What Goes Behind Shower Tile or Wall Panels?

Water-resistant and waterproof are two different words, and in a bathroom the difference is not academic. One delays damage. With the other, water is simply not the thing that ends it.

Is greenboard allowed behind a shower?

Not as a tile backer in a wet area. Greenboard is moisture-resistant drywall — a gypsum core with treated paper facing. The core softens when it gets wet, and the facing is paper with a wax coating.

That wax is the part people get backwards. It is there to slow moisture down, not to stop mold. Building scientist Joseph Lstiburek, writing at buildingscience.com, described the waxes added to protect the paper as, in his words, “mold food.” Paper is organic and wax is organic. Behind a tiled wall, both are lunch.

Is cement board waterproof?

No, and this is the most common misunderstanding on the subject. Cement board is Portland cement and aggregate with fiberglass mesh and no organic material, so there is nothing in it for mold to eat. It is a genuine step up and it is what code accepts as a substrate.

But the US Department of Energy’s Building America Solution Center puts it plainly: “Unless specifically manufactured as a waterproof product, cement board is water resistant but not waterproof. You must coat it with a fluid-applied waterproofing or apply a water-resistant barrier behind it that drains.”

So cement board needs a second product and a second labour step. That second step is exactly the thing that gets skipped when a job is running behind.

What about plywood?

Plywood is organic. It absorbs water, swells, warps, rots and feeds mold. In a wet area it has no place.

What does a panel system change?

A waterproof foam backer board is substrate and waterproofing in one panel, which removes the second step rather than relying on someone to remember it. Our wall sheets are 4ft by 9ft — floor to ceiling, so there is no horizontal seam, one barely visible vertical seam on the back wall, and no grout anywhere.

No grout matters more than it sounds. Grout is the part of a tiled shower that a homeowner ends up maintaining forever.

How long does a wall take to fail?

That depends entirely on how much water gets behind the surface and how often. What can be said is which way each material fails: greenboard swells and feeds mold, cement board passes vapour through unless it is sealed, plywood rots, and a closed-cell foam panel does none of those because there is nothing in it to soften or feed on.

The question to ask

Ask what the backing is, and then ask the follow-up: is it the waterproofing, or does it need a waterproofing layer applied over it? Those are two different answers, and only one of them is finished when the board goes up.