggy summer days the dehumidifier may run almost continuously. If you allow the dehumidifier to run for only one or two hours a day, it won’t reduce the relative humidity to the comfort level.

A space heater is effective in heating a single room. If you want to heat more than one room, you will need a fan to circulate the air between the rooms. Even in a single room, a fan will be helpful in achieving a uniform heat distribution. A thermostat that's mounted on a wall across from the heater, rather than on the heater itself, will also improve heat distribution within the room.

Mold Creeps In

I have just noticed this since I had my rooms remodeled. I had them paneled, and I had insulation put on first. Now I find mold spots forming inside glass picture frames on the shelves. My basement is a dirt floor under the living room. Could the dirt floor be causing this problem? Any help would be appreciated.

I believe this problem is caused by the dirt floor in your basement. Even when the dirt feels dry to the touch, it wicks up subsurface water; and this is released into the area under the living room and eventually into the living room itself.

Before you remodeled your rooms, there apparently were enough open joints in the walls through which the moisture could escape to the outside. After you remodeled, those joints were sealed, causing the moisture to remain in the rooms.

In order to control the moisture buildup, you should cover the dirt floor in the basement with a vapor barrier, such as 4- or 6-mil-thick polyethylene plastic sheets. Overlap the sheets and tape the joints shut.

Persistent Musty Odor

What can we do to rid our home of the persistent musty odor in the basement? Books and other items stored there develop a smell that's usually retained. We have tried fans and dehumidifiers, to no avail.

A musty odor is quite common and is caused by mildew, a tiny, simple plant also known as fungus or mold. Mildew grows wherever it’s damp, dark, and poorly aired. It also feeds on cotton, linen, wood, and paper.

Mildew can be prevented by keeping an area or an item dry, usually with adequate air circulation. In a basement, this is often done by decreasing humidity with one or more dehumidifiers or by heating the basement. Mildew can be removed from an item using chlorine bleach, but clean a test patch first to deter mine whether the bleach will damage the item.

Mold spores can appear as black, brown, blue, orange or white specks, but they are not always visible to the naked eye. They often grow in carpeting, upholstered furniture and even on the back of wall paneling. If you’ve done all of the obvious things to eliminate the musty odor, and it persists, it’s because mold is growing in areas that are not readily visible.

At this point, you may have to call a company that specializes in treating mold conditions in “sick houses,” a phrase that describes houses with a range of air-quality problems. Unfortunately, these companies are scarce.

Beating Below-Grade Mildew

I have a second home, which we use mostly on weekends. The house is four years old and is built into the side of a hill. My problem is mildew in the closet, bathroom and laundry room on the lower floor. Last summer every thing in the closet mildewed—clothes, shoes, walls. The bathroom and laundry also mildewed around the baseboards, and on some of the walls.

We washed the walls and ceilings with a bleach solution, which seems to retard growth of mildew but doesn’t stop it. I’ve been told to vent the area by puffing in small fans, or to run a dehumidifier. But since it’s only a weekend house, I don’t want to leave anything electrical on when the house is empty. I have found this to be a common problem with weekenders all over our area, but no one seems to know what to do.

Mildew thrives in a damp environment, and in order to prevent it, it’s necessary to control the dampness. When the dampness is the result of condensation of the warm, moist summer air, it can be controlled by a dehumidifier and ventilation as was suggested to you.

However, it sounds as though your problem is caused mainly by moisture buildup on the foundation walls and floor slab because of the hydraulic pressure of wet soil adjacent to the house. In this case, additional measures must be taken.

Since your house is built into the side of a hill it’s likely that the ground on the uphill side is not graded properly. The ground, for at least several feet, should slope away from the house so surface water won’t accumulate against the foundation. Also, gutters and drain pipes should channel roof rain runoff away from the house, and toward the downhill slope of the land, rather than letting it saturate the soil adjacent to the foundation.

Since you don’t want to run a dehumidifier and fan during the week when the house is empty you might try getting rid of the dampness using chemicals that absorb moisture, such as silica gel and activated alumina. These chemicals can be placed in open containers or cloth bags in the problem areas. The chemicals have the capacity to absorb half their weight in water. After they have become saturated they can be heated to draw off the water, and then reused.

Overflowing Window Well

Please advise us on the problem of an over flowing window well. When there is heavy rain, the water builds up in the well, goes through the window frame and flows into the basement. How much crushed rock should be placed so the water will seep through?

You should prevent the rain from entering the window well rather than let it seep through a gravel bed. You can buy clear plastic domes that fit over the window- well opening at home centers.

If your window well is an odd size, you can build the equivalent of a plastic dome. Buy a sheet of clear plastic, and cut it to size so that it overlaps the sides of the window well by about 6 inches. Install a ledger board on the wall above the window well, and lay the plastic so it’s inclined with the top resting on—and secured to—the ledger. The bottom of the plastic should overlap the outside edge of the window well by a few inches.

If the window well is filled with gravel, during a sustained rain, water will seep down through the gravel and accumulate around the base of the foundation, where it could seep into the basement via a crack in the foundation wall.

Floor Tiles Come Loose

Most of the resilient floor tiles on my basement floor are lifting at the corners. Underneath the tiles is a saltlike substance. The tiles were placed just five years ago, and this condition is occurring on about one-third of the floor. Is there a solution?

In most cases, this is caused by moisture rising up through the floor. The white substance is indeed a salt. It’s called efflorescence. As moisture passes through masonry it dissolves salts present in it. When the moisture evaporates, salt crystals are deposited on its surface. The condition is probably caused by water accumulation under the basement floor slab. It may help to identify and eliminate the causes of the accumulation. Downspouts that empty roof runoff too close to the foundation, improper grading around the house and clogged foundation footing drains can all cause water to build up under the floor.

It’s hard to say whether the basement floor will be suitable for tile after the moisture problem is corrected. Residual moisture that remains may cause tiles to lift and may make it difficult to establish a bond firm enough to hold the tile in place.

If you want to tile the floor again, the loosened tiles will need to be removed and the surface below cleaned and prepared to ensure a strong bond. Consult with a flooring store about which products to use.

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Updated: Monday, December 26, 2016 2:02