Beat the Heat: If You Want a Cool House, Celebrate Thermal Mass

Austin office
©. Wiss, Janney, Elstner Associates offices/ Larry Speck

Another in our series on how to design with climate in mind.


© Termessos/ Larry Speck

Many parts of the world, from China to Iran to Texas and Arizona, have a " high diurnal swing", where it is really hot in the daytime and cool at night. Larry Speck, a principal at PageSoutherlandPage wonders if there isn't a better way of designing in such conditions than our current standard practice of insulating and air conditioning. He writes:

I became interested using high thermal mass as an alternative while traveling in Turkey with my son Sloan eight years ago. He and I visited remote Roman ruins on the south coast and the interior, where the sites are in raw states and are not much frequented by tourists. The summer climate in Turkey is very hot and humid, not unlike Texas. But it was strikingly comfortable inside the stone ruins with their high thermal mass.
Ping Yao

© Ping Yao/ Larry Speck

I noted the same effect working beautifully in the all-masonry city of Ping Yao in western China, where homes have thick, stone walls and massive, stone beds that kept us amazingly cool on hot summer nights.

When hired to do a small office building for an engineering firm, he proposed walls of high thermal mass.

Austin office
©. Wiss, Janney, Elstner Associates offices/ Larry Speck

© Wiss, Janney, Elstner Associates offices/ Larry Speck

This had to be a pretty inexpensive building—a small office for a small business. Our budget was equivalent to a stud wall building with brick or stone veneer. The structural loads on the walls were modest and, of course, greater thickness worked to our thermal advantage. So we built unreinforced concrete walls—pure compression structures. The absence of rebar along with the fact that all of the formwork was kept to simple rectangles meant labor costs became low enough to be affordable. When we were pouring the walls, everyone said the finished pieces looked like Stonehenge.

Office building

© Wiss, Janney, Elstner Associates/ Larry Speck

The building isn't finished and the air conditioning hasn't been installed, but Larry says that " the building interior is surprisingly cool."

Wall section

© Wall section/ Larry Speck

This is a really remarkable wall section for the 21st century: no interior finish, no exterior finish, no reinforcing bars, nothing but a big honking 18" thick wall of concrete. It will be really interesting to find out how this works out. More at Larry Speck Thinking

Rammed earth

Eden Project Rammed Earth Wall/ Wikipedia/CC BY 2.0

I have described this form of construction as a thermal rechargeable battery, charging up with heat in the daytime and releasing it at night, and vice versa; Wikipedia calls it "the thermal flywheel effect". Not usually being a fan of concrete, I show my students rammed earth installations like this one at the Eden Project. Really, you can get thermal mass out of almost anything, even water.


© GroundHog from RainwaterHog

In Australia, RainwaterHog offers the GroundHog, which you can build into your floors. Really, you don't have to go to church to celebrate mass.