What is gypsum board?

What do you get when misconceptions and “common knowledge” cause project difficulties, payment issues, owner dissatisfaction and sometimes to the point where knifes start flying? You get informed fast…the best approach for this type of battle is not a bigger knife or a gun. Seek accurate, expert information. Let’s start at the beginning before considering the paint finish.

Always elevate your expertise from reliable, knowledgeable sources other than and hopefully before going legal. Our subject specialists are industry veterans and active Construction Specifications Institute (CSI) members, Andy Vegter from USG and Thad Goodman from National Gypsum.  For each project, it is a best practice to recommend process, scheduling and/or specification improvements, while following the plans and specifications. However, with both eyes wide open realize the best plans and specs leave room for improvement. That “room” should be filled with specification, contract qualifications and generous pages of written documentation and verification.

What is gypsum/gypsum board?

It is worth noting that in addition to the paint and coating application specifications, professionals should have at least a basic understanding of other project specification sections that are related to the paint scope. Examples include structural steel, stair steel, miscellaneous steel/metals; concrete, exposed metal decking, gypsum ceilings and walls, millwork finishes and installation, etc.

Gypsum Crystal Formation Underground Photo courtesy of the Gypsum Association

09 29 00 – Gypsum Board

Question to Andy and Thad:

“When you think about the questions and comments you hear from design professionals across all levels of experience, what misconceptions about gypsum products do you find that you most commonly have to dispel?”
Gypsum is a natural mineral, chemically made up of calcium and sulfur bound to oxygen and water.  It is found naturally in sedimentary rock formations, with some of the world’s largest natural reserves in North America. A synthetic version, which is a byproduct of coal burning electric power plants, is chemically identical to natural gypsum.

Some gypsum board manufacturing plants are fed with mostly synthetic gypsums and others are built over a mine where the gypsum is coming out of the ground.  Synthetic gypsum is considered a recycled material by sustainability rating systems, so projects seeking LEED or similar certification can specify that gypsum panels be made up of 90% recycled content.  It’s important to remember that not all products are available from plants that use synthetic gypsum.

Gypsum board is manufactured when gypsum is mixed with water and additives to form a slurry, which is then fed between continuous layers of paper or another type of facer.  Through a chemical process, the slurry hardens to its original rock state, and the facer becomes bonded to the gypsum core.  The boards are then cut to size and dried.

Here are some of the most frequent misconceptions and misunderstandings related to gypsum board:

Type X Gypsum Board is Fire Rated

Gypsum board by itself has no fire rating.  Ratings are conferred by testing agencies on full assemblies, which include everything that goes into the construction of a given assembly: gypsum panels, studs, insulation, resilient channels, etc. Although gypsum board is not fire rated by itself, a single layer of drywall acts as a thermal barrier, protecting concealed foam plastics from ignition in a fire for a period of fifteen minutes as required by model codes.

Mold-resistant gypsum board is mold proof

In reality, nothing is mold proof.  Mold spores are everywhere, and they just need a food source and moisture in order to grow.  Mold-resistant gypsum board products have paper faces that are treated to retard mold growth, but the best and most cost-effective strategy to manage the risk of mold is to prevent the intrusion of water into the building and to encourage wet materials to dry rapidly.  In addition to mold-resistant paper-faced gypsum board, which is recommended for use in conditions where incidental moisture is present, there are glass mat-faced products that are tolerant of direct water and prolonged humidity.

Sound Transmission Class ratings on standard details

Sound Transmission Class (STC) testing is done in a perfect lab environment on a wall with no penetrations or places where sound flanking can occur.  In the real world, sound flanking can occur with electrical boxes, lack of acoustical sealant at the base or top of the wall, and around doors and windows.  Expect an STC performance loss of at least five points compared to the STC tested result in the lab.

Type X Drywall can be used in all fire-rated assemblies

Most rated gypsum ceiling assemblies require Type C, but Type X is frequently used and accepted by mistake.  Type X does not match the fire performance of Type C.  As a rule of thumb, remember “C” for “Ceiling” when the ceiling is part of the fire rated assembly, but read the tested design requirements carefully because there might be exceptions to that rule.

No wall cracks if the 30 foot spacing rule for control joints is specified

In reality, control joints reduce the risk of cracking, but do not eliminate it.  The amount of movement anticipated is what should dictate control spacing and placement.  Installation specifications and a guide for spacing control joints is described in ASTM C 840, “Standard Specification for Application and Finishing of Gypsum Board.”  Also, cracking can occur if deflection from structure above the top of a wall is not accommodated in the wall assembly.  Gypsum board and stud manufacturers have numerous recommended methods for taking up the deflection.
Level 5 Drywall will be perfectly smooth

The levels of gypsum board finishing are very confusing and mostly overlooked until there’s a problem.  The five levels of finish were developed in a collaboration of manufacturers and associations within the paint and gypsum industries as a way to manage the expectations of the appearance of a finished wall by the architect and owner.  Details are described in ASTM C 840, which is based on Gypsum Association’s document GA-214-15.  All walls have blemishes and imperfections that can be exaggerated by critical light, glossy paints, quality of framing, quality of finishing, quality of primer, quality of paint and workmanship.  GA-214-15 includes mock-up provision, which provides an opportunity to set expectations and receive Owner acceptance before the work is complete.

No building material is free of complicated qualifications regarding its use.  Architects and specifiers often collapse these qualifications into rules of thumb that they use when they generate designs and specifications.  Many of these rules of thumb are commonly useful, but when you dig deeper, they are often based on obsolete or incorrect information.  Experienced product reps have seen and helped solve many real-life problems that have arisen through incorrect product selection, bad detailing and poor specifications. Product experts are a critical resource that can help us update our knowledge and dispel our misconceptions.

Thanks to Andy and Thad for participating.

By Elias Saltz

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The reason Type C and Type X gypsum boards perform differently during a fire relates to the composition of the product. In the manufacturing process of gypsum boards, crystallized water becomes chemically bonded within the gypsum. As fire begins to affect gypsum board in a wall assembly, the temperature of the product is raised in a linear fashion until the water within the product turns from a solid component directly into a vapor, at which point a tremendous amount of heat energy is required to achieve the change in state. In fact if you are able to witness the ASTM E-119 test of a gypsum assembly, you probably will be able to see steam being released from the product on the non-fire side of the test.

With regular gypsum board, as the embodied water coverts to steam, the composition of the gypsum breaks down as the total mass of the board is reduced through loss of the embodied H2O. Type X board improves the time the gypsum stays together by adding glass fiber reinforcement to the mix to partially offset the shrinkage of the product. In fact, it is easy to visually determine the difference between regular board and Type X by closely examining the scored and snapped edge of a scrap of board – you can see the fibers in Type X.

Type C also has glass fibers, but it adds vermiculate, which compensates for the shrinkage caused by the escaping steam by expanding at approximately the same rate as the board is shrinking. Therefore Type C gives, performs better – 1/2” of Type C equals 5/8” of Type X for that rule of thumb “fire rating” (agree the product itself really doesn’t have) for most assemblies. Like Type X, it’s easy to visually identify Type C by examining the scored and snapped edge.

With all gypsum boards, the change in state from solid to vapor within the material consumes a tremendous amount of heat energy from the fire side without raising the temperature further on the safe side, and thus allows a couple of thin layers of gypsum board to provide fire resistance quantifiable as code requires. Even regular board provides some fire resistance, Type X is better because it stays in place longer due to the added reinforcing fibers, and Type C stays in place even longer still because it includes vermiculate to offset the shrinkage.

Comment from  George Everding

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