Basements start with water
A basement slab sits in the ground, and the ground is wet. Moisture moves up through concrete as vapor, and where groundwater rises against the slab it can push liquid water through cracks and joints. A coating that's sealed over a damp slab traps that moisture underneath, and the pressure can lift the coating in blisters. So a basement coating project starts with two questions: is liquid water coming in, and how much moisture vapor is the slab giving off?
Fix leaks before you coat
A floor coating isn't a waterproofing system for a leaking basement. If water seeps through walls, cracks or the joint where the wall meets the floor, that needs to be dealt with first, with drainage, grading, gutters, crack repair or a waterproofing contractor. Coating over an active leak hides the problem until the coating fails. Tell the specialist about any history of water, even if the floor is dry today.
Test the slab
Two standard tests are used. ASTM F1869 measures the moisture vapor emission rate at the surface with a sealed dome of calcium chloride, reported in pounds of moisture over 1,000 square feet in 24 hours. ASTM F2170 measures the relative humidity inside the slab with probes set in drilled holes. Either result only describes the slab at the time and places it was tested, so testing should cover the floor, not one corner. The coating's data sheet says what level that product tolerates.
Moisture-mitigating primers
When a slab tests above what the chosen coating tolerates, the usual answer is a primer or basecoat made specifically to be applied over damp, high-emission concrete. These products are rated for particular moisture levels, and the rating is on their data sheet. Ask which product the specialist will use, what level it's rated for, and how that compares with your test results.
Decorative options
- Solid color. A pigmented epoxy or urethane system in one color, often with a clear topcoat. Simple, clean and easy to maintain.
- Flake or quartz broadcast. Texture and color variation that hides dust and small imperfections, the same approach used in garages.
- Metallic epoxy. Pigments that move as the coating levels, producing a swirled, marbled look. Each floor is one of a kind, and the look depends heavily on the installer's technique, so ask to see the installer's own previous floors in person.
- Self-leveling systems. Thicker products that flow out to a smoother surface. They need a floor that's been prepared and repaired carefully, because they reveal every low spot.
Air, odor and the people upstairs
Basements are enclosed and often connected to the rest of the house. Many coating products give off strong odors while they cure, and some contain solvents. Ask for the product's safety data sheet, ask about low-VOC or low-odor options, and plan for ventilation and for keeping people and pets out of the space while it cures. Follow the specialist's instructions on when the space is safe to use.
Temperature and humidity during the job
Coatings cure more slowly in cold rooms and can fail to cure properly when the slab is close to the dew point. One manufacturer's floor epoxy data sheet, for example, requires the substrate to be at least 5°F above the measured dew point to reduce the risk of condensation, which it says can cause adhesion failure or "blushing" (Sika). Basements are often cool and humid, so ask how the specialist will check conditions during the job.
Questions to ask
- Is any liquid water coming in, and should that be fixed first?
- Which moisture test will you use, and where on the floor?
- If the result is high, which moisture-mitigating product will you use, and what is it rated for?
- What are the product's odor and VOC characteristics, and how long should we stay out?
- How will you check temperature and dew point during the job?