Today’s homeowners won't tolerate damp, wet, musty-smelling, and mold-infested basements, especially in newly constructed homes. 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. Builders have a need, therefore, to understand what causes moisture in basements and to use construction methods and products that eliminate moisture problems.
To build a basement that’s protected against moisture, it helps to take a whole-house approach. Consider how each aspect 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. The Best Practices® are listed in the typical construction sequence:
Apply a capillary break to the top of footings to prevent water from wicking up. 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 to collect any rising groundwater and carry it away from the foundation. 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 downward. Place the top of the highest section of pipe no higher than the top of the footings to prevent standing water 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 piping that has an integrated filter sock to keep out silt and clay soils.
Seal all penetrations in the foundation walls, creating 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 it 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 at the specified thickness, consistently 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” 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, for example, dimples outward.
Install a capillary break under the slab. To control water infiltration via capillary action, place a 4” to 6” 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 to prevent water vapor from rising through the gravel layer and into the slab and 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” up the foundation wall, fixing it to the wall with construction tape or adhesive. Overlap all seams at least 12”, and seal the seams with 3” construction tape. Also, seal all punctures from 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 environmental conditions. Make sure the aggregate doesn't separate from the cement paste during placement. 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.
Protecting the basement from water intrusion is a significant step toward constructing a durable home. Keeping water away from the foundation helps to ensure that the basement level is a safe, healthy, comfortable environment for your homebuyers.
More Best Practices® for basement construction can be found in two online training courses from BuildIQ University: Foundations: Basements & Crawlspaces and Water Management: Basements & Crawlspaces.
