Basements Are Wet
Unless it’s built like a boat, with proper exterior waterproofing and radon control systems in place, a basement is a dangerous space to finish. The ground around a basement is almost always wet, and the walls and concrete slab are porous; CMUs and mortar joints are super-porous. A basement may look dry, but that’s because the walls and floor are being heated, converting liquid water into vapor, which is pulled into the air. As soon as you cover up those surfaces and insulate, which restricts airflow and moves the condensation point to the interior surface of the foundation wall, you’ll have wet walls and floors.
A basement should be built like a boat to keep water out. But most houses are built on porous walls and floors with nothing between the wet and the dry. In a house that is built like a sieve, if you want finish materials in the basement, you first have to manage the moisture in the walls and floor. To be successful, there are a few simple things you absolutely must do, and a few you absolutely must avoid.
Here’s a list of Dos and Don’ts, plus some illustrations that show everything you need.
DO install a drain-tile system with a sump pump that discharges any water within a minimum of 5 feet from the foundation.
DO extend interior drainboard from the drain tile up to at least 6 inches above grade. Best is a dimpled sheathing.
DO frame with steel studs set on a capillary break, such as a sill seal (this also helps with uneven floors).
DO hold framing 1 inch away from the foundation walls—no exceptions!
DO spray high-density polyurethane foam 2 to 3 inches thick behind the stud wall and all the way up onto the rim joist. If you choose to use batt insulation, don’t let it touch the foundation walls.
DO install paperless drywall, and hold it ½ inch off the floor.
DON’T let the wall framing touch the walls or the floor (treated lumber wicks water).
DO install an active Sub-Slab Depressurization System (SSDS) to control moisture and radon. The SSDS can remove up to 10 gallons of water a day from under your home, helping to improve air quality and reduce the potential for mold and musty odors. As a side benefit, it will also reduce radon levels in your home!
DO use moisture-resistant flooring, such as ceramic tile or vinyl composition tile. If you want the look of wood, cover the slab with a vapor-control layer with taped seams and install a glued or dry-fit engineered click-lock floating floor system.
DON’T install carpet.