Thin Slab Radiant Heat Systems
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.
Poured gypsum slabs are water-resistant not
waterproof. The slab will eventually soften if exposed
to water for prolonged periods. They should not be
installed under conditions where rain or other sources
of moisture can accumulate. They should also not be
installed in areas that are likely to experience
flooding.
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