DAMP AND WET BASEMENTS

Damp and wet basements are perennial old-building problems, and if not attended to, can encourage insect infestation and decay of the building's wooden structure. Many old-house structural problems that I am called to deal with are in one way or another related to this condition. Since it's that pervasive, it seemed like it might be a good subject to write about again.

Many people lump "wet" and "damp" together when thinking about cellars, but they are actually two different conditions, with different causes. "Wet" refers to the presence of liquid water, seeping in through basement walls, standing on the floor, and more spectacularly, dripping from the structure overhead or running like a brook across the floor.

"Damp," on the other hand, refers to water vapor in the air. If it would only stay there it would not be a problem, but it tends to condense on (and in) objects, making them wet and corroding metals, damaging bricks, and encouraging insects and rots to set up housekeeping in wood.

To help understand ways of attempting to correct some of these conditions, it is useful to know how they come about. Damp basement air is partly a consequence of the construction of old basements: rubble stone walls and dirt floors (sometimes covered with a thin wash of cement to keep down the dust) easily admit the water vapor that is present in the soil.

If the basement is not ventilated or otherwise dehumidified, the relative humidity will gradually rise to an equilibrium, which sometimes can be one hundred percent (Basements tend to be cooler than outdoors in summer, which means that they tend to be more humid than the outside air). The vapor then condenses into liquid water and drips from every available surface.

It is difficult to prevent water vapor from entering a basement, so control of dampness typically focuses on mitigation by ventilation or mechanical dehumidification. Neither approach is ideal: Mechanical dehumidification is expensive to operate, and is not readily usable at temperatures below about sixty degrees, while under some circumstances, ventilating with outside air can introduce water vapor into the building, rather than remove it.* The worst example of this phenomenon that I have seen was in the basement of an old house where, one hot, humid summer, the outlines of the first-floor framing had been drawn in the dirt floor by water brought in with the outdoor air, condensing, and dripping from the floor structure.

Condensation is not the only route water can take into an old basement. Structures sometimes have been built over underground watercourses that result in streams flowing through the basement. I have seen cellars constructed with inlets and outlets in the walls to encourage the water on its way with minimum damage.

If the building was poorly sited, or if the basement was unwisely dug deeper, the water table can rise above floor level at wet times of year. Short of making sure that the heating plant and other mechanical and electrical equipment are set safely above high water, building cofferdams to protect them, or filling in the basement until its floor is above the high-water level, there is little that can be done about this condition.

More common, and responsible for most cases of liquid water flowing into the basement are poor exterior grading and failing or absent gutters and downspouts. Most problems with water in the basement involve water from rainfall and melting snow on the ground surface, not underground streams and high water tables, although those problems certainly do exist, and can be extremely challenging to mitigate.

Water flows downhill. This simple fact accounts for most preventable water-in-the-basement problems. The first cause of water getting into a basement is poor grading around the building. If the ground around a building is level, or worse, if it slopes down toward the building, surface water is going to flow toward the basement wall, and if the wall contains any openings, no matter how small, water is going to get into the building. This situation is more obvious with basement walls laid up with stone and brick, but it affects walls made of all materials.

Prevention involves keeping the water from getting into the basement wall. Once it penetrates the wall, it is almost impossible to stop water from going all the way into the building. Inside-the-wall waterproofing treatments rarely work, even on monolithic concrete walls; on joint-rich stone walls they don't stand a chance.

Since water can only flow downhill, the grade must slope away from the building. Maintenance is important, too. Even a well-graded building perimeter can be compromised over time. Runoff from the roof dripping onto the ground can erode the earth alongside the building, providing a perfect setting for puddling to occur. The roots of plantings near the building often raise the grade a short distance away from the building, as well, making another puddling place.

Hardening the ground around a building--with a concrete or brick apron, for instance--can help prevent erosion, but water splashes back onto the building from this kind of surface, and can over time cause deterioration of the building walls, particularly on northern exposures other places where drying is slow.

Gutters and downspouts reduce the quantity of water dripping from the eaves, and so reduce the amount of erosion and consequent puddling that will occur. But there is a trade-off with gutters: In exchange for reducing the eaves drip, they concentrate runoff at the places where the downspouts discharge onto earth. Using splash blocks (available at hardware and garden stores) under the downspout discharge, or better, taking that discharge underground to drywells located away from the building, is essential.

