The sealed-dish moisture emission test measures how much moisture a slab gives off over a set period. A dish of calcium chloride is sealed under a dome on the floor and weighed before and after. The result is reported in pounds per 1,000 square feet per 24 hours. It’s an older method, and many installers now use relative humidity testing instead.
What a Calcium Chloride Moisture Test Tells You
You’ll often hear this called the calcium chloride test. The installer seals a small dish of the salt under a plastic dome on a clean patch of concrete and leaves it for 60 to 72 hours. The salt soaks up moisture coming off the floor; think of a sponge left under an overturned bowl. More weight gained means a wetter slab.
The catch is that it really only tests the surface of the slab, and it’s easily thrown off by room temperature and humidity. Concrete is usually wetter deeper down, so a decent surface number doesn’t promise the whole slab is dry. That’s why relative humidity testing with probes is often preferred.
What to Ask Your Installer About the Sealed-Dish Test
You don’t need to run this test yourself. Ask your installer:
- Which test will you use, and why that one for my floor?
- Will the space be at normal temperature and humidity while it runs?
- What number does the coating or primer you’re using allow?
- If readings come back high, what’s the plan, such as a moisture-blocking primer?
Sealed-Dish Test FAQ
What are the acceptable results for a calcium chloride test?
It depends on the product going down. Every coating, primer, and flooring glue lists its own limit, so the installer should compare the result to that product’s number rather than a single “safe” reading.
Is the calcium chloride test still accurate?
It gives a useful look at moisture leaving the surface, but it doesn’t see deeper into the slab and room conditions can skew it. Many installers pair it with, or replace it with, in-slab humidity probes.