Drywall is a low-cost, easily obtainable gypsum-and-paper material that helps you quickly add wall covering to framed walls. Drywall is invaluable when putting up walls in just about any area of the home—kitchen, dining room, bedroom, general areas, and garages. But one place where standard drywall falters is in areas that experience a great deal of humidity, such as bathrooms, kitchens, and laundries. Here, a better choice is a special type of drywall that has been treated with a moisture- and mold-resistant coating.
These products, available from many manufacturers, are known collectively in the building trades as "greenboard." But while greenboard drywall was once commonly used in showers as a tile backer, there are now other products that perform much better in showers.
What Is Greenboard Drywall?
Greenboard drywall is a type of drywall with a paper facing that has been treated with a waxy coating and chemicals that resist moisture and mildew. It is well suited for finishing walls in humid locations, but should be avoided for very wet locations, such as showers and tub surrounds.
Greenboard is the term that's often used, especially by trade professionals, for a unique type of drywall that is used for walls in damp but not wet zones in a building. It is identical to standard drywall, except for the waxy coating that makes the paper facing resistant to mold and water.
Originally, greenboard was frequently used as a direct backer board behind ceramic tile in highly wet areas such as showers or bathtubs, but this usage began to be phased out in the 1990s with the introduction of cement board. When combined with a waterproof membrane or a brush-on water-proofing coat, cement board makes an excellent code-compliant backer for ceramic tile in showers and above tubs. Later, all-in-one waterproof backer boards such as DensShield fiber mat panels were introduced, further simplifying the process with a building material with an integral waterproof layer.
Greenboard, while no longer common as a tile backer, is still widely used as a wall surface in high-humidity spaces, such as bathrooms and laundries. Greenboard's popularity may be gradually waning, however. Greenboard's paper facing alone does not prevent all mold growth, nor is the material fully waterproof. While it is still being manufactured and sold (and likely will continue to be available for several decades), greenboard is slowly being supplanted by paperless glass-mat drywall that is genuinely waterproof and mold-proof.
Greenboard vs. Conventional Drywall
Greenboard's inner core of gypsum is the same as regular drywall. Several things about the outer covering of greenboard differentiate it from conventional drywall:
- Greenboard's outer paper covering resists water but is not considered to be waterproof.
- Some greenboard is impregnated with compounds that deter the growth of mold. While the inner core of both greenboard and conventional drywall is gypsum and thus inorganic, the outer paper facing is organic cellulose, and thus susceptible to mold when subjected to moisture over time.
- Greenboard's paper cover is a sea-foam green on one side. The color imparts no special water-resisting properties, but it does serve two purposes. For one, it identifies this as moisture-friendly drywall. For another, it helps the installer remember which side should face out during installation.
Installation of greenboard panels is nearly identical to installing conventional drywall, The panels are attached directly to framing members with drywall screws or nails. Care needs to be taken while nailing or screwing, as the paper facing on greenboard is slightly more fragile than the facing on conventional drywall panels. Some professionals prefer to attach greenboard with closer spacing between nails or screws.
Some professionals advise against hanging greenboard on ceilings, since the paper facing is less secure and the panels are heavier in weight than conventional drywall. For bathrooms ceilings, many pros continue to opt for conventional drywall.
Water-resistant drywall commonly comes in 1/2-inch and 5/8-inch thicknesses and conventional 4-foot by 8-foot sizes. It is also available in 4 x 10 and 5 x 10-foot sheets, which are sometimes preferred by professionals.
Even though greenboard drywall is the same thickness (5/8-inch) as Type X, it should not be confused with the Type X fire-resistant product. Type X should be used in areas such as kitchens where a fire might be expected. Type X drywall is required by building code for garages.
Water-Resistant Drywall Brands
Greenboard is sold under various brand names such as GP's ToughRock Mold-Guard Drywall Panel and American Gypsum's Aquabloc. Greenboard costs slightly more than conventional drywall. But don't bother to look for products labeled "greenboard": the term is a common trade colloquialism for this type of sheet good product but is not used by manufacturers.
Is Greenboard Allowed for Wet Applications?
Technically, greenboard and other water-resistant drywall panels are approved for use in highly wet places. According to ASTM C 1396, Section 7, water-resistant drywall can be used in wet locations such as behind tile in bathtubs or shower stalls. According to ASTM C473, after two hours of water immersion, the average water absorption for panels should not be more than 5 percent by weight.
However, individual communities can, and increasingly do, prohibit the use of greenboard for this kind of application. While greenboard drywall's paper covering is water-resistant, it is not waterproof. Since local code always supersedes national standards, always check with your local building inspection office when choosing materials.
Caution should be taken when using greenboard and other water-resistant drywall panels for ceiling applications. Ceiling panels can sag when subjected to high humidity conditions or finishing materials that contain moisture.
Alternative Materials for Wet Locations
Water-resistant drywall is quite similar to standard drywall, with a gypsum core and paper facings. In the case of greenboard, the outer paper facing has been treated with a water- and mold-resistant waxy coating. Note that no manufacturer describes these products as water-proof, thus they are not a good choice as a tile backer in showers. Like regular drywall, you can paint greenboard panels, so they do make good wall surfaces for bathroom areas outside the shower.
Cement-based backer boards Durock, Wonderboard, and Hardibacker are all mineral-based or cementitious materials. When combined with a waterproofing sheet or coating, cement board is perfect as a backer for tile-covered wet applications like showers and tub surrounds. It is also common to use cement board backer panels as the backer for tile in other wall applications, such as behind kitchen backsplashes, or on bathroom walls outside the shower. Another advantage of cement board is that it can be used as a base for tile flooring. Cement board is only used as a backer material, however. You can't install it as a surface wall panel and paint it.
Increasingly, though, the use of cement board is being supplanted by a newer building material, glass mat panels. These are gypsum-based panels that replace the paper facing with a fiberglass surface layer that is entirely waterproof and utterly immune to mold. Because these products (DensShield is one well-known brand) require no waterproofing layer, and because they can be cut in the same way as drywall, they are quicker to install, making them greatly preferred by pros. DensShield can also be painted, so it makes a viable choice for non-tiled walls in bathtub alcoves, which receive a lot of splashing water.
Where to Use Greenboard Water-Resistant Drywall
It is best to reserve greenboard drywall for the large areas of bathrooms, kitchens, and other areas where the drywall may be subjected to lightly damp conditions, humidity, and the occasional minor splashes of water. Locations where greenboard water-resistant drywall is acceptable:
- Bathrooms (outside shower and tub alcoves)
- Basement walls, which can be prone to mold
DO NOT use greenboard in these spaces:
- Saunas and steam rooms
- Shower stalls
- Rooms with spas or pools
For a tile backing material in wet areas such as showers or tub alcoves, cement board (combined with a moisture barrier) or glass mat panels are far better choices. Sag and outright failure often occur when the greenboard has been subjected to wet conditions over time.