Drywall
Drywall is a panel made of calcium sulfate dihydrate, with or without additives, typically extruded between thick sheets of facer and backer paper, used in the construction of interior walls and ceilings. The plaster is mixed with fiber ; plasticizer, foaming agent; and additives that can reduce mildew, flammability, and water absorption.
In the mid-20th century, drywall construction became prevalent in North America as a time- and labor-saving alternative to lath and plaster.
History
Sackett Board was invented in 1890 by New York Coal Tar Chemical Company employees Augustine Sackett and Fred L. Kane, graduates of Rensselaer Polytechnic Institute. It was made by layering plaster within four plies of wool felt paper. Sheets were thick with open edges.Gypsum board evolved between 1910 and 1930, beginning with wrapped board edges and the elimination of the two inner layers of felt paper in favor of paper-based facings. In 1910 United States Gypsum Corporation bought Sackett Plaster Board Company and by 1917 introduced Sheetrock. Providing installation efficiency, it was developed additionally as a measure of fire resistance. Later air entrainment technology made boards lighter and less brittle, and joint treatment materials and systems also evolved. Gypsum lath was an early substrate for plaster. An alternative to traditional wood or metal lath was a panel made up of compressed gypsum plaster board that was sometimes grooved or punched with holes to allow wet plaster to key into its surface. As it evolved, it was faced with paper impregnated with gypsum crystals that bonded with the applied facing layer of plaster. In 1936, US Gypsum trademarked ROCKLATH for their gypsum lath product.Image: Drywall.jpg|thumb|Vertically hung drywall with joint compound
In 2002, the European Commission imposed fines totaling €420 million on the companies Lafarge, BPB, Knauf and Gyproc Benelux, which had operated a cartel on the market which affected 80% of consumers in France, the UK, Germany and the Benelux countries.
Manufacture
A wallboard panel consists of a layer of gypsum plaster sandwiched between two layers of paper. The raw gypsum,, is heated to drive off the water and then slightly rehydrated to produce the hemihydrate of calcium sulfate. The plaster is mixed with fiber, plasticizer, foaming agent, finely ground gypsum crystal as an accelerator, EDTA, starch or other chelate as a retarder, and various additives that may increase mildew and fire resistance, lower water absorption, reduce creep. The board is then formed by sandwiching a core of the wet mixture between two sheets of heavy paper or fiberglass mats. When the core sets, it is dried in a large drying chamber, and the sandwich becomes rigid and strong enough for use as a building material.Drying chambers typically use natural gas today. To dry of wallboard, between is required. Organic dispersants and plasticizers are used so that the slurry will flow during manufacture and to reduce the water and hence the drying time. Coal-fired power stations include devices called scrubbers to remove sulfur from their exhaust emissions. The sulfur is absorbed by powdered limestone in a process called flue-gas desulfurization, which produces several new substances. One is called "FGD gypsum". This is commonly used in drywall construction in the United States and elsewhere.
In 2020, 8.4 billion square meters of drywall were sold around the world.
Construction techniques
As an alternative to a week-long plaster application, an entire house can be drywalled in one or two days by two experienced drywallers, and drywall is easy enough to be installed by many amateur home carpenters. In large-scale commercial construction, the work of installing and finishing drywall is often split between hangers, who install the wallboard, and tapers who finish the joints and cover the fastener heads with drywall compound. Drywall can be finished anywhere from a level 0 to a level 5, where 0 is not finished in any fashion, and five is the most pristine. Depending on how significant the finish is to the customer, the extra steps in the finish may or may not be necessary, though priming and painting of drywall are recommended in any location where it may be exposed to any wear.Image: Drywall-screws-v1.png|thumb|Drywall screws for wood, with parallel-threaded woodscrew shanks and bugle heads
Drywall is cut to size by scoring the paper on the finished side with a utility knife, breaking the sheet along the cut, and cutting the paper backing. Small features such as holes for outlets and light switches are usually cut using a keyhole saw, oscillating multi-tool or a tiny high-speed bit in a rotary tool. Drywall is then fixed to the structure with nails or drywall screws and often glue. Drywall fasteners, also referred to as drywall clips or stops, are gaining popularity in residential and commercial construction. Drywall fasteners are used for supporting interior drywall corners and replacing the non-structural wood or metal blocking that traditionally was used to install drywall. Their function saves material and labor costs, minimizes call-backs due to truss uplift, increases energy efficiency, and makes plumbing and electrical installation simpler.
