Question: We will be remodeling our bathroom and one of the changes will be to install new tile in the tub area. Someone suggested we replace the wallboard with a drywall that has a green exterior. I had never heard of that and don’t understand the reason or need for it. Will you please explain the difference? Thank you,

Frank S., Spokane

Answer: I assume that when you refer to a green exterior you are referring to water-resistant gypsum wallboard, commonly known as “green board.” This is one product that is frequently misunderstood and misused.

Similar to regular gypsum wallboard and available in the same sizes, green board is commonly used behind tub and shower enclosures because its paper face and back and its core are chemically treated to withstand the effects of moisture and humidity. Unlike gypsum wallboard, green board is water-resistant all the way through. It is designed for the adhesive application of ceramic tiles as well as tub and shower fiberglass surround kits and other nonabsorbent finish materials like Corian or Swanstone.

However, “water-resistant” is not the same as “waterproof,” so this product cannot be applied to an exterior wall over insulation with a vapor barrier, a situation you will encounter on an exterior wall. Spokane building codes require a vapor barrier over exterior wall studs, so use cement board, also known as cementitious backer board, instead.

Cement board is a lightweight panel composed of aggregate, Portland cement, and coated glass-fiber mesh reinforcement. It is resistant to water, moisture and steam, and it will not decay, warp or soften. This is the best kind of wallboard for this type of application because it is approved for wet areas.

Cement board panels can be scored and snapped like gypsum wallboard. I suggest that you cut the panels outdoors or cover the work floor with heavy cardboard to protect it from cement grit.

These panels are a great substrate for tile, but they do not have a good finish surface, so if you apply them to the exterior wall in the shower area, you’ll have to make some decisions.

What should you do with any remaining wall space above the enclosure? I recommend that you apply cement board clear up to the ceiling and then install tile or other nonabsorbent finish material to the same height. This way you won’t have to deal with peeling paint on the remainder of the wall.

If you still want to use an enclosure kit, then determine the height of the kit and install standard drywall from the ceiling to ½-inch below that height. Finish the wall with cement board down to the top of the tub.

On the remaining two shower walls you could use green board, but I would recommend cement board on all walls, as there is not much difference in cost and it’s a better product for wet rooms.

To prep the cement board and green board for tile installation, I recommend spreading a uniform skim coat of ceramic tile adhesive over the surface where tile will be installed and allow it to fully dry before installing it. Alternatively, you could apply two coats of an oil-based primer to properly seal and prepare the surface for adhesive application. If you are using an enclosure kit, then prep the walls with two coats of oil-based primer.

If Kraft-faced insulation is going to be used in a tub/shower area involving an exterior wall, then its face will act as a vapor barrier, providing that the insulation tabs are stapled to the stud faces as required (not to the sides of the studs). Adding a plastic vapor barrier over Kraft-faced insulation is not a wise choice because moisture can get trapped between the Kraft face and the vapor barrier, causing mold or mildew growth on the Kraft face and water damage to the bottom plate.

If you want to use a plastic vapor barrier, then I recommend that you use unfaced insulation in conjunction with an approved barrier.

An exhaust fan should be installed to help remove moisture from the bathroom. If you have a window in the shower area, I recommend that you remove it because they cause nothing but water damage down the road. Since most individuals will not open windows in the winter to ventilate the bathroom, the best solution is to install an exhaust fan.

As you plan your remodeling project, keep in mind that a building permit is required to replace the drywall and insulation. A plumbing permit is required if a fixture is replaced or moved. An electrical permit is required if an outlet is moved or installed, or if an exhaust fan is installed. Also, a mechanical permit will be required for an exhaust fan installation.

I hope this clarifies the differences in the wallboard material. Good luck with bathroom project!

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