Understanding The Problem

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 California 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's 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, unvented clothes dryers, bathrooms and cooking, as well as the 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 nonfunctioning subsurface drainage systems are also found relatively frequently. These problems can all be addressed and corrected if a systematic approach is used.

This page 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.

Air leakage through walls and floor

Rain and ground water

Defective or missing gutters and downspouts

Basement moisture sources

Rain and groundwater

In a 1-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.

Interior moisture sources

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.

Ventilation with humid outside air

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.

Moisture movement mechanisms

Capillary suction

Capillary suction moves moisture through porous materials. The water can be drawn upward through small pores in the concrete footing and slab and laterally through walls. This effect creates the ring of dampness seen at the base of many basement walls. This is very common at cold joints. Water can rise by capillary draw significantly as shown below:

Soil type and amount of capillary rise:

Gravel - Less than a few inches.
Sand - 1 to 8 feet.
Silt - 12 to 16 feet.
Clay - 12 to 20 feet.

Air leakage through walls and floor

In most houses, a stack effect is created because warm air rises. This induces a negative pressure on the basement and draws moist air in through any cracks or openings in the foundation including open sump pits. For this reason, sumps should have an airtight cover. With a concrete block foundation, moist air is drawn through the block cores, especially if they are left open at the top course.

Vapor diffusion through foundation walls

Vapor diffusion is the movement of moisture in the vapor state through a material. It's dependent on the permeability of the material and the driving force of vapor pressure differential. In a basement, vapor can diffuse from the wetter ground through concrete walls and floors toward the dryer basement interior. Vapor retarders such as foundation waterproofing and polyethylene slow down this process.

Typical causes of basement moisture problems

Inadequate grading

PROBLEM: If the ground around a foundation is level or slopes toward the house, water is directed into the basement. The soil next to the house is often backfilled without proper compaction and later settles. This is especially true under stoops where water can collect next to the basement wall.

Defective or missing gutters and downspouts

PROBLEM: Missing gutters and downspouts cause rainwater to be directed toward the foundation perimeter. A downspout without an extender or splashblock is worse than no downspout at all. It is depositing the huge volume of rainwater from the roof in a single concentrated location near the basement.

Improperly designed window wells

PROBLEM: Window wells are like a drain right next to the basement wall. Often they are improperly built so that any water is directed toward, rather than away from the foundation.

Ineffective drain tile and sump pit

PROBLEM: Many existing houses simply have no subsurface drainage system. This comes from a time when basements were not used as habitable space. In other cases, the systems don't work for a variety of reasons, such as collapse of the pipe, clogging of the pipe with silt and/or tree roots or a broken connection to the sump. The sump pit usually contains a pump designed to lift the water to the ground surface outside the foundation wall. This pump can fail.

Improper drainage with underslab ducts

PROBLEM: If heating ducts are installed beneath a basement floor slab, the drainage system may be inappropriately left at a level higher than the duct. In effect, the duct becomes the drainage system. With standing water within the heating duct, there are potentially serious health consequences from mold contamination.

Structural cracks

PROBLEM: Concrete and concrete block foundations usually develop some cracks. They can be severe if floor joists are not properly connected to the foundation wall, thus permitting the wall to move. Also, soil settling causes cracking. Places where walls meet rigid structures like the fireplace often crack as well. Usually, drainage removes the water from cracks, but repair may be necessary.

A problem that can damage your health and home