When driven fully home, drywall screws countersink their heads slightly into the drywall. They use a 'bugle head', a concave taper, rather than the conventional conical countersunk head; this compresses the drywall surface rather than cutting into it and so avoids tearing the paper. Screws for light-gauge steel framing have a sharp point and finely spaced threads. If the steel framing is heavier than 20-gauge, self-drilling screws with finely spaced threads must be used. In some applications, the drywall may be attached to the wall with adhesives.
After the sheets are secured to the wall studs or ceiling joists, the installer conceals the seams between drywall sheets with joint tape or fiber mesh. Layers of joint compound, sometimes called mud, are typically spread with a drywall trowel or knife. This compound is also applied to any screw holes or defects. The compound is allowed to air dry and then typically sanded smooth before painting. Alternatively, for a better finish, the entire wall may be given a skim coat, a thin layer of finishing compound, to minimize the visual differences between the paper and mudded areas after painting.
Another similar skim coating process is called veneer plastering, although it is done slightly thicker. Veneering uses a slightly different specialized setting compound that contains gypsum and lime putty. This application uses blueboard, which has specially treated paper to accelerate the setting of the gypsum plaster component. This setting has far less shrinkage than the air-dry compounds normally used in drywall, so it only requires one coat. Blueboard also has square edges rather than tapered-edge drywall boards. The tapered drywall boards are used to countersink the tape in taped jointing, whereas the tape in veneer plastering is buried beneath a level surface. One coat veneer plaster over dry board is an intermediate style step between full multi-coat "wet" plaster and the limited joint-treatment-only given "dry" wall.
Properties
Sound control
The method of installation and type of drywall can reduce sound transmission through walls and ceilings. Several builders' books state that thicker drywall reduces sound transmission, but engineering manuals recommend using multiple layers of drywall, sometimes of different thicknesses and glued together, or special types of drywall designed to reduce noise. Also important are the construction details of the framing with steel studs, wider stud spacing, double studding, insulation, and other details reducing sound transmission. Sound transmission class ratings can be increased from 33 for an ordinary stud-wall to as high as 59 with double drywall on both sides of a wood stud wall with resilient channels on one side and glass wool batt insulation between the studs.Sound transmission may be slightly reduced using regular panels, but it is more effective to use two layers of drywall, sometimes in combination with other factors, or specially designed, sound-resistant drywall.
Water damage and molding
Drywall is highly vulnerable to moisture due to the inherent properties of the materials that constitute it: gypsum, paper, and organic additives and binders. Gypsum will soften with exposure to moisture and eventually turn into a gooey paste with prolonged immersion, such as during a flood. During such incidents, some, or all, of the drywall in an entire building will need to be removed and replaced. Furthermore, the paper facings and organic additives mixed with the gypsum core are food for mold.The porosity of the board—introduced during manufacturing to reduce the board's weight, lowering construction time and transportation costs—enables water to rapidly reach the core through capillary action, where mold can grow inside. Water that enters a room from overhead may cause ceiling drywall tape to separate from the ceiling as a result of the grooves immediately behind the tape where the drywall pieces meet becoming saturated. The drywall may also soften around the screws holding the drywall in place, and with the aid of gravity, the weight of the water may cause the drywall to sag and eventually collapse, requiring replacement.
Drywall's paper facings are edible to termites, which can eat the paper if they infest a wall cavity covered with drywall. This causes the painted surface to crumble to the touch, its paper backing material being eaten. In addition to the necessity of patching the damaged surface and repainting, if enough of the paper has been eaten, the gypsum core can easily crack or crumble without it, and the drywall must be removed and replaced.
In many circumstances, especially when the drywall has been exposed to water or moisture for less than 48 hours, professional restoration experts can avoid the cost, inconvenience, and difficulty of removing and replacing the affected drywall. They use rapid drying techniques that eliminate the elements required to support microbial activity while restoring most or all of the drywall.
It is for these reasons that greenboard, a type of drywall with an outer face that is wax- and/or chemically coated to resist mold growth, and ideally cement board are used for rooms expected to have high humidity, primarily kitchens, bathrooms, and laundry rooms.