The Cause and Effects of Basement Water Intrusion
The University Of Minnesota published a comprehensive study about the causes of water intrusion, as well as the consequences. Here is a valuable excerpt of their study.
“Moisture problems in existing basements are very common, but often are not understood or properly treated. In a basement that is seldom used and separate from the living spaces above, this may not present a great problem. However, most basements in Minnesota are connected to the rest of the house through ductwork or other openings. In addition, basements are increasingly used as finished living and bedroom spaces. In these cases, moisture problems are not only annoying and uncomfortable, but can lead to significant health problems. Molds and mildew can grow in damp carpets and beneath wall coverings. Finishing a basement without first dealing with the moisture problems can result in making health conditions worse and lead to significant damage as well. Basement water problems are solvable, but there is a cost to doing it right.”
To correct basement moisture problems, it is necessary to understand where the water is coming from, and what mechanisms permit it to enter the basement.
There are just three sources of moisture:
- Liquid water from rain or ground-water
- Interior moisture sources such as humidifiers, un-vented clothes dryers, bathrooms, cooking, moisture in concrete after construction
- Exterior humid air that enters the basement and condenses on cooler surfaces –
Moisture is transferred from the outside of the building to the basement interior by four mechanisms:
- liquid water flow
- capillary suction
- vapor diffusion
- air movement
Sometimes problems are traced to poor construction with cracking, settling foundations. In many cases, however, houses and basements can be structurally sound but are often not properly built to handle water drainage. Failure to slope the ground surface away from the foundation or lack of a good gutter and downspout system is common. Missing or non-functioning subsurface drainage systems are also found relatively frequently. These problems can all be addressed and corrected if a systematic approach is used.
This publication briefly describes moisture sources, moisture movement mechanisms, and typical basement moisture problems. Then, a step-by-step process for addressing each problem is presented along with several detailed approaches to solving the problem.
- Water trickling out of walls
- Standing water on floor
- Saturated base of concrete block walls— a ring of dampness
- Damp, humid air
- Condensation on cold walls and floor in summer
- Odor, mold, and mildew
- Deterioration of carpet or wood
- Rot and decay of wood headers, joists, sill plates, and columns
- Staining and blistering of wall covering
- Efflorescence, spalling of concrete or masonry
Rain and groundwater
In a one-inch rain, 1,250 gallons of water fall on the roof of a 2,000-square-foot house. Without proper grading, gutters, and downspouts, some of this water flows into the basement. The below-grade water table can also rise due to flooding or seasonal site conditions. This is why drain tile systems are recommended around basement walls even in sandy or gravel soils.
Moisture is generated inside of basements from people and their activities. Common sources are humidifiers, unvented clothes dryers, showering, and cooking. When basements are finished, these activities increase. Another source that can be thought of as internal is the moisture contained in new concrete after construction. In a typical house, this can amount to 0.2 gallons per square foot of wall, and 0.1 gallons per square foot of floor. It may take many months or even years for a new house to come into equilibrium with its environment.
In the summertime, basement windows may be opened for fresh air. If the outside air is warm and humid, it will condense on the cool basement wall and floor surfaces. Many homeowners see this moisture and believe they are experiencing basement wall leakage, when in fact the accumulated moisture is from condensation.
The best way to approach any building problem is to first do the things that are easy and low cost. Then proceed in a logical order doing the next least costly technique with the most positive likely result. With moisture problems, the best approach is almost always to remove or control the source of the moisture, not to try to stop it at the last line of defense.
First, the simplest and least costly techniques are to remove excessive internal moisture sources in the basement (humidifiers, cooking) and ventilate other sources (clothes dryer, bathroom). Second, if condensation in the summer is the problem, do not ventilate the basement directly with warm, humid air. Ventilation through an air conditioning system or with a dessicant-type heat exchanger is recommended.
Dehumidification can be used as a means of reducing the symptoms of humidity and odor in a basement, but it is not a permanent or complete solution. In fact, if a dehumidifier is used in a basement with moisture problems, it may cause greater damage. By drying out the basement air, moisture is drawn into the basement more rapidly causing efflorescence and spalling of concrete and further damage to interior finishes.
It is appealing to solve a basement moisture problem with a membrane or coating on the inside. It is less expensive than a drainage system and seems to work for a time in some cases. The water is still there, however, and eventually these systems deteriorate or simply move the water to another pathway into the basement.
The recommended approach after removing interior moisture sources is to evaluate the gutters, downspouts, and surface grading around the house. These should be corrected first and may solve the problem. Then, if a moisture problem persists, proceed with an interior or exterior drainage system. All of these techniques are described in the remainder of this publication. If your goal is to finish a basement that has water problems, it is recommended to first deal with the water problem.
Step by step process
Control interior moisture sources.
If summertime, don’t ventilate with outside air.
Correct grading, gutter and downspout system.
Provide an interior or exterior drainage system.