University of Minnesota Extension

FO-07051     Revised 2006  

Moisture in Basements: Causes and Solutions


Moisture source diagram

A Problem that Can Damage Your Health and Home

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.

Understanding the Problem

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:

1. Liquid water from rain or ground-water.

2. Interior moisture sources such as humidifiers, unvented clothes dryers, bathrooms, and cooking, as well as the moisture in concrete after construction.

3. 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:
1. liquid water flow
2. capillary suction
3. vapor diffusion and
4. 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 nonfunctioning 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.

Symptoms

  • 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
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The information given in this publication is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by University of Minnesota Extension is implied.

Produced by Communication and Educational Technology Services, University of Minnesota Extension.

In accordance with the Americans with Disabilities Act, this material is available in alternative formats upon request. Please contact your University of Minnesota Extension office or the Extension Store at (800) 876-8636.


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