Yes, gypsum board does shrink due to changing heat and humidity levels. But don’t jump to conclusions. We review articles, reports and technical documents from a variety of sources at the Gypsum Association, and some contain some “unique” conclusions. An article that recently appeared in a national periodical discussing “shrinking gypsum board” falls squarely in that category.<

Yes, gypsum board does shrink due to changing heat and humidity levels. But don’t jump to conclusions.

We review articles, reports and technical documents from a variety of sources at the Gypsum Association, and some contain some “unique” conclusions. An article that recently appeared in a national periodical discussing “shrinking gypsum board” falls squarely in that category.

The article discussed “nail pops” and theorized that the cause of the pops was “shrinking” gypsum board. According to the article, the board absorbed moisture as the result of exposure to high humidity conditions during transit and storage. Then, after installation in a conditioned environment, the board shrank, thus causing the heads of the application fasteners to become exposed.

It is an interesting supposition. Unfortunately, it is completely incorrect on a number of levels. Here’s why.

Getting the numbers straight

Virtually all common building materials expand and contract as the result of two primary physical influences: thermal expansion and hygrometric expansion. Thermal expansion occurs when a material is exposed to heat. It can be quantified for a solid object by identifying its Thermal Coefficient of Linear Expansion (call it TCLE, for short). Hygrometric expansion occurs when a material absorbs moisture, including water vapor.

This characteristic is particularly true for materials of organic origin and is usually expressed by quantifying the Hygrometric Coefficient of Expansion (let’s call it HCE, for short).

Gypsum board exhibits both thermal and hygrometric expansion characteristics. It has a TCLE of 9.0 x 10-6/in. and a HCE of 7.2 x 10-6/ % RH. In lay terms, that means that an inch of gypsum board will expand 0.000009 inches when the temperature of the board is increased by one degree Fahrenheit. Likewise, an inch of gypsum board will expand 0.0000072 inches when the relative humidity in the area of exposure is increased by one percentage point.

Materials with higher TCLE or HCE coefficients expand and contract due to changes in temperature or moisture to a greater extent than materials with lower coefficients. Comparatively, the TCLE coefficient for steel is somewhat less than that of gypsum board and the coefficient for aluminum is significantly greater. Simply stated, when exposed to heat, a gypsum panel will expand slightly more than a steel panel, but less than an aluminum panel. Mercury has a very high TCLE, which makes sense when you think of its traditional use in thermometers.

Digging deeper

So, if gypsum board can expand and contract, why doesn’t the movement cause fastener pops? Essentially, there are two reasons. First, an individual sheet of gypsum board does not expand or shrink very much (note the numbers above; they are not very large). Second, fastener pops are a function of stresses placed on the fasteners from myriad other sources, all of which are significantly greater than the thermal or hygrometric stress placed on a board.

The first concept can be most readily explained by simple math and comparative analysis using the HCE for gypsum board. Since the board in question was installed in the southeastern United States, let’s assume that it was transported, stored and installed in a hot, humid Georgia summer with a relative humidity of 90 percent. Then, let’s assume a cold, dry winter with a relative humidity of 20 percent. That’s a swing in relative humidity of 70 percent over six months, easily a worst-case scenario.

Also, note that gypsum board is isotropic, meaning that it moves uniformly in all directions when it expands and contracts. As a result, a board will expand from side-to-side and top-to-base at the same rate that it expands from back face to front face.

Take the HCE for gypsum board of 0.0000072in./% RH; multiply it by 70 (using an integer for the percentage change in relative humidity); multiply it by 0.5 (for the thickness of the board) and you arrive at a figure of 0.00025, or 25 ten-thousandth of an inch. That is how much a layer of 1/2-inch-thick gypsum board will shrink from front to back when exposed to a decrease in relative humidity of 70 percent - not much of a change and not perceptible to the naked eye.

Now assume that the fasteners in question are either standard drywall nails – manufactured to ASTM C 514 – or standard drywall screws – manufactured to ASTM C 1002. Per standard, ASTM C 514 nails have a head that is a minimum of 0.015 inches thick and ASTM C 1002 screws have a head that is a minimum of 0.02 inches thick, both measurements taken at the edge of the fastener. Compare the gypsum board shrinkage to the fastener head thickness and you quickly arrive at the conclusion that even if the board did shrink due to an extreme change in relative humidity, the total shrinkage was less than 2 percent of the thickness of the head of the thinner-headed fastener; movement that is undetectable if the fasteners are properly installed and not sufficient enough to expose the head of the fasteners. The movement also is less than the thickness of the face paper of the board and less than the suggested thickness of the joint treatment that is applied to the board.

Wood you have guessed?

So what causes fastener pops? The biggest culprit is lumber that is installed when it is still “wet” and does not attain environmental equilibrium until well after the gypsum board has been applied. Most wood industry documents recommend (and increasingly most building codes require) that the moisture content of framing lumber not be in excess of 19 percent at the time of installation. Even if installed “per standard,” lumber that shrinks to a 6 percent moisture content (a common winter environment moisture content for lumber) will decrease by almost 5 percent in cross-section. For a properly manufactured 2-x-4, that’s more than 1/8 of an inch of shrinkage.

Furthermore, as the lumber shrinks, it places pressure on the shank of the fasteners used to attach the gypsum board. That pressure, coupled with the receding stud, forces the fastener out from the stud and causes the head of the fastener to “pop” out from the surface of the board. Use of over-long fasteners increases the shank area – thus increasing the area where pressure may be placed on the fastener – and exacerbates the problem, as does the use of plain shank nails. Ring shank nails and screws are less affected.

Worker error is also another prime cause of fastener pops. Fasteners that are not properly seated or that completely miss the framing member; use of the wrong-size fastener; and not holding the board tightly to the framing members when it is installed all can cause fastener pops. Fastener pops can also occur in metal framing; however, unlike wood framing fastener pops - which are most often a function of the moisture content of the lumber - metal framing pops are most often attributed to worker error and are caused by fasteners that are not properly seated in the metal studs, or by the use of fasteners that have a thread pattern that is too large for the thickness of the metal studs. Building stresses, in particular the downward pressure placed on walls by truss movement, can also aggravate the problem.

Based on the description of the situation as it appeared in the original article, it is difficult to determine exactly what caused the rash of fastener pops that occurred. To be fair, the author may not have been able to obtain all the background information on the situation – information about stud moisture and application conditions that might have allowed a different conclusion. However, what is clear is that the inference that was expressed in the article was incorrect and that gypsum board doesn’t shrink as claimed.