Epoxy Floor Blisters: The Moisture Problem Nobody Tests For
You walk onto a recently coated floor and there it is: a cluster of blisters, some popped, some still raised. The client is not happy. The instinct is to blame the epoxy — but in the overwhelming majority of cases, the epoxy is innocent. The cause is moisture, and it was there before you ever mixed a pot.
Why moisture causes blisters
Concrete is never truly dry. It wicks moisture up from the ground its whole life. When you seal a surface with a moisture-impermeable coating like epoxy, that rising moisture has nowhere to go. It builds up vapour pressure under the coating until the bond gives way — and you get blisters, delamination, or adhesion loss.
The key point: the moisture was always in the slab. The coating just trapped it. A good slab in dry conditions can pass a moisture test and still carry a lot of moisture — the test measures the emission rate, and that’s what the coating has to live with.
How to confirm it’s moisture
Before you tear anything out, confirm the diagnosis:
- Pop a blister and look. If the underside is damp or there’s a dark moisture ring, that’s your answer.
- Test the surrounding slab with a calcium carbide (CM) test or in-situ probe. If the emission rate is above the system’s limit, moisture is confirmed.
- Check the pattern. Moisture blisters tend to cluster near external walls, over drains, or in the wettest zones — not randomly across the floor.
If the test is clean and the blisters are random, look elsewhere — poor profile, contamination, or an application error (mixing, pot life) are the other candidates.
The fix (and the real lesson)
- Localised blisters: cut out the failed area, dry and prepare the substrate, and recoat the patch. It works for small clusters.
- Widespread failure: the slab needs a moisture mitigation system — a vapour barrier under the coating, or a moisture-tolerant 100% solids system. There’s no shortcut around a slab that’s genuinely over the limit.
But the real lesson is in the prevention, because fixing it is expensive and preventing it is a test:
Always test moisture before you specify or apply a coating. It costs a fraction of a day and a fraction of the money of a failed floor. Test in the worst spots, under real conditions, and read the result against the system’s actual limit — not a guess.
The applicators who never get called back are not the ones with the best epoxy. They’re the ones who test the slab first.
The full method — testing, profiling and the decision tree — is in our Epoxy & Resin Floor Systems course.