Why Stone Needs Breathable Protection
A Straightforward Guide for Stone Pros By Frederick M. Hueston
Stone Is Porous — and That Matters
Even the densest granite or marble has a network of tiny pores and capillaries. They’re microscopic, but they’re there. These pores allow moisture vapor to move in and out of the stone. This is what we mean when we talk about “breathing.”
Moisture doesn’t just come from spills on top of the floor. It can come from the substrate below, from setting materials, or from water vapor migrating up through the concrete slab. If that moisture can’t escape, it gets trapped, and trapped moisture almost always leads to trouble.
What Happens When Stone Can’t Breathe
When we block or significantly reduce the stone’s ability to let vapor pass through, we set the stage for a whole range of problems:
- Spalling or Flaking: Moisture trapped inside the stone can freeze and expand (in cold climates) or create salt buildup that pops off the surface.
- Efflorescence: Those white, crusty deposits you see on stone or grout are salts being pushed to the surface by water vapor. Blocking the pores can cause those salts to get trapped just below the surface, which can eventually break the stone apart.
- Discoloration: Moisture under a coating can darken the stone unevenly or create blotchy patches that won’t go away until the coating is removed.
- Softening of Setting Materials: Prolonged moisture entrapment can weaken the thinset or mortar bed, leading to loose or hollow-sounding tiles.
Coatings and Crystallization
This is where coatings and crystallization processes get controversial.
Coatings: Topical coatings sit on the surface like a layer of plastic. They may look shiny, but they can completely block vapor transmission. Once moisture is trapped under that coating, it has nowhere to go. Eventually the coating can delaminate, turn cloudy, or peel.
Crystallization: Crystallization, when done repeatedly, can create a micro-layer on top of the stone that blocks or partially blocks the pores. The result is the same—moisture can’t escape, which leads to problems over time.
Impregnators and Breathability
Even impregnators can be an issue if they aren’t breathable. Most quality impregnators are designed to allow vapor to pass through while still protecting against liquids. But some older or cheaper products can create a partial barrier, especially if over-applied.
When you use an impregnator, make sure it’s marketed as “vapor permeable” or “breathable.” And always wipe off the excess—leaving a film on top defeats the purpose and can turn it into a pseudo-coating.
Why It’s Critical on Flooring
Stone flooring is especially vulnerable because it usually sits directly on a mortar bed or concrete slab, which are both moisture reservoirs. If that moisture is blocked from escaping, you can end up with a floor that looks fine for a while but slowly develops spalling, staining, or hollow spots.
I’ve seen plenty of expensive marble and limestone floors fail prematurely simply because someone applied a non-breathable coating to make it “low maintenance.” The maintenance problem turned into a replacement problem, and an expensive one at that.
Best Practices for Stone Pros
- Choose Products Carefully: Look for sealers labeled breathable or vapor-permeable. Avoid heavy topical coatings unless there’s a very specific reason (and a maintenance plan).
- Avoid Over-Crystallizing: If you use crystallization, don’t overdo it. Too many applications can clog the surface.
- Educate Your Clients: Explain why natural stone works best when it can release moisture vapor. This not only protects their investment but also reduces the chance they’ll blame you when the stone starts spalling or discoloring later.
- Check for Moisture: Before sealing or coating, check the slab with a moisture meter if possible. If it’s still wet, wait until it’s dry enough to minimize problems.
Final Word for Stone Pros
Stone doesn’t actually breathe, but it does exchange moisture with its surroundings. If you cut off that exchange, you’re asking for trouble. Coatings, crystallization, and even some sealers can block the pores if used incorrectly. As stone pros, it’s our job to keep the stone protected while still letting it “breathe,” and that’s the difference between a floor that looks good for a year and one that lasts for decades.
