Common Misconceptions
September 2nd, 2007

September-October 2007

Cork is a highly misunderstood material in the US market. Since it has an appearance similar to particleboard in some ways, many people assume that it has similar characteristics. We’d like to take this opportunity to dispel any misconceptions.

Cork will absorb water like a “sponge” if it gets wet.

Incorrect. Think of the most common use for cork, wine stoppers. Cork has been used for wine stoppers for hundreds of years primarily because it does not absorb water or liquids. Cork has also been used for years in buoys, lifejackets and other floatation devices, again because it does not absorb water and can remain buoyant for years. A cubic inch of solid cork immersed in water for 48 hours will gain less than 3% in weight due to water absorption. A cubic inch of solid wood or unglazed clay-bodied ceramic tile would gain many more times this percentage in weight of water if immersed for 48 hours.

Composition cork material will fall apart if it gets wet after it has been installed.

Incorrect. That may have been the case over 25 years ago when animal protein binders were used, but not anymore. Since the early 1980’s non-water-soluble polyurethane binders have been used to adhere the granules together to make quality composition cork products. These polyurethane binders also produce no post-installation off gassing and do not leach into ground water supplies. Products constructed with these types of binders can be totally immersed in water for 30 days or more and show no signs of structural deterioration.

Cork will “swell” with exposure to moisture and cause finish floor coverings to fail.

Incorrect. Because cork absorbs so little water, it is very dimensionally stable. When exposed to 100% Relative Humidity conditions for 30 days the dimensional change in the grade of materials used to manufacture certified underlayment-grade product is less than 3%. In a 6mm thick piece of material this would represent an increase in thickness of less than 1/120 of an inch.

Cork will compress and crumble under heavy loads and traffic.

Incorrect. Unlike open or even close-celled synthetic foam materials, cork consists of an interlocking structure of 14-sided polygons called tetracadecahedrons. These totally sealed gas-filled cells have a very tough outer surface that is almost impossible to break. Because of this unique natural attribute, cork has a compression/recovery rating of close to 100%. Unlike many foam and fiber-based products, it will not collapse over time with traffic. The binders used to adhere the granules of certified underlayment-grade cork products together are designed to create a permanent structural bond between the particles.

Cork will support the growth of mold and mildew if used in a moist environment.

Incorrect. Going back to the traditional use of cork in wine stoppers, solid cork is used for sealing fine vintage wines precisely because it does not readily support the growth of mold and other biological agents that can cause spoiling of the wine. Additionally, the granules of cork used to make certified underlayment-grade products are coated with a polyurethane binder, which enhances their natural microbial resistance.

“Cork is cork” and all cork underlayment products are the same.

Incorrect. Properties such as density, particle size and consistency of particle size are very important and vary widely from one manufacturer to another. Density affects the structural stability of the product and the sound attenuation quality. A product that lacks density will be too delicate to use as an underlayment and a product that is too dense will have poor sound attenuation characteristics. A product that has too large of particle size or a wide range of particle sizes in the mix may lack the structural integrity to be effectively used as an underlayment.

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