Keep Slabs Dry
Provide a capillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. break between the slab and the soil.
Ground moisture entering a building not only corrodes metal, but also causes fungi and bacteria growth. Groundwater and vapor pass through a concrete slab in several ways: a rising water table, capillary action, diffusion of water vapor through concrete, and as moisture-laden air through gaps or air ducts located beneath the slab. (For this reason, do not locate ducts beneath slabs.)
The solution to the ground moisture problem is to provide a capillary break. A drainage pad of clean 3/4-in. stone (no fines) topped with 6-mil polyethylene plastic directly beneath the concrete slab will drain rising water through gravity or a mechanical pump. The 6-mil plastic serves as a vapor barrier.
Green Points
LEED for HomesLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. : 1/2 point available for fly ashFine particulates consisting primarily of silica, alumina, and iron that are collected from flue gases during coal combustion. Flyash is employed as a substitute for some of the portland cement used in the making of concrete, producing a denser, stronger, and slower-setting material while eliminating a portion of the energy-intensive cement required. More info or slag as replacement for 30% or more of Portland cement in foundation under MR 2.2 (Materials & Resources).
NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. /ICC-700: Under Chapter 6 — Resource Efficiency: 3 pts. for use in foundation systems, such as slabs, that are considered resource-efficient (601.8); 4 points for interior and exterior foundation perimeter drains sloped to daylight, dry wellUnderground structure that captures, then slowly releases storm-water runoff so that it can be absorbed by the soil., or sump pit (602.3); up to 4 pts. for recycled-content (fly ash or slag substitution for Portland cement in concrete) (604.1). Under Ch. 7 — Energy Efficiency: up to 4 pts. for use in slabs as part of a passive solar design (704.3.1.3, 704.3.1.4).
Learn more in the Green Building Encyclopedia:
Further Resources
Building Science Corp.
Primer on problems with concrete slabs and how to prevent them:
