Insulating a radiant slab (C) Daniel Friedman Flooring Choices for Use Over Concrete Slabs Using Thermal Mass

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This article discusses the best choices for finish flooring to use over a concrete floor slab intended to provide direct solar heat gain and heat storage.

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Our page top photograph shows a well-designed radiant-heat floor slab being constructed in Two Harbors, Minnesota where it gets too cold to fool around.

What Finish Flooring to Use to Cover a Concrete Floor Slab

Brick interior passive solar floor (C) Daniel FriedmanThe question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

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What Flooring Materials are Best to Use Over a Concrete Floor Slab


I'm in the process of building a home.

Are there any floor-covering products I can install over a pre cast concrete second floor (see THERMAL MASS in UPSTAIRS)

and a cast-in-place poured concrete first floor slab (see THERMAL MASS in BUILDINGS).

I want a resilient (soft clay, brick?) finish floor that will yet retain the thermal mass properties of the concrete slab below.

I understand the necessity for thermal mass [in my passive solar design home] but I would like to find a more pliable floor covering [than ceramic tile].

Our photograph (left) shows a passive solar brick interior floor, constructed over a poured concrete slab. This building is on the Vassar College Campus in Poughkeepsie, NY.

Our photograph (above) shows ceramic tile installed over a concrete floor slab intended to provide both passive solar gain and heat storage as well as delivering heat from a backup in-floor radiant heating system. Readers constructing an insulated slab with radiant floor heating, whether by passive solar or any other means, should not fail to read RADIANT HEAT FLOOR MISTAKES.


Tile floor over radiant heat and passive solar slab (C) Daniel FriedmanAssuming you are interested in using these floors for direct-gain passive solar heat storage, we see two possible approaches:

  1. Try to find a finish flooring material that has a high heat capacity itself
  2. Look for a finish floor material that has negligible heat capacity but has good absorptive and good thermal conductivity.

Either of these approaches will allow the finish floor covering to transfer heat into the concrete floor slab which itself has a high heat storage capacity.

We know of no soft floor covering with high heat capacity, but the second approach shows some promise. A 1980's study by industry researchers concluded that any floor covering whose thermal conductance exceeded about 10 BTU/hr-ft2 would not impair the thermal storage capability of the concrete slab.

This eliminates using even the thinnest floor carpeting (whose conductance C is about 0.8 BTU/hr-ft2), and some vinyl floor coverings.

But this criteria permitted use of vinyl-asbestos floor tiles (C = 43 BTU/hr-ft2) until that product was discontinued in buildings, and it probably permits use of the modern replacements of those products. It also permits use of sheet vinyl flooring on felt backing (C = 21.2 BTU/hr-ft2).

Beyond C=10 flooring products, increasing the absorptive does more than increasing the conductivity.

Light flooring materials absorb less solar radiation (heat) and are thus less efficient (as much as 40 percent less for pastel colors).

Also, it is important to install the finish floor covering so that it makes a very good thermal contact through a continuous bond over the floor slab.

Low-density brick and other "soft" masonry materials are not ordinarily considered resilient. If they are well-bonded to the slab, however, they will not reduce its thermal performance much.


Also see FLOOR, CONCRETE SLAB POURED FINISH where we describe pour-on finish flooring for use over thermal mass concrete slabs.




Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Original article:

The link to the original Q&A article in PDF form immediately below is preceded by an expanded/updated online version of this article.


Continue reading at FLOOR, CONCRETE SLAB POURED FINISH or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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