Basement moisture, associated symptoms and solutions

A problem that can damage your health and home

Basement moisture problems 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 are connected to the rest of the house through duct work 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.

Where basement moisture comes from

Moisture in a basement falls into two categories:sources of a basement moisture

  • Liquid water from rain, melting snow, overflowing drainage ditches, etc., surface water and underground water
  • Water vapor from steam, evaporation, condensation, etc. and moisture in the ambient air

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.

In order to determine how to deal with a damp basement the most important information you can have is the cause(s) of the moisture. It is not unrealistic to have more than one source of moisture in any basement since some of these moisture sources, which result in basement dampness, contribute to high moisture levels in virtually every basement.

Symptoms of a damp basement

The sources of basement moisture that we listed above will result in problems and exhibit symptoms to a greater or lesser extent. The most likely problems and symptoms a homeowner will encounter are as follows:

  • Water leaking through the basement wall(s)
  • Water penetration of the building envelope above grade
  • Efflorescence, spalling of concrete or masonry
  • The saturation of the base of concrete block and poured concrete foundation walls
  • General dampness / high humidity felt when entering the basement
  • The presence of condensation on foundation walls and basement windows
  • Standing water on floor
  • Visible water pooling in the insulation blanket(s)
  • Odor, mold, and mildew
  • Damp and/or wet carpeting
  • Swelling of laminate flooring
  • Buckling of hardwood flooring
  • Discoloration of the grout between floor tiles
  • Peel and stick tiles lifting or no longer adhering to the floor
  • Rot and decay of wood headers, joists, sill plates, and columns
  • Staining and blistering of wall covering
  • Discolored baseboards (usually a yellowish color)
  • Rotten paneling (at the bottom)
  • Wooden furniture is swollen or rotting at the bottom

Step by step process

  1. Control interior moisture sources.
  2. If summertime, don’t ventilate with outside air.
  3. Correct grading, gutter and downspout system.
  4. Provide an interior or exterior drainage system.


  • A dehumidifier can help reduce the symptoms of humidity and odor, but does not solve the problem.
  • A membrane or coating on the interior without providing drainage generally will not solve the problem in the long term.
  • Walls must be dry before insulating. Slabs must be warm and dry before carpeting.

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.

SOLUTION: Place earth around the house so that it slopes away from the foundation wall a minimum of one inch per foot for at least six feet.

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 splash block 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.

SOLUTION: Place a minimum of one downspout per 50 linear feet of roof eave. Extensions should discharge water at least four feet beyond the wall. Sloped concrete sidewalks around basements are very effective in directing rain runoff.

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.

SOLUTION: Window wells should be filled from the footing to the window sill with 3/8- to 3/4-inch coarse aggregate. A supplemental drain tile extension should extend from the footing to the base of the window well.

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 do not 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.

SOLUTION: Installing an exterior drainage system at an existing building is the most costly, but also the most effective water control approach.

In most cases when water is entering the basement, an interior drainage system is installed. The simplest and least costly approach is a drainage channel adhered at the base of the wall and the floor slab. Water is collected and drained into a sump using another channel placed on top of the slab, then through a trap to the sump basin. The sump should have an airtight, childproof cover. This system is best suited to a concrete wall with cracks. It does not solve the problem in masonry walls because water remains in the block cores at floor level and the water level is only lowered to the top of the slab. With this approach, the water is not completely removed from the space. The result is that humidity, mold, and mildew can still be a problem. This system cannot drain groundwater from under the floor slab.

Another technique is to place a drainage channel at the base of the wall on top of the footing. This requires removing and then replacing the concrete along the slab edge. The drainage channel is connected to a drain pipe leading to the sump. The sump should have an airtight, childproof cover. This approach is effective for concrete masonry walls with water problems because it drains the block cores completely. Holes must be drilled at the base of every block core to permit drainage. This may require removing more than the minimum amount of concrete, as shown, to fit the drill in. These systems have different shapes and prices depending on the product installed. Because moisture is allowed to penetrate the block cores, it is essential to cap the tops and place a vapor-retarder coating on the interior basement walls.

The most effective of the interior drainage systems is a perforated drain pipe installed inside the perimeter of the footing. This requires removing and replacing concrete at the slab edge. By placing the drain pipe beneath the slab, it drains the area to a lower level. Similar to an exterior system, the drainage pipe connects to a sump. The sump should have an airtight, childproof cover. A critical component of this approach is the dimpled plastic sheeting placed at the base of the wall and beneath the slab edge. Dimpled sheeting is similar to a small egg crate and permits free drainage of the wall into the drain pipe. It is less expensive than many specialized drainage channel systems. In low permeability soils, this system cannot accept rising groundwater unless there is an aggregate layer under the slab.

Improper drainage with under-slab 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, and with standing water within the heating duct, there are potentially serious health consequences from mold contamination.

SOLUTION: Heating ducts placed beneath the basement floor must be insulated, watertight, and sloped to collection points for drainage and cleaning. A drain tile and coarse aggregate can be placed under the duct work.

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.

SOLUTION: Proper footing design and proper connection between the foundation wall and the structure above are required (e.g. anchor bolts or straps at the sill plate and floor joists nailed to the sill plate).

If you have no idea why your basement is damp or why there is visible moisture, contact us.

Metrolinx Construction services Highlands Ranch and surrounding area.