In situations where it is not possible to control water fully with these means, it may be possible to construct a subsurface dam around the building to divert water away from the basement wall. This is an old technique; the original dam material was blue clay, which is relatively impenetrable to water. The present-day version uses geotextile fabric (Enkadrain is one brand):

The ground around the building is excavated at a slope of one vertical to two or three horizontal for a distance of six to ten feet away, and geotextile fabric laid on the slope, which is then backfilled (In new construction, the fabric is laid down the outside of the basement wall itself, but when adding a dam to an existing building, the gentler slope avoids the need to expose and possibly de-stabilize masonry walls, and provides for a safer excavation)

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To summarize, preventing surface water from getting into the basement involves these measures:

Now let's return to water vapor, or humidity. Even if a basement is completely isolated from sources of liquid water, during warm weather it will tend to be more humid than the space above it, for the simple reason that it will be cooler. Since cool air can hold less moisture than warm, the relative humidity in the basement will rise. This is as true for newly-built waterproofed basements as it is for leaky old cellars. It's just worse in old cellars, because their walls and floors are porous, and admit more vapor from ground water.

The question then becomes, how can we best remove excess humidity from the air before it builds to levels that corrode metals, damage textiles and paper, rot wood, and invite wood-destroying insects?

The "simplest" and theoretically least expensive approach is to ventilate with outside air. Bringing in dry air to replace moist air will effectively reduce the relative humidity. When the weather is cool, this method works well. The problem comes as the weather warms up:

Outdoor air will reduce the relative humidity in the basement only when its dew point is below the temperature of the indoor air. If the outside air is warm and moist, its dew point will be higher than the basement air temperature, and rather than removing humidity, it will add it, in extreme cases resulting in condensation raining from the overhead structure.

In warm weather, therefore, the dew point of the outside air and the temperature of the basement air must be closely monitored, ventilating openings opened and closed, and ventilating fans switched on and off as conditions change. To do this effectively requires either a great deal of personal attention or a sophisticated system of automatic controls. Even then, natural ventilation will fail when it is needed the most--during hot, humid weather.

For this reason, most attempts to control warm-weather basement humidity end up using mechanical dehumidifiers. These machines are readily available and reliable. They are not without their own drawbacks, however:

Since dehumidifiers operate by cooling the air below its dew point to cause the water vapor in it to condense and precipitate out, they are relatively expensive to operate--in effect, they are like refrigerators running with open doors. They also produce significant quantities of liquid water, which must be disposed of.

Most portable dehumidifiers are fitted with removable collection buckets. It is preferable, though, to connect them to gravity drains (or sumps). Having to empty the bucket daily (or more often during especially humid weather) is a nuiance, hauling water always involves the risk of spills, and if the bucket is not regularly emptied, the dehumidifier will shut off and defeat the very purpose for which it is being used.

In unheated basements and other spaces, one should use a dehumidifier designed for low-temperature use. When the air temperature drops below about sixty to sixty-five degrees, the dehumidifier coils tend to ice up. If this process is not stopped, this ice will block the air flow through the machine and stress the motors, causing premature failure.

Low-temperature dehumidifiers are designed to shut off the condenser (cooler) as the coil begins to ice. The fan remains on, drawing air over the ice and melting it. Odd as it may sound, ice is an excellent dehumidifier, since the air flowing over it is cooled down near the freezing point, where it can hold very little water vapor. Once the ice has melted, the condenser switches back on and starts the cycle over again.

Heating the basement will both allow using dehumidifiers into the winter and, by reducing the temperature differential between the basement and the space above, lower the relative humidity.*

I noted earlier that during cooler weather the outdoor dew point lies well below the indoor temperature, and natural ventilation can effectively be used to reduce basement humidity. Unfortunately, as temperatures drop, outside air may cool the basement more than is acceptable.

To minimize that problem and avoid wasting heat, heat-recovery ventilators can be used. These readily-available devices bring in outside air and exhaust inside air, but retain inside the space most of the heat that would otherwise be dumped outdoors with the exhaust air.

Over the past couple of decades, heat-recovery ventilators have improved greatly in effectiveness, reliability, and cost, as increasing pressure to conserve energy resources has opened the market for devices that not so long ago were the stuff of science fiction.

To summarize, controlling water and humidity in basements is a challenging and not inexpensive proposition. The alternative, though, is to risk serious damage to, or even the loss of, the buildings and their contents.

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Copyright 1999-2008 Allen C. Hill