If you are doing a full bathroom, kitchen, or laundry room renovation, you may have heard the term “green board” thrown around while discussing plans with a contractor; sometimes, they’re referred to as “water-resistant gypsum drywall.” But are they necessary?
Green boards are often required by local building codes to be used as a backing for tiles and wall panels in areas that are exposed to moisture; this includes bathrooms, laundry rooms, and kitchens. However, they are not allowed for use in parts of the home that have excess humidity, like near indoor pools.
Although green boards are usually associated with water-resistant use, some types are uniquely designed for use in areas that need fire-resistant drywall.
We’ll be discussing this and traditional green boards and how they relate to building codes in the remainder of this article, as well as how to install green boards properly.
What Is Green Board?
Green board—commonly referred to as a “water-resistant gypsum backing board”—is a type of drywall that has a greater resistance to moisture than traditional plasterboard drywall (source). It is commonly used in bathrooms and other areas that are susceptible to moisture.
It is not drastically different from the gypsum board commonly used in drywall. The only significant difference that you will notice is a thick coating on the material protected by wax; this is the feature that makes this type of drywall resistant to moisture.
Green board panels are also designed to be resistant to mold. They have anti-microbial properties, both in the core of the drywall and on their faces (the exterior of the panel). They are built to handle mildew better than traditional drywall options.
Finally, green boards are thicker than standard drywall. Standard drywall comes in options that are ¼” thick and ½” thick. By comparison, green boards are sold in options that are ½” thick and ⅝” thick.
Why the difference in thickness? Despite being designed to be more resistant to moisture-related problems, green boards are not completely water-resistant. In fact, the material will rapidly deteriorate if it is frequently exposed to water; this is the primary reason that their panels are made to be thicker.
When is Green Board Not Required by Code?
The reason discussed above is the same reason that building codes state that green boards should not be used in places with excess humidity, like saunas, indoor pools, and steam rooms. The material that the backing board is made from will not hold up very well under these circumstances.
Green boards are also not recommended for exterior applications. If you are looking for a material to use on the outside of a building, you will need to seek out cladding that is designed to handle higher amounts of moisture.
When is Green Board Required by Code?
Building codes will often dictate that water-resistant gypsum backing board (also known as green board) should be installed in bathrooms where moisture can find its way through tiles in tub and shower areas.
Per the American Society for Testing & Materials code ASTM C1396, a “water-resistant gypsum backing board is to be used as a base for the application of ceramic or plastic tiles on walls or ceilings” (Source: City of Philadelphia).
However, these materials are also well-suited for laundry rooms and kitchens. In fact, local building codes may actually require that green boards be used in these locations as well; this is because these locations may also have ceramic tiles or wall panels that make up the construction of the room. See Can You Tile Over Drywall?
Green Boards & Fire Building Codes
Some building codes may require special fire-resistant gypsum backing board in some situations. For that reason, there are also types of green boards that are made to be fire-resistant.
These types of gypsum boards are referred to as “Type X” backing boards. The material can provide fire ratings of up to 4 hours. To meet fire-resistance requirements, the boards need to be assembled in a manner that follows Universal Laboratories’ (UL) requirements. This includes strict guidelines for fasteners and fastener-spacing.
ProRoc Moisture-Resistant Type X Gypsum Board is an example of a product that has both moisture-resistant properties and fire-resistant properties. If you are looking for a product that holds both these characteristics, you will need to look for a product that has Type X in the description.
Note: When fire-resistant backing is required, building codes dictate that it must be installed from floor to ceiling.
Green Boards & Noise Attenuation Codes
Local building codes may also require builders to dampen or attenuate noise within structures (source). This is often the case with renovations or full builds within apartments, townhomes, condos, or other adjoining types of construction.
With this additional requirement, one question you may have is whether green boards would interfere with a builder’s ability to follow it. Because green boards are thicker in nature, often they can meet the noise attenuation guidelines without having to sacrifice their water or fire-resistant features.
Sound attenuation is measured in sound transmission class, abbreviated STC. At an STC rating of 50, loud speech is not audible. Type X green board has been tested and shown that some products have a rating of up to 60 STC, making it a suitable material to help attenuate noise as required in some building codes.
Ensuring You Install Green Boards According to Code
Proper installation of your green boards is critical for them green boards to serve their purpose adequately.
At the end of the installation process, you may notice that there are edges and openings left in your layer of green boards. These spaces need to be addressed as openings and edges will allow moisture to make its way through the wall cavity, defeating the purpose of including the green board in the first place.
CertainTeed encourages builders to seal these openings with a water repellent sealant of some kind. This will ensure that the green board serves its purpose.
There are a variety of water-repellant sealants available on the market. One of the more popular choices is Gorilla Clear 100 Percent Silicone Sealant Caulk (Link to Amazon). This product is waterproof and also resistant to mold and mildew.
It can be used to seal cracks and edges in green boards that moisture would otherwise be allowed to escape through.
There are some places where builders are not permitted to use green boards. These restrictions often stem from the fact that the water-restart gypsum board contains a protective layer that can wear down under particular circumstances, such as with high humidity.
Some areas of the home that will not need green boards—and is often not allowed according to building codes—include:
- Over a vapor retarder in tubs and showers
- In areas where there will be continuous exposure to water
- Indoor pools
- On ceilings where frame spacing exceeds limits
However, there are several locations where you are required, or at least strongly encouraged, to use green boards in drywall construction:
- As a backing for ceramic tiles in tubs and showers
- In conditions where mold is of concern
- In locations where fire-resistant Type X gypsum board is required
- Laundry rooms
Note that these restrictions and guidelines will generally be more specific than this but vary from community to community. Be sure to check your local building codes before you purchase materials, as one locality may require green boards where another does not.