by Green Building Supply's Joel Hirshberg

This article explores questions frequently asked about underlayment and provides links to the underlayment products we recommend.

If you install carpeting without a pad, it will not be as comfortable to walk on and it will wear out faster. If you install tile on a cracked concrete subfloor it will enventually crack the tile and grout. If you install a wood, cork, or bamboo floating floor on an unlevel subfloor, the edges will not fit well together. Consequently you will stub your toes and eventually destroy that part of the floor.

If you install any type of floor on a damp concrete slab it may absorb into the flooring causing buckling. It may also create mold and mildew resulting in serious health issues. If you install any type of floor on a structurally weak subfloor it may squeak and result in uneveness in the surface. Need we say more!

No matter what product you install on the floor, the subfloor must be 1) level, 2) dry, and 3) solid. That holds true for today AND in the future. These factors are critical if you want your investment to last. Don't underestimate its importance and don't skimp. Installing a beautiful floor without proper preparation is like building a home without a foundation or like painting without primer. Sure it can be done, but the results will not be satisfactory in the long run.

That depends upon the nature and age of the subfloor. Always consult with your installer first. If adjacent floorboards are at different heights, sanding may be required. If they have serious flaws, a self-leveling cementitious product or a premade 1/8"-1/2" hardboard or cement board may suffice. If the concrete slab is cracked, a filler needs to be used in conjunction with an isolation membrane. If the floor is damp it needs to be dried thoroughly. If mildew or mold exists they need to be removed. If the floor is squeeky or weak, structural work may be required or hardboard or cement board may suffice.

Prep work is critical but underlayment is equally important for the following reasons: 1) To smooth out any undetected rough edges, small dips, or rises in the subfloor, 2) To make the floor softer to walk on, 3) To provide sound insulation, 4) To provide protection against moisture, 5) To prevent problems in the future due to settling.

There are many types of underlayment such as cork, rubber, and recycled. Some have vapor barriers built into them such as the QuietWalk recycled underlayment but cork and rubber do not. Almost every flooring manufacturer requires a vapor barrier to protect against moisture coming up from the subfloor. Therefore, a 6 mil polyethylene sheeting is required to be placed beneath the underlayment.

Our Moisture Block vapor barrier is a recycled plastic sheeting that is ideal for this purpose. All of our underlayments and vapor barriers are non-toxic and are listed along with each flooring product. Please read the installation instructions of each product for further details of what type of underlayment is required.

In-floor radiant heating, either hydronic or electric, will penetrate through any underlayment except lose with reflective foil. Depending upon the R-value of the underlayment the infra-red heat will penetrate through more or less quickly. For radiant heat, it is best to use an underlayment with the lowest R value. Our SCI 1/8" synthetic rubber pad is an excellent non-toxic pad with a very low R-value of .097 making it well suited to in-floor heating.

Can you use the other pads? Yes, but the response time will be slower. This means, if someone opens the windows or doors in the winter and the house gets cold, the response time to bring the temp back up to normal conditions will take a little longer if the heat has to fight its way through the underlayment. Having said that, we have confirmed with numerous customers that radiant heat works fine with cork flooring or marmoleum click flooring, both of which have cork underlayment built into them.

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