In many ways, cork floors are a good solution to the impact noise control problem. They are relatively inexpensive; squares of cork are available for as little as a dollar per square foot (as of Spring, 2010). Better grades can cost up to five or six dollars a foot, which is no longer inexpensive, but about the same as many other flooring choices. You need to get a cork flooring tile that is thick enough to do the job; 1/2" is a minimum, but 3/4" or 1" is better--at which point it is no longer a very inexpensive floor.
Cork flooring works reasonably well for impact sound control when it is newly installed, but the floor does not wear particularly well. You may be tempted to improve durability by applying a protective coating like urethane. In doing this you're also reducing the ability of the cork to absorb sound without transmitting it. Cork has an organic open cell structure. This is why it's spongy when you press it, and also why it absorbs sound well. When you put a coating on it like urethane, the liquid, unless the cork is very dense (and therefore very expensive) tends to seep down through the interstices of the cork, both stiffening it and filling the open spaces with what dries to a durable, hard substance. Sellers of cork for sound control purposes will dispute this, but experimental results confirm the lessening of impact sound control. Even wax, which hardens the surface of the cork, reduces its sound-absorbing capabilities, although not as much as urethanes, paints and shellacs.
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