Combatting Moisture Issues in Homes with Concrete Slabs

It’s important to acknowledge that as it is curing the typical concrete slab is giving up approximately 1,000 gallons of water the first two years after being poured. This is equal to about 1.3 gallons of water a day. Even after it has largely cured the typical slab is an excellent moisture transfer mechanism for its lifetime, due mostly to vapor diffusion and capillary action. In cases of high water table conditions bulk water can easily push through gaps in the slab.
 
Air circulation and dehumidification is critical to removing moisture. Proper HVAC design can greatly enhance these elements. But before we discuss those strategies let’s address the slab preparation. Our #1 recommendation to combat basement moisture levels is to fully insulate under the slab.
 
Installing full insulation under the entire slab, with an R15 slab edge break (the R15 slab edge perimeter insulation is Oregon code, found under the prescriptive code table M1101.1 in OR ORSC chapter 11), will keep the slab warm by de-coupling it from the cold ground. This will greatly reduce excess basement moisture levels by eliminating the ability of warm, moist basement air from condensing on the cold slab. If you’re installing carpet over slab that is cold (ie- un-insulated) your condensation problems, and hence moisture levels, will be worse. This is because carpet and pad de-couples the slab from the room, making that slab even colder. Use two (2) inches of XPS rigid foam under the slab for an R value of 10, and be sure to air seal (spray foam or high quality construction tapes) the gaps/seams in the rigid foam board (because XPS rigid foam is vapor impermeable air sealing the seams will practically eliminate vapor diffusion).
 
Increased air circulation, dehumidification, and balanced mechanical ventilation will further reduce moisture in the basement. Air conditioning removes latent moisture from the house, while air circulation will keep stagnant air from accumulating moisture and condensing in the basement. We strongly recommend not only HVAC supply registers in the basement, but also the inclusion of a return HVAC register to bring that air quickly back to the air handler and (in the summer time) remove the moisture. This basement return register is very important if there’s a door separating the basement from the first floor.