While Dr. Fred was preparing to teach his hands-on workshop in Las Vegas,  we received a question from a frustrated home owner. Dr. Fred was kind enough to offer some answers by email, and his video blog on The Tile and Stone Show…

Dear Stone Detective, 

I have a question for you. I installed a marble-lined shower about 3-1/2 years ago, and I am wondering if it is time to be resealed. I am attaching some pictures – you can see my ADA compliant roll-in shower has no threshhold, a built-in bench, and pony walls topped with glass that separates it from a walk-in tub and the rest of the bathroom. I supplied the contractor with recycled 6x6 inch polished Carrara tiles, and larger pieces of honed Carrara for the bench top and shelves on the pony walls. As you can see, this means there are a lot of seams!

The floor in the shower is a different composition: small tiles of Carrara and black Carrara Bianco basket weave, purchased from a commercial tile house. It coordinates with a larger basket weave pattern used for the rest of the bathroom floor.

So: after over three years of water and steam, I can see where tiles on the lower walls are darkening, and that worries me. Should I seal everything in the shower? I don’t know how much the installer actually applied. I gave them a full, one liter bottle of Akemi Sealer, and they didn’t use it all.

You might be wondering why I don’t ask the original installer?  Because this was part of a larger renovation, which ran over about two months past what it should have taken. On the bathroom project alone, two different teams of tile guys worked on it. I stipulated in the original contract that the floor and wall tiles be installed over a Noble membrane system, with a slot drain, instead of a central sloping drain. They said they were familiar with Schluter system membrane installation, and were able to finish the job, but I’d give the marble tile shower installation about a B–minus grade, and now think it was too hard for them. I’d rather trust the recommendation of someone more knowledgeable.

So, Stone Detective: in your experience with wet environments, what do you recommend?

Thanks, M. H. in Tennessee

Hello M. H.  –

This is a problem I have seen before. Not the bad tile job, but the marble shower sealing question.
(Okay, I have also seen plenty of bad installations, too – LOL). 

I receive several calls a week with questions on the use of impregnating type sealers for use on outdoor stone as well as interior wet areas such as showers, water fountains, etcetera. The question is simple: Should I seal my stone in these conditions?

I happen to have a video blog (#23) and a short article that address this very question. In my Tile and Stone Show number 23, I do an experiment starting with unsealed white marble, which can be a fairly porous stone. I demonstrate how water is absorbed into unsealed marble (see above, top right), while water will bead up and not be absorbed on sealed stone. Watch it – my blog demonstrates and examines the case against sealing stone in wet areas.

Above, left: Marble showers: How often should the stone be sealed – or not be sealed, at all? Above, middle: Absorbed water and water vapor show up as darker areas in the unsealed marble (top) and the sealed marble steam test. Above, right: Darker wall tiles at the bottom of this marble shower show signs of water absorption.

Above, left: Marble showers: How often should the stone be sealed – or not be sealed, at all?

Above, middle: Absorbed water and water vapor show up as darker areas in the unsealed marble (top) and the sealed marble steam test.

Above, right: Darker wall tiles at the bottom of this marble shower show signs of water absorption.

But I’m getting ahead of myself. 

What most installers fail to acknowledge is that you also have to deal with the long-term effects of condensation and steam in a shower.

In my video experiment, I used  a handheld steamer to simulate the effect that steam has in a steam shower, or the effects of vapor from hot water in the shower.

I steamed a piece of sealed marble for about a minute and a half. The video shows how dark the stone gets with absorbed moisture. What happened is that the steam was penetrating right through the sealer.

This is a pretty compelling reason why stone in wet areas should not be sealed.

But before we discuss the reason why we shouldn’t seal stone in these conditions, a few definitions are necessary.

Impregnators or penetrating sealers
are designed to penetrate below the surface of the stone and deposit solid particles in the pores of the stone or to coat the individual minerals below the surface of the stone.  Water, oil, and dirt are restricted from entering the stone.  Impregnators can be solvent- or water-based. Most impregnators are vapor permeable.

Vapor Permeable
refers to the stone’s ability to “breath.” Vapor permeability describes a stone’s ability to allow water vapor to pass through it.

The case for not sealing stone in wet environments

When stone is exposed to unregulated humidity and temperature fluctuations, like it would in an outdoor environment or in a shower where the air contains vapor in what we know as humidity, temperature along with humidity can result in condensation as well.

Most of the impregnators on the market today are breathable. This simply means that the stone will be protected from water entering the pores of the stone in liquid form, but will allow water vapor to pass.

In a wet environment, vapor can be present for several reasons: rain, high humidity, temperature fluctuations, and steam. Since these impregnators are breathable, this vapor can easily penetrate into the stone.  One would think that this is a positive. The fact is, once the vapor enters the stone it can condense and become a liquid. Since impregnators protect against water in its liquid phase it becomes trapped within the stone’s pores and will not escape until it evaporates, or in other words, turns into a vapor.

Once this water becomes trapped it can result in all kinds of problems. Stones with iron content can begin to oxidize, natural salts within the stone can dissolve and cause pitting and spalling. Aesthetically, the stone will appear darker since it is constantly wet. 

This problem is becoming more of an issue with the increased use of stone in showers and exterior environments. Besides me, there are currently several people doing experiments to demonstrate that sealer in wet environments can cause these issues. I strongly believe that care should be taken when sealing stone in these wet environments.

What I concluded in my experiment is that if you don’t seal, the stone tends to dry out much quicker than if you seal them.

Now, in the situation with marble walls AND floor in a shower, walls are vertical and moisture can escape through the back–whereas a floor is (usually) directly on a slab and hence holds water longer. Also, the floor has a thicker substrate and that’s where all the water collects.

So, in the case of this project: my advice is that it’s OK to seal the walls and bench, but I wouldn’t seal the floor. And, it is always best to have a professional inspect the problem. Your issue might be the result of poor installation, after all.

The Stone Detective is a fictional character created by Dr. Frederick M. Hueston, PhD. Dr. Fred has written over 33 books on stone and tile fabrication and installation, and also serves as an expert witness. Fred has also been writing for the
Slippery Rock for over 20 years.
Send your comments to

To see the video blog mentioned in this article, visit
BlogTalkRadio.com/DrFred  on Youtube.