Drywall is the most common interior wall surface in Canadian homes. It has replaced the use of plaster and is inexpensive and easy to install. It is also referred to as “wallboard”, or by the brand name “Sheetrock”. Drywall has a gypsum core with a coarse paper on the back and a smooth paper on the finish side. There are several types that have been developed for specific applications. All types of drywall are vulnerable to both moisture and mildew.
Types of Drywall
Water resistant drywall is sometimes called “greenboard”. It has the same gypsum core as drywall but has a water-resistant facing. It is not waterproof and will deteriorate with long-term exposure to moisture. Greenboard is typically used in bathrooms around bathtubs and shower enclosures. The water-resistant facing makes it less vulnerable to moisture damage than standard drywall but exposed edges must be properly sealed for maximum effectiveness.
Concrete backerboard is used as a backing for ceramic tile. It has a grey coloured, solid concrete core and is faced on both sides with fibreglass. It also goes by the name “Durock” and “Wonder Board”. Concrete backerboard is used in conjunction with ceramic tile in kitchens and bathrooms. It is heavier and stronger than standard drywall or greenboard so it keeps it’s shape when the tile is applied to it. It can be cut with a utility knife. Once the tile is applied all exposed edges of the backerboard must be sealed. If water is allowed behind the backerboard, it will decompose and invite mould and mildew.
Type X drywall represents the highest standard in drywall and is the most expensive. It is 5/8″ thick and is used in commercial buildings such as hospitals and schools where fire-proofing is important. It has a fire code C rating which means it will block fire for approximately 90 minutes.
Drywall comes in 4’x8′, 4’x10′ and 4’x12′ sheets. The most commonly used drywall is either 1/2″ or 5/8″ thick. When walls have a 16″ on-centre framing standard 1/2″ drywall is used. If the wall has a 24″ on-centre framing 5/8″ drywall is used. Existing surfaces can be recovered with 1/4″ drywall but it is not recommended for single layer applications.
Curved walls are generally done with 1/4″ drywall because of it’s flexibility. 3/8″ drywall can be used in tight spaces where 1/2″ may be too thick. It is more expensive than and generally has to be specially ordered.
Currently, new technologies are being developed in the drywall industry. A new product called “Lite-Rock” appears to be far superior to traditional drywall. It substitutes another mineral for the use of gypsum and uses improved paper coverings and adhesives which make the product much lighter and easier to install. In addition, Lite-Rock is stronger, more durable, does not discolour, expand, warp or crumble when subjected to adverse weather conditions and moisture. It is also a better sound barrier and has superior thermal insulation and fire resistant properties. Lite Rock is not available on the market as of yet, but it is in the final stages of product development.
A product called “Fiberock Abuse-Resistant Panels” resists denting breaking and puncturing in high-traffic areas. It has no paper face; instead it is made from a combination of gypsum and cellulose. It can be finished and painted like regular drywall and can be used in conjunction with standard drywall. It costs more than regular drywall but has a longer life expectancy.
When drywall becomes damp or is installed in a humid area, it can become vulnerable to mould damage. Mould is a fungus composed of micro-organisms that cause allergies, asthma and other health problems. If mould has appeared on a painted drywall surface and has not caused the paper surface to blister or bubble, the mould may not have penetrated.
To test if a stain on painted drywall is mould, dab the stain with chlorine bleach and watch for any change in colour. If the colour is entirely or largely removed, the stain is likely organic and probably mould. If it is only a small area, the wall can be washed with pure chlorine bleach and rewet so that the surface stays moist for a minimum of 15 minutes. This ensures that the spores of the mould are killed. The wall must then be dried rapidly using plenty of ventilation to prevent new mould growth.
When mould appears on unfinished drywall it may indicate a moisture problem in the room. The drywall should be removed and replaced because the mould has penetrated and is growing within the paper face of the drywall.
Regardless of the type of drywall used in an installation, once it has become water-damaged it should be removed and replaced. Drywall will not regain it’s original properties if it has become wet and then has been allowed to dry. Moisture absorption can cause expansion and powdering of the gypsum core and delamination of the paper face.
Standard drywall on ceilings or in basements, that has become wet, can buckle and twist from the supporting studs as it dries out. If the drywall does not have an opportunity to dry thoroughly it becomes susceptible to mould and mildew which can spread to the framing and carpets. Likewise, greenboard and backerboard, though they are water-resistant, can still crumble from within if water penetrates any exposed edges. The more porous the material is, the more opportunity for mould to infiltrate and spread to the framing. It is cheaper and easier to remove and replace the drywall than it is to repair water and mould damage.
The best strategy in regard to mould and moisture damage to drywall is to avoid it. Keep surfaces dry and the relative humidity of the air reasonably low (below 70% all year and below 55% in the winter). Proper insulation and vapour barriers should be in place on interior walls behind the drywall. This prevents moisture from condensing inside the walls. The vapour barrier goes between the insulation and the drywall. To prevent moisture problems around sinks and bathtubs, leave a 1/4″ space between the drywall and the surface of the tub, shower floor or return. Use water-resistant gypsum board in damp areas. The National Building Code requires that “moisture-resistant backing” be used for ceramic tile. Regular drywall cannot be used.
In basements, be sure to examine walls for moisture and mildew BEFORE drywalling. Install appropriate insulation and vapour barriers and strap concrete walls with framing lumber with the lowest moisture content possible. Framing lumber should not exceed a moisture content of 19% but it is often much higher. As the wood dries, a 10% change in moisture can cause a spruce or fir stud to shrink up to 1/4″. If obtaining quality lumber is difficult, consider the use of metal studs.