Preventing Basement Moisture

Homeowners expect warm, dry basements that can be finished to extend the home's livable space. At a minimum, they expect a moisture-free area for storage. So it’s crucial to understand what causes moisture in basements and use construction methods and products that eliminate moisture problems.

To build a basement that's protected against moisture, take a whole-house approach. Consider how all aspects of the foundation and surrounding construction — perimeter drain, footings, exterior walls, slab, backfill, grade, flatwork — should be installed to work together to achieve a dry basement. Following are the best practices for constructing the foundation to prevent both bulk water and water vapor intrusion, listed in the typical construction sequence.

Apply a capillary break to the top of footings. To prevent water from wicking up through the footings, apply a polyethylene sheet or an elastomeric asphalt coating to the top of the footings to create a capillary break between the footings and the foundation wall and slab.

Install a perimeter drain. The drain will collect any rising groundwater and carry it away from the foundation. To install, lay a bed of clean gravel fill in the excavation and place the perimeter drain alongside the footings. Orient the pipe with the holes facing down. Place the top of the highest section of pipe no higher than the top of the footings to prevent standing water from collecting at the floor level, and slope the pipe slightly toward the main storm sewer or to an area above ground that's lower than the footings. Protect the perimeter drain from damage and clogging by surrounding it with gravel fill. In addition, use pipe that has an integrated filter sock to keep out silt and clay soils.

Seal all penetrations in the foundation walls. The goal when sealing penetrations is to create a barrier against water and air intrusion. Use an approved mortar or low-shrink sealant. When sealing around a cast-in penetration that has a sleeve, such as a gas or water line, seal the space between the service line and the sleeve. If a penetration doesn't have a sleeve, make sure the penetration is protected from any chemical reaction with the concrete.

Apply a waterproofing system to the exterior of the foundation walls. This system has two components: an elastomeric asphalt coating to seal the porous concrete surface of the foundation walls and a drainage medium to direct water down the walls and into the perimeter drain, relieving hydrostatic pressure on the foundation. Apply the coating consistently to the specified thickness from footing to grade. Allow the coating enough time to cure according to the manufacturer's specifications. Next, install the drainage medium over the asphalt coating. The drainage medium can be a 1-inch insulative drainage board or a dimpled membrane. Confirm that all courses are attached securely to the walls, that no breaks or gaps exist between the panels and that the product is oriented and installed per the manufacturer's specifications — e.g., dimples outward.

Install a capillary break under the slab. To control water infiltration via capillary action, place a 4- to 6-inch thick layer of clean gravel in the foundation footprint. Thoroughly tamp the gravel with a plate compactor to create a level surface for the concrete.

Install a vapor barrier under the slab. This prevents water vapor from rising through the gravel layer and into the slab and prevents water from wicking into the slab during heavy rainfall. Position a 10-mil polyethylene vapor barrier to fully cover the foundation footprint, and extend it 2 to 4 inches up the foundation wall, fixing it to the wall with construction tape or adhesive. Overlap all seams by at least a foot, and seal the seams with 3-inch construction tape. Also seal all penetrations around plumbing and electrical stubs with construction tape.

Pour and finish the slab properly. Specify concrete with durable, well-graded aggregate, with a low water-to-cement ratio and with adequate compressive strength. Use a low-shrinkage concrete mix, and use admixtures only if they're appropriate to the environment. To help prevent the concrete from segregating, pour the first batch into the farthest area of the foundation footprint, and pour subsequent batches against the previously placed concrete. Work concrete thoroughly around reinforcements and embedded fixtures, into the corners and against the sides of the footprint to move aggregate into these areas. Finish the concrete once all bleed water has evaporated. Don't add water to increase the working time, because it weakens the slab surface and leads to spalling and other surface issues. Install control joints to direct inevitable cracking; locate them no farther apart than 30 times the thickness of the slab.

Backfill the foundation properly. Prepare the overdig by removing any debris. Use backfill material that allows for good drainage, is free of organic materials such as tree roots and vegetation, and contains no large rocks, pieces of concrete or sharp objects that could puncture the insulation or waterproofing system. Place the backfill material carefully in the corners of the foundation first for the greatest support. Backfill the remainder of the excavation in layers to promote even settling and minimize stress on the foundation, taking care not to puncture the waterproofing system.

Slope the rough grade away from the home. Establish the rough grade immediately after backfilling to keep water from collecting against the foundation and to drain water away from the home. Typically, the rough grade should fall a minimum of 6 inches within the first 10 feet and slope to the sidewalk or street.

Establish swales. Swales are particularly important in areas of high rainfall to direct large amounts of water away from the home and into the storm-drain system. Locate and slope swales per the lot plan, typically at least 2 percent.

Establish the correct final grade. Follow the slope established by the rough grade, maintaining the top of the foundation at a minimum of 8 inches above grade.

Install flatwork that promotes drainage. All sidewalks, driveways and patios must allow water to drain through or direct water away from the home into a swale, an area drain or the street. Before pouring concrete for flatwork, install a level, tamped bed of clean gravel fill as a capillary break between the soil and the flatwork.

Install landscaping correctly. Maintain the top of the foundation at a minimum of 8 inches above the soil to prevent moisture from damaging the exterior finish. Place plants a minimum of 2 feet from the foundation and low enough in the soil so as not to direct water toward the foundation. Don’t overwater the ground during landscaping.