Moisture- and Mold-free Basement Living

Many basements are damp, musty, leak air and have little natural light. As a lifetime drywall contractor, I have hung more than my share of drywall in basements. I would just hang and tape some moisture-resistant drywall and be on my way. In 2009, I became a building-performance analyst through the Malta, N.Y.-based Building Performance Institute and a year later a green-home verifier through the National Association of Home Builders, Washington, D.C. My building-science training helped me realize I needed to spend more time thinking about solving the moisture/water problem first and incorporating construction techniques that prevent moisture from moving into unwanted areas.

When dealing with a moisture/water problem, first eliminate the source; then, remove or divert the pathways; and finally, install resistant materials. Understanding building science is very important. The practical purpose of building science is to provide predictive capability to optimize building performance and understand or prevent building failures. When creating living space in a basement, the failures could involve moisture intrusion, air quality and energy efficiency.

Moisture Intrusion

If a moisture problem is discovered in a basement, you must solve the moisture problem before doing any construction work. First, identify the cause. To tell if walls are damp from exterior water or condensation from humid interior air, tape a 2-foot square sheet of plastic to the masonry on a below-grade area of the wall.

Condensation can appear as water droplets, wet spots or puddles on basement floors and walls. It happens when moist, warm air hits cool foundation walls or uninsulated cold-water pipes, dampening carpets, rusting appliances and making the basement clammy. If moisture collects on the front of the plastic, it is condensation.

There are several ways to solve condensation problems:

  • Insulate: Insulate surfaces where condensation is likely to occur, such as cold-water pipes and ductwork. Installing continuous insulation along the foundation wall will help prevent air from cooling and reaching its dew point, which is the temperature at which moisture changes to liquid form based on the relative humidity of the air before it cools. Warm air can hold more moisture than cool air; 80 F air with high humidity will reach its dew point at a higher temperature than 80 F air that has a lower humidity. Having a continuous air barrier at the same location as the insulation also will help.
  • Air seal: Because air movement can carry potentially moist air into walls and ceilings, caulking and foam sealing any gaps in the building envelope is a must.
  • Dehumidify: Install a dehumidifier to remove excess moisture from the air. For continuous dehumidifying, connect the unit’s collection reservoir to a drain.

Capillary action is the problem if moisture collects on the backside of the plastic after a few days. This indicates water is wicking through the foundation wall from outside. Water can move through solid concrete. The basement should be treated the same as if it were leaky. If there is regular seepage or puddles appearing after storms (even once every few years), the problem has to be fixed permanently before finishing the basement. The moisture may be present because of runoff or subsurface sources.

Runoff: Rainwater or melted snow that isn’t routed away from a house is a common cause of basement moisture. Runoff percolates through porous topsoil and then stops at the compact soil near the base of the foundation. Accumulated pressure forces the water through gaps or cracks in walls and footings.

There are several options to solve runoff problems:

  • Proper grade: If the ground near the foundation slopes toward the house, you will need to grade the slope away from the house to move water away from the foundation. Grade the slope at least 1 inch vertically for every 1 foot of horizontal for at least 4 feet.
  • Gutters: Clean gutters and make sure they are sloping in the proper direction with no low sections. Install an extension for the downspout to redirect the runoff away from the foundation. A 4-foot extension would be ideal if space allows.
  • Permeable materials: Having fewer solid surfaces on the property near the house will reduce the likelihood of runoff water reaching the foundation where it could accumulate.

Subsurface sources: In tougher cases, capillary action may be caused by rising groundwater, which may be fed by an underground spring. Groundwater is by far the most difficult water source to stop. If the wet-basement symptoms act like runoff but occur all the time, high groundwater is likely to blame.

