Coefficient of thermal or moisture expansion in drywall or plasterboard:

This article cites and explains the thermal coefficient of expansion (or movement) of plasterboard and the coefficient of moisture expansion (or shrinkage) of drywall due to humidity or moisture changes. Drywall cracks in ceilings or walls are often blamed on gypsum board expansion or shrinkage due to temperature and moisture changes.

But plenty of experts think that other building factors such as framing or wood shrinkage or building structural movement are more significant causes of plasterboard cracks.

We cite industry sources, technical research and plasterboard or gypsum board standards for various countries. We include example calculations showing how to convert a movement or coefficient of expansion into a distance or the size of an expected crack or tear as moisture or temperature vary and if other forces are excluded.

Sketch at page top: common expansion or shrinkage crack risk points in ceiling drywall - where a control joint may be required.

How Much Does Drywall Expand or Shrink in Response to Temperature or Moisture Changes?

Depending on the individual drywall control joint product design and materials (width and plastic vs zinc metal) expansion control joints for plasterboard can handle up to about 4/10" of horizontal movement due to thermal or moisture-related expansion or contraction.

The required drywall or gypsum board (or plasterboard) control joint width between the abutting drywall panels ranges depending on the specifications of the drywall expansion control strip's manufacturer, typically somewhere between 1/4" and 1" in width.

In the United States, the Gypsum Association (2010) gives an extensive table of drywall properties, including the dimensional stability of drywall, expressed as coefficients of linear expansion. This is for unrestrained gypsum board in the temperature range of 38 °F to 90°F (3.3° to 32°C)

Photo: severe cracking and tearing in a drwyall interior corner. A crack like this one is caused by building movement, not by thermal or moisture changes in the drywall itself.

[Click to enlarge any image]

But in any large expanse of drywall such as along a thirty-foot ceiling or wall expanse, you may see cracks caused by moisture and temperature changes alone if control joints are not installed. Our next photo below illustrates that sort of drywall or plasterboard cracking.

• Temperature effects on drywall expansion: Thermal Coefficient of Linear Expansion of unrestrained drywall (gypsum board or plasterboard) due to temperature variation:
  
  9.3 x 10^-6 in/in °F
  
  or for centigrade,
  
  16.7 x 10^-6 mm/mm.°C.
  
  What the heck does that mean?
  
  Example: if our gypsum board temperature changes from a low of 40°F up to a high of 90° F that's a 50° swing.
  
  Keeping the math clear, 10^-6 = 0.000001 we can write the thermal movement per linear inch over a 50° change as
  
  50° x 9.3 x 10^-6 in = (50° x 9.3 x 0.000001) = 0.000465 inches of dimensional change per inch of length - which doesn't look like much.
  
  But for a 30-foot long section of drywall ceiling or wall we can calculate
  
  360 inches x .000465 = or about 0.167 inches of movement - about 2 tenths of an inch - enough to show up on your ruler or possibly to cause a tear or a gypsum board butt-joint ridge formation.

Our drywall ceiling photograph above shows a crack that appeared in a continuous drywall expanse, a ceiling that runs the length of a Poughkeepsie New York home, for 64 1/2 feet in total distance.

• Moisture effects on drywall expansion: The Gypsum association gives the Hygrometric Coefficient of linear expansion of unrestrained gypsum board (drywall or plasterboard) due to moisture level changes as
  
  6.5 x 10^-6 in./in/%RH  (or also for mm/mm/%RH)
  
  Example: if our gypsum board humidity changes from a typical low and dry building interior of say 10% to a whopping 50% (over a wet or flooded basement, for example but the board is not immersed in water) that's a 40% change in relative humidity or RH for which we calculate the total gypsum board movement (expansion when we are getting wetter or shrinkage when our plasterboard is getting more dry) as
  
  6.5 x 10^-6 x 40 x 360 inches of total length = .09 inches of change or about one tenth of an inch - which would produce a very noticeable tear or crack.
• Combined forces: if our gypsum board has to endure both the temperature change and the moisture level change we calculated above we've got about
  
  3 tenths of an inch of shrinkage or expansion over 30 feet - a large opening, tear or crack.

Other Citations of Plasterboard Dimensional Stability

For Australia, according to Boral, a producer of plasterboard, under normal temperature and humidity conditions, plasterboard dimensional properties are as follows: - [4][4a]

• Thermal Coefficient of Linear Expansion = 16.2 x 10^-6 mm/(mm°C) –at temperature range 4°C to 38°C
• Hygrometric Coefficient of Expansion = 7.2 x 10^-6 mm/mm/%RH – at 5% to 90% RH

Other sources use the Boral constants.

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. Here is the math: .0000072 x 70 x .5 = .00025

Now let's do the same calculation substituting the length [of a 12-foot gypsum board panel] (144 inches) for the thickness:

.0000072 x 70 x 144 =.0725 or a little over a 16th of an inch in 12 feet.

A sixteenth of an inch is a very noticeable crack. - JLC forum discussion t-35269.html

Effects of Restrained (fastened in place) vs Unrestrained Drywall Movement due to Temperature & Moisture

Note: these data are for unrestrained gypsum board panels. In a "real world" application the panels are secured with nails, screws, adhesive, or a combination of these, fastening the plasterboard panels to structural members typically spaced 16" o.c. or 24" o.c. This means that the movement stresses are distributed over fastener points across the panel surface.

One would expect actual movement results to be more constrained than in the unrestrained movement data given above.

Some experts comment that wood (framing lumber) shrinkage and structural movement are more likely to be the dominant forces producing cracks or tears in drywall. I agree - DF. - JLC forum discussion t-35269.html

...

Continue reading at DRYWALL CRACKS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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