Walls wet? No Sweat
By: Don Carter, MS, P.E.
One of the things we are occasionally tasked to assess is a basement that sweats. This is fairly common in spring and fall when neither the air conditioner nor furnace is running to circulate and dry the air. The principle is well known of course, same thing that happens when a glass of ice water sweats. Condensation forms as warm and humid room air encounters a cooler surface and changes into water droplets. In basements, that cooler surface is the earth-side wall. Dry air is the solution and even a small dehumidifier can generally do the job.
Floors are a different matter. Often they sweat for the same reason walls do but sometimes they produce water even when surrounding air is dry. You may have seen this as boxes, toys, or other objects placed on the floor, leaving a wet footprint after they are moved. The wet spot is caused by water beneath the floor passing through concrete as vapor then getting trapped under objects. If you have this situation, here are some things to know:
Concrete is porous just like a sponge and water always tries to pass from its wet to dry side. Since I can remember, concrete and block basement walls have had an exterior bituminous coating (the black stuff) to keep ground water from passing through walls. Not true for floors. Building codes now call for a plastic sheet under basement floors to act as a vapor barrier, but this requirement is relatively new, and older homes don't have it.
If you see wet footprints, place a 1' square of clear poly (think laundry bag) on a clean bare spot of the basement floor and duct tape all around. If there is standing water under the floor, vapor will condense on the poly underside.
Standing water typically comes from 1 of 4 sources - treated city water as in broken service line or lawn irrigation system, leaky sewer pipe, natural spring or rain runoff. A common source in homes over 30 years old is a dribbling sewer line. Many of the old sewer lines were slip fit cast iron and have now begun to disintegrate from rust and roots.
If there is water under the floor, you should obtain a sample and have it laboratory tested for fluoride, chlorine, sulfates and fecal coliforms to help identify its origin. Knowing where water comes from is knowing how to stop it. Standing water has the potential to heave a floor or undermine foundation footings, either of which will throw the house out of alignment. Stop the problem early, before it causes expensive secondary damage.