There are several options for solving subsurface problems:

  • Foundation sealing: If water infiltration is minor, inspect the interior walls and seal all seams, and caulk cracks and holes in the foundation with a polyurethane masonry product. For larger cracks or gaps, use hydraulic cement. When water infiltration is more extensive, exterior waterproofing is far more effective than interior waterproofing but is more costly. Typically, exterior waterproofing is done in conjunction with outside foundation drains. The exterior foundation can be waterproofed, and an insulation board to improve the basement’s thermal performance can be installed simultaneously.
  • Footing drains, sump pumps, drainage planes: For water that can’t be eliminated or diverted, the best approach from the interior is to collect the water in a French drain, conduct it to a sump pump and pump it out. A French drain is a bed of stone that has replaced the soil around the interior perimeter of the basement just below the floor. Water that attempts to enter from along the perimeter through the wall or from below will flow in this channel, enter a sump pit and be moved out of the house quickly. Keep in mind a sump pump requires electricity; if power is lost during a storm, the system will not work. It would be ideal to have the water drain out a pipe that slopes to daylight.

Construction Materials

Mold needs water, oxygen and nutrients to grow. Once you have eliminated any basement moisture, air sealing will reduce the amount of oxygen available for mold growth. Construction with mold-resistant products where possible will help reduce the food source necessary for mold growth.

Unless jack posts are being taken out, most basement walls are not structural support walls, so light-gauge steel is a good choice. The metal is inorganic so it is not a food source for mold, and metal does not absorb moisture. I suppose if it were exposed to moisture often enough the metal would corrode. There also is a recycled-resin stud that is marketed for basement remodeling because it is mold-, moisture- and corrosion-resistant. (For more information about the resin-based EcoStud Track and Stud System, see July 2011 issue, page 74, or visit ForResidentialPros.com/10239491.) Metal or resin studs are lightweight compared to wood so getting the materials into the basement is easier. If you are going to use wood, a treated plate is usually installed along the floor.

Floor

The existing concrete floor should have a vapor barrier under the concrete. In case moisture appears on the floor, it is a good idea to have a material in contact with the floor that provides a barrier and space so the moisture can drain away. Look for 2- by 2-foot interlocking wood panels with a plastic bottom that holds the wood away from the concrete and allows any water to drain away. If an additional layer of concrete is poured over the existing concrete, lay a plastic membrane made to be both a vapor retarder and a drainage plane before pouring the new concrete to allow water to drain away in a similar manner.

Insulation

Keep the higher R-value insulation to the outside. If using foam panels, they should be a continuous surface from floor to ceiling (sealed on all edges). The rim joist area should also be insulated and connected with the wall insulation. A common practice is to have the best insulation and the air barrier next to the outside concrete wall. This prevents warm air from reaching the cooler wall. The closest the air can get to the concrete wall is the warm insulation surface. If a vapor retarder is used it should be next to the warm side. Note: A vapor retarder, such as a poly membrane, is not necessarily an air barrier. Time must be spent sealing edges and splices with a good-quality flexible caulk. I prefer to make the drywall an air barrier and prime it with a vapor-retarding primer when a vapor retarder is required.

Drywall

Even if the block or concrete wall appears dry, it is not recommended to attach drywall directly to it in a basement. Poor adhesion and moisture and mold problems could occur. Instead, build a separate wall with metal, plastic or wood framing. The same criteria should be followed if a plaster finish is desired.

Most if not all drywall manufacturers sell a mold- and moisture-resistant drywall. If the goal is to avoid mold growth in the assembly, then all materials—or as many as possible—should be mold-resistant. Paperless drywall, fiberglass-mat joint tape and mold-resistant joint compound are designed to increase the longevity of an assembly built in a damp environment.

Optimize the Basement

When I was a kid, my parents’ house had a full basement that contained the washer and dryer, a huge wood and oil furnace, ductwork and a 60-gallon hot water heater. My mother canned in the basement and stored her vegetables and jams. Every fall, we would pack about nine full cords of firewood down there. We needed the basement in those days. Recently, I have done a lot of energy audits and find most basements are full of junk. These basements are conditioned spaces that can be put to much better use than storage. For much less money than building an addition, homeowners can use the same footprint as the house itself to create some quality living space, but they can’t do it without a remodeler who is willing to take the time to ensure the basement and homeowners’ belongings stay dry.

Myron Ferguson is owner of Ferguson Drywall Innovations Inc., Middle Grove, N.Y. He has 30 plus years’ experience as a drywall contractor, is a member of the National Association of Home Builders, and is an author and teacher who provides instruction about drywall and related topics.

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