The science behind ceiling fan blade slump and sustained humidity.
When a catastrophic event breaches the building envelope, solid and composite wood materials swell and deform in response to moisture exposure and absorption. In addition to separated crown molding, cupped wood flooring, and ill-fitting cabinet doors, you may notice that ceiling fan blades tend to drop downward. A couple questions arise: What types of ceiling fan blades drop and under what conditions of humidity? How long does it take to show a drop?
Early ceiling fans were powered by a stream of running water in the 1860s. Then, in 1882, Philip Diehl invented the ceiling fan as we know it today. By the 1920s, ceiling fans were found in homes and businesses throughout the country due to mass production and distribution commencing throughout the world.
Since then, ceiling fan design has remained relatively unchanged. However, over the last 90 years, the operational efficiency has improved, and their weight has decreased by replacing solid wood blades with plastic, canvas, and composite materials.
This study evaluated 52-inch blades from the ceiling fans of six different manufacturers (Photo 1, samples FB1 through FB6), and modestly priced models were used that ranged from $90 to $120. The blades were exposed to elevated relative humidity (RH) of greater than 80 percent and a stable temperature range of 70 to 80 degrees Fahrenheit for four months (Figure 1).
Among the six ceiling fans purchased, each came with a set of five blades. Four of the five blades were exposed to test conditions, while the fifth blade from each set served as a control and was exposed to conditions of 76 degrees Fahrenheit and a relative humidity of approximately 50 percent for the duration of the test period.
The test blades were bolted horizontally to a stainless steel support apparatus (Photo 2) and covered with a plastic shroud of eight-millimeter clear polyethylene over the entire apparatus to sustain humid conditions. Two water reservoirs equipped with an aeration pump were placed inside the apparatus to encourage evaporation. HOBO data loggers documented temperature and humidity conditions. A measuring stick and laser were used to measure progressive changes in blade deflection at regular intervals. The study’s objectives were to evaluate how different blades responded to elevated RH and to determine the relative rate of blade decline as an estimate of duration of exposure.
Relative humidity and temperature measurements confirmed that the fan blades were held under stable conditions (average 76 degrees Fahrenheit and 83 percent RH) over the test period (Figure 1). In a previous test, no appreciable deflection (less than one millimeter) was observed when the blades were exposed to RH of less than 70 percent over a 12-month period.
In this study, the test fan blades were identified as FB1 through FB6 and ranged from least to most deflected, respectively. Over the study period, the measured downward deflection ranged from 1/16 inch (FB1 at 1.59 mm) to 3 1/16 inches (FB6 at 77.78 mm).
The outer covering of each fan blade did not allow one to visibly distinguish the materials used in construction other than shape, color, and design. To understand what materials were used and to distinguish and explain one fan blade’s performance from another, we removed thin cross sections of each blade and examined them using scanning electron microscopy.
Measurements of Decline
Measurements of fan blade drop increased when the relative humidity exceeded 80 percent (Figure 2). The fan blades made of fibrous/adhesive composites (FB4 through FB6) exhibited the poorest performance averaging 2/100th of an inch (0.5 mm) per day to 1/4 of an inch (0.65 mm) per day of deflection. Measurements of fan blade drop among those made of laminate material (FB1 through FB3) showed better performance, with either a slight elevation or decline. Those that declined expressed an average drop of 5/10,000 of an inch (0.013 mm) per day to 5/1,000 of an inch (0.132 mm) per day.
Scanning Electron Microscopy
In examining the cross sections, we found that three of the six blades were made of compressed fibrous materials, while the other three were made of natural laminated wood. The fibrous material fan blades, which were 6 mm to 6.5 mm thick (Photo 3), consisted of compressed fibers ranging in size from 5 to 50 microns in diameter (Photo 4). The fibers were held in place by an adhesive in a plate-like structure.
The laminated fan blades, which were 6 mm thick, were made of natural wood (Photo 5) and constructed in a manner consistent with plywood using three, five, or seven sheets of wood veneer, from 600 to 1,300 microns thick, running in at 90 degrees from each other (Photo 6). The natural construction of wood was revealed in the cross-section showing the vascular arrangement of the wood, which was not compressed.
Laminate vs. Composite Wood
Comparisons of moisture resistance between laminate and composite wood products consistently reveal the vulnerability of composite products to moisture. Solid and laminate wood materials retain their elasticity when exposed to moisture because they retain the inherent cellular structure of the wood and return near their original configuration when wetted. As a result, moisture absorption causes only temporary swelling and little or no loss of structural integrity.
In contrast, composite wood products (e.g., medium-density fiberboard, particle board, and oriented-strand board) tend to absorb moisture, expand, and release potential energy created during wood fiber compression and bonding with adhesives.
Fan blades made of composite wood materials will drop when subjected to sustained conditions of elevated relative humidity. The use of natural wood materials to construct fan blades will produce greater resistance to humidity than fan blades made of compressed wood or fibrous material. The construction of fan blades using multiple sheets or veneers of natural wood with the grain running in opposite directions will offset the tendency for the blades to warp and drop following exposure to elevated RH and gravity. Ceiling fan blades made of natural, unmodified wood materials will retain their shape and provide a longer, consistent service life in a humid environment.
Ceiling fan blades made of composite materials start to drop within one or two weeks of sustained elevated humidity exposure.
The materials used to construct the blades have a profound effect on drop performance.
Ceiling fan blades made with composite materials drop when exposed to elevated humidity conditions of greater than 80 percent.
Ceiling fan blades did not drop when exposed to lower humidity conditions of less than 70 percent RH.
Laminate wood fan blades had the best performance under sustained conditions of elevated humidity.
Composite fibrous wood fan blades had the poorest performance above 80 percent RH.
The authors would like to thank the following colleagues for their editing comments and suggestions: Nicholas Albergo, P.E. DEE, Bruce Bosserman, P.E., John Marquardt, P.E., Paul Castellano, P.E., and John Barkey.