Insulating a radiant slab (C) Daniel Friedman

Slate Thermal Mass for Heat Storage - Design Details for Passive Solar Heating Systems

  • Use slate as thermal mass for storing solar heat in passive solar design?
    • What is the coefficient of expansion for slate - how much will slate expand in length per degree in temperature rise?
    • How should slate be bonded to the concrete slab for thermal mass storage and heat transfer?
  • Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
  • Questions and answers about the use of slate as thermal mass for storing solar heat in passive solar design

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Thermal mass effects of slate: this article discusses the properties of slate as a medium for thermal mass in passive solar heating system design and greenhouse or sunspace thermal mass material.

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Active Solar Heat Storage Using Rock Bed Storage Systems

Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. Our page top photograph shows a ceramic tile floor installed in Buenos Aires. Using ceramic tile finish flooring over a rock bed heat storage system is one method of design for a solar-heated radiant heat floor system.

Readers should also see BLOCKBED RADIANT FLOORS - SOLAR DESIGN for an alternative to block bed solar heat storage, and for supporting heat storage design details see FLOOR, CONCRETE SLAB CHOICES and PASSIVE SOLAR FLOOR TILES, PHASE CHANGE. Also see SLAB INSULATION, PASSIVE SOLAR and other articles at SOLAR ENERGY SYSTEMS. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

Thermal Properties of Slate for Solar Heat Storage and Radiant Heated Solar Floors

Question: how to install slate for use as thermal mass for sunspace or greenhouse passive solar heat storage

A client of mine has a unique storage material to use in his solarium - 30 one-inch thick slate pool table tops.

Will the thermal expansion and contraction affect the thermal bond to the concrete slab below?

Should we buffer the joints between the slabs of slate to prevent damage to the edges? - Tom Deal, alternative energy consultant, San Francisco CA


Slate's coefficient of expansion is 0.0000058, meaning that for every foot of length slate will expand 0.0000058 feet, or about 0.00007 inches for every degree Farenheit of temperature rise.

A 60 degF temperature rise will cause an 8-foot long slate slab to increase in length by 0.0336 inches - roughly 1/32 inch.

If the sunspace experiences wider swings in temperature, the expansion and contraction of the slate will of course be greater.

Kevin Callahan at the National Concrete Masonry Association recommended using Type M mortar, which has a higher compressive strength than mortar normally used for concrete blocks. He commented, though, that the mortar might not bond very well to the slate, which does not have a very porous surface.

Bedding the slate in mortar should, however, create a fine thermal bond.

Also see


For an alternative design approach to storing heat in solar heating designs see BLOCKBED RADIANT FLOORS - SOLAR DESIGN, and and ROCK-BED SOLAR HEAT STORAGE DESIGN, and for solar designs that combine greenhouse solar heat gain and trombe wall designs see GREENHOUSE DESIGN for SOLAR HEATING.

For more on radiant slab floors see "Radiant Floors", Solar Age 5/82, and the following articles online:


Other articles you'll want to see on passive solar floors, ceilings, and walls, include

The question-and-answer article above, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

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

  • Q&A on Slate as Thermal Mass for Solar Heat Storage- PDF version, use your browser's back button to return to this page. Original article, Solar Age Magazine, November, 1985, adapted and updated for December 2010.


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