News Center Home > Flooring > Technical Bulletin LEVELROCK Brand Floor Underlayment Technical Bulletin 002 TOPIC Radiant Heat Performance PUBLISHED Fall 2001 Printer-friendly version of this page. Gypsum Underlayment Performance in Radiant Heat Applications LEVELROCK® Brand Floor Underlayments are specially formulated binders that are mixed by USG licensed applicators with sand and water in the field to pour floor underlayments. The specially formulated binders consist of gypsum, Portland cement, and other additives. Since gypsum is the primary component of the binders, it is commonly referred to as a "gypsum underlayment". USG's entry level product with its nominal compressive strength of 2500 psi comes in two types - Regular and Radiant Heat. This technical bulletin explains the differences between the two kinds of the nominal 2500-psi product and emphasizes the importance of using the "Radiant Heat (RH)" binder for radiant heat applications. For radiant heat applications, use LEVELROCK Brand Floor Underlayment RH (Radiant Heat) to minimize calcination. Chemically, gypsum is chemically calcium sulfate with two water molecules attached to it (CaSO4. 2 H2O). When exposed to temperatures above 300 degrees F., the water molecules are removed to varying degree from the chemical structure. This phenomenon is called "calcination". The speed with which calcination occurs depends on the actual level of temperature and the duration of exposure. However, even at temperatures between 150 to 300 degrees F., slow removal of water molecules take place. The removal of water molecules results in loss of strength and hardness of gypsum casts over time. Research conducted by USG Corporation and others over the last 50 years has shown that there are special additives that can be added to gypsum casts to dramatically slow down and control the removal of the water molecules from the chemical structure, especially in the temperature ranges of 150-200 deg F. These special additives that dramatically slow down the "calcination" of gypsum casts and the resultant strength loss are present only in the "Radiant Heat" class of floor underlayment binders. Thus, use of regular binders in radiant heat applications is an invitation to failure of the floor system. Over time, the use of regular binders in radiant heat applications will result in gradual loss of strength of the floor and may lead to problems such as powdering and chalking. It also will lead to poor heat transfer performance. USG Corporation only recommends the use of LEVELROCK Brand Floor Underlayment-RH (Radiant Heat) in radiant heat applications (either hydronic or electrical). Specifiers, contractors, and licensed applicators need to make sure that the "Radiant Heat" formula is used in Radiant Heat applications. If you have specific questions on the use of LEVELROCK Brand floor underlayments, please call 1-800-USG-4YOU and we will be glad to help you in deciding if your application is suitable.