Very few
people have ever heard of "hydrous calcium sulphate." And even
fewer are aware that hydrous calcium sulphate - CaSO4.2H2O as
chemists know it - is gypsum, the rock that nobody knows.
In addition to being found in the
Egyptian pyramids, gypsum rock is referred to in the ancient
cuneiform scripts of the Assyrians. Its use was probably developed
by the Greeks, whose influence certainly remains in the name by
which the rock is known.
They called it Gypsos, the source of our word "gypsum."
Gypsum is a rock that, like limestone, occurs naturally in many
parts of the world, and like limestone, in an absolutely pure form
it is white. However, the usual presence of darker impurities
produces rock in varying shades of gray, brown, and even black.
Since it normally is found close to the surface, gypsum can be
mined or quarried easily.
The biggest part of the gypsum rock used in this country goes into
wallboard for homes. Gypsum "boards" are formed by sandwiching a
core of wet plaster between two sheets of heavy paper. When the
core sets and is dried, the sandwich becomes a strong, rigid,
fire-resistant building material. Fire-resistant because in its
natural state, gypsum contains water, and when exposed to heat or
flame, this water is released as steam, retarding heat transfer.
Manufactured in unbelievable quantities on continuous machines
almost a quarter mile in length, gypsum wallboard and lath,
prefinished wallboard, and gypsum sheathing for use under exterior
finishes are among the most important materials used in housing.
Here are a few other ways in which gypsum is seen during an average
day.
First of all, there is toothpaste. Gypsum forms the basis for some
of the well-known brands of toothpaste, and thereby helps to pay
the salary of some of television's best-known entertainers.
Most of the fixtures in bathrooms are made of clay formed in a mold
of gypsum plaster, and chances are that gypsum board is behind the
wall tile.
At the breakfast table, plates, cups, and saucers are present which
were shaped on or in a gypsum plaster mold, a mixture of plaster of
paris and water. So were the sterling silver handles of knives,
forks, and spoons.
On the way to work or school, people step on sidewalks made of
Portland cement, which contains a percentage of gypsum to regulate
the setting time.
Maybe their walk is covered with chalk marks from the game the
children played the night before. But of course, they weren't made
by chalk at all, but by a molded stick of, you guessed it, soft
gypsum plaster.
Many streets and highways also contain their share of gypsum.
Doctors and dentists, too, use gypsum for everything from dental
molds to splints.
Benjamin Franklin, the great experimenter, was one of the first to
introduce it in this country when he used ground raw gypsum, called
land plaster, on his farm.
Today, thousands of tons of land plaster are used in the U.S. each
year.
(This information is from a booklet originally written by the late
John C. Best, Vice President of National Gypsum Company).
FAQ's
What is the "R" Value for gypsum board?
For purposes of calculating "U" values, the "C" factor for 1"
gypsum board is 1.2; Resistance "R" for 3/8" board is 0.32; for
1/2" board 0.45; for 5/8" board 0.56; and for 1" board 0.83.
What's the difference between "regular" and "type X" gypsum
wallboard?
ASTM C 36 designates two types of gypsum wallboard, regular and
type X.
Type X wallboard, which is typically required to achieve fire
resistance ratings, is formulated by adding noncombustible fibers
to the gypsum. These fibers help maintain the integrity of the core
as shrinkage occurs providing greater resistance to heat transfer
during fire exposure.
By ASTM definition, type X gypsum wallboard must provide: not less
than a one hour fire resistance rating for 5/8" board or a 3/4 hour
fire resistance rating for 1/2" board applied in a single layer,
nailed on each face of load-bearing wood framing members, when
tested in accordance with the requirements of ASTM E 119, Methods
of Fire Test of Building Constructions and materials.
Additionally, the Gypsum Association requires 1/2" type X gypsum
board to achieve a one hour fire resistance rating when applied to
a floor ceiling system, as described by GA File Number FC 5410, in
GA 600, the Gypsum Association Fire Resistance Design Manual.
What is Hi-Impact Wallboard?
Hi-Impact wallboard is a specially designed product consisting of
5/8" Fire-Shield Type X wallboard with a stronger core and face
paper that is backed with Lexan polycarbonate film manufactured by
GE Plastics.
What is "Green board"?
Green board is designed as an indoor tile backer board, & can be
used as sheathing.
Gypsum sheathing has a moisture-resistant paper and a treated or
non-treated gypsum core. Green board also has a moisture resistant
paper and a treated core. Green board has a tapered edge while
sheathing is a square edge product.
If you plan to attach vinyl or steel siding over the sheathing,
there will be a dip at the taper; therefore, this application is
unadvisable.
If the substrate is a stone or brick cavity wall, however, the
application can work.
We strongly suggest using building felt over this type of
sheathing. It appears that for any high humidity application,
Moisture-Guard® (green board) should be the preferred product. But
that’s not necessarily the case.
For ceiling applications, green board requires extra support.
According to Gypsum Association guidelines, green board in a
ceiling application needs 12" o.c. support.
If supports are 16" o.c., 5/8" green board should be used. Why is
this additional support needed? Moisture repellents, such as the
asphalt and wax emulsions in the green board core formulation, act
as lubricants. These lubricants cause the gypsum crystals to slip
easily, resulting in board sag.
For bathroom ceiling applications, we recommend regular or Type X
board. Simply apply a good quality sealer and two coats of latex
based paint after installation.
