A Good Foundation
Since houses are typically attached to the ground, managing groundwater is essential to support better health and wellness. This applies to slabs, crawlspaces and basements in both new and existing homes.
Most foundations are built with brick or concrete. Both are sponges that wick water like a Bounty paper towel. The process is called capillary action. A dry foundation must include damp proofing to stop capillary action. Any material that touches the ground should include damp proofing or be de-coupled from the ground. Damp proofing material fills the pores in brick and concrete to stop the wicking water. De-coupling uses materials and methods that will not allow the material to stay wet.
The photo to the right shows ground water suction in the basement of a new home. This is evidence of poorly installed damp proofing. If this area eventually becomes living space, the moisture becomes trapped inside the wall cavity. Once the wall is insulated, the trapped moisture can quickly spawn fungal growth, hidden inside the wall. This hidden biological growth is frequently the cause of most musty smells in finished basements.
Damp proofing the top of the footing (shown below) will reduce capillary action. Unfortunately, the capillary rise through footings is typically ignored.
Exterior foundation walls must include a damp proofing material and ideally be de-coupled from the backfill with some type of drainage mat. A “drainage mat” is a semi-rigid dimpled moisture-impermeable polymer material that comes in sheets. The dimples form an air gap between the foundation and the membrane. This directs water downwards to a perimeter drain, where it is piped away from the building.
It’s interesting to note that building code requires damp proofing on below grade walls. However there is NO requirement for a system to manage bulk groundwater. Damp proofing is typically not able withstand significant pressures of ground water force, which makes drainage a mat critical. Even below-grade walls that are damp or water proofed with tar, asphalt or sprayed on membranes will eventually fail at seams. A properly designed water management system will solve this issue.
The detail to the left shows an alternative method of damp proofing by encapsulating the entire footing with a moisture proof membrane.
Stopping capillary action is also required for slabs and crawlspaces. A 4-inch layer of wash stone underneath concrete slabs will decouple the concrete from the wet soil. Then a robust vapor impermeable ground cover between the stone and the slab provides more protection. Wash stone under a slab also makes radon mitigation easier. Sand is not a proper material to be used under a slab.
Before pouring a concrete slab, always consider radon resistant construction techniques. The detail to the upper left shows a simple passive radon vent that can be installed underneath a concrete slab. The riser pipe is routed up through the house and out the roof like a plumbing vent. If a radon test shows mitigation is required, the system is completed with the addition of a simple fan.
If your home is one of the 25,000,000 or so that has a crawl space, you should know that crawl space thinking has changed over the past 20 years. Science and research has now established that the vents need go. Vented crawl spaces are associated with a variety of problems, including excessive moisture, mold and mildew, wood rot, corrosion, poor indoor air quality, and insect and pest infestation. Venting a crawl space also increases heating and cooling bills, create uncomfortable cold floors and greater wear and tear on heating and cooling equipment.
The discussion regarding closed crawl spaces contained herein is not comprehensive. There are numerous details required for best practice. A positive drain, capillary break and an appropriate humidity control strategy are also critical. A thorough discussion of those details can be found at
Both board foam and spray foam are acceptable insulation materials for closed crawl spaces.
Where possible, best practice is always to install insulation on the exterior of a wall. Insulation around the permieter edge and underneath concrete slabs improves comfort, reduces heat loss and the risk of condensation under floor finishes.