by Kevin Slattery, CSI
Paper-faced exterior gypsum, governed by ASTM C79 - Specification for
Gypsum Sheathing Board, has been used for quite some time as an economical
sheathing, but it does have its limitations. The face, which is made of
paper, is stronger in the long direction than it is across the width. The
paper face, even though intended to accept some exposure to weather, needs
to be covered by building paper within thirty days after installation to
protect against moisture absorption and delamination. Also of concern in
today's construction is the fact that the paper can support mold and
mildew growth. Typically this product carries a thirty-day warranty from
the day it is shipped, not from day it is installed.
Glass mat-faced sheathing
About 15 years ago, another type of sheathing was introduced. This product, which now complies with ASTM C1177 - Specification for Glass Mat Gypsum Substrate for Use as Sheathing, fast became a popular substrate for the manufacturers of EIFS. In this new type of sheathing, the gypsum core is combined with silicone and a wax emulsion to prevent moisture intrusion and wicking into the board. The glass fiber facer is embedded into the treated core and then the exterior side is coated with an alkali resistant coating.
The popularity of this type of sheathing has now grown beyond the EIFS market, and glass faced, treated core sheathing is now being widely used behind brick and other claddings. There is no need for an additional layer of building paper to protect the board. Joints are simply treated with tape or a bead of silicone sealant. This material comes with a 6 month warranty, allowing it to be in place, keeping out bad weather, while work is completed on the inside.
There is now yet another product on the market, a fiber-reinforced gypsum panel. It has glass fiber embedded into the core, and like the glass mat product, delamination is not a problem as it is for paper-faced products. Although gypsum is incorporated into the board, it does not have the fire-resistive qualities that the other boards have.
While this new product meets the requirements of ASTM C1177, it has not been tested by ASTM. It seems that this board may have been developed to be the substrate for its parent company's own EIFS system.
Each project you design has its own unique qualities, needs, and budget. However, as always, good research and product knowledge are the best methods to fit materials to a building project and prevent unforeseen costs or material problems somewhere down the road.
Kevin Slattery is an Architectural Sales Manager for Edwards Sales Corporation, www.EdwardsSales.com. Edwards Sales has been the representative for quality building products in the Twin Cities area for over 45 years.
© 2000 Kevin Slattery, kslattery@EdwardsSales.com.
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