Sheets of drywall, also known as sheetrock or gypsum board, laid out in a room.
The typical drywall installation involves one layer of gypsum board over a wooden, stud-framed wall. Sheetrock with a thickness of ½ inch is normally used on walls while board 5/8-inch thick is commonly used on ceilings. Not all drywall applications are typical, however, and there are certain scenarios where two layers of drywall are necessary.
Curved Framing
Typically, 2-layer drywall installations are used on specially designed curved walls or ceilings. Because a 1/4-inch sheet of drywall is needed to bend around the curvature of the wall without breaking, two or three layers are sometimes employed to meet thickness requirements.
Fire Resistance
Some builders and homeowners use two or more layers of drywall for added fire resistance, which is especially true on walls that separate the garage from the living area of a home, otherwise known as firewalls. While Type X and Type C drywall is manufactured to a higher standard of fire resistance than a standard sheet of drywall, improving the fire resistance of a wall or ceiling by doubling up the layers of standard drywall is possible.
Sound Dampening
Although specialty sheets of drywall are manufactured with sound-dampening attributes, these sheets can be expensive and may require special ordering, which is why some homeowners and builders use two layers of standard drywall instead of one layer of sound-resistant drywall.
Heating
Heating is another reason why a homeowner or builder may want to use two layers of drywall. Drywall provides a limited amount of insulation on its own, never mind the fiberglass or foam insulation inside the walls. Using two layers instead of one will moderately improve a room's ability to retain heat.
Cost
Cost is a factor that must be factored in when determining whether to use two layers of drywall instead of one. Specialty sheets of drywall, which are meant for fire or sound resistance, are more expensive than standard sheets of drywall. If your local gypsum supplier doesn't have these specialty boards in stock, special ordering them can be even more expensive. Some homeowners and builders opt to use two layers of standard, inexpensive drywall in place of one layer of special, costly drywall.
Convenience
Sheer convenience also leads some builders and homeowners to use two layers of drywall, which is especially true during long, drawn-out remodeling projects where simply covering up a layer of old drywall is quicker, easier and more cost-effective than tearing out the old layer beforehand.
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