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Radiant Heat Thin Slab System

There are several methods of installing hydronic radiant heat systems over a conventional wood-framed floor. One of the most common is called a thin slab system. Thin slabs consist of either a specially formulated concrete or poured gypsum underlayment. Both types of slabs have installation requirements that must be carefully coordinated with the building design process.

One requirement that must be accommodated is that thin-slabs typically add 1.25 to 1.5 inches to the floor height. This requires adjustments in the rough opening heights of windows and doors as well as the height of door thresholds. It will also affect the riser heights on stairs.

Another issue that must be addressed is the added weight of the thin-slab. Poured gypsum thin-slabs typically add 13 to 15 pounds per square foot to the “dead loading” of a floor structure. Standard weight concrete thin slabs add about 18 pounds per square foot (at 1.5” thickness). Never assume the proposed floor structure can simply support the added weight of either type of thin-slab. Have a competent designer or structural engineer verify what, if any, changes are necessary to support the added load.

The additional floor thickness and weight are easily managed if planned into the building as it is designed. However they can present obstacles in retrofit situations.

Poured Gypsum Thin-Slab Radiant Heat System

Poured gypsum underlayments have been used for many years for floor leveling as well as to enhance the acoustic and fire resistance properties of wood-framed floors. They also function well as the slab material for thin-slab floor radiant heating systems. In most cases, the slab is installed by a subcontractor trained and equipped to mix and place the materials.

Installation begins by stapling the tubing to the subfloor. A pneumatic stapler with a special attachment allows the staples to be quickly placed without damage to the tubing. It’s the preferred attachment method for all but very small thin-slab areas.

Once all tubing circuits have been installed they should be pressure tested as described earlier.

Next the floor is sprayed with a combination sealant/bond enhancement coating. This minimizes water absorption into the subfloor as well as strengthening the bond between the slab and subfloor.

The poured gypsum underlayment consists of gypsum cement, masonry sand, admixtures and water. The product is prepared is a special mixer usually placed outside the building, and is then pumped in through a hose. As the product is poured, it self-levels with minimum floating.

Some installers prefer to install the gypsum slab in two layers (or “lifts”). This minimizes any differential shrinkage in the slab, resulting is a very flat finish surface.

When poured gypsum underlayment cures, it resembles plaster and is almost as hard as standard concrete. However, unlike concrete it is NOT intended to serve as a permanent “wearing surface.”

With the proper preparation, a poured gypsum slab can be covered with almost any finish flooring including carpet, sheet vinyl, ceramic tile and glue-down wood flooring. Always follow the gypsum underlayment manufacturer’s procedures to verify that the slab is adequately cured that and the surface is properly prepared before installing finish flooring.