Thermal Mass and Building Energy-Efficient Houses

Building energy efficient houses is all about R-values: higher R-values mean lower energy-loss, and lower energy bills. Simple. Or maybe not. Maybe it is a little more complicated than that. Maybe there is more in play than just R-values.

And indeed there is more at play: thermal mass. Thermal mass is an often overlooked factor in building energy efficient houses. This is understandable: with all the hoopla about R-values, air-tightness, Ep and McGeo, thermal mass just adds to the confusion. And then when you try to find out what it actually is, or what it means, you end up at Wikipedia-pages with stuff like this: thermal-mass-on-wikipedia

Right, not very helpful for most of us. So let’s try to explain what exactly thermal mass is, and how it can help to lower our energy bill.

Thermal mass is a bit like normal “weighted” mass.fiat-500-1957-edition--1 It’s heavy, and it doesn’t want to move. Compare a truck with a Fiat 500: you can push both with the same force, and while the Fiat starts moving, the truck will not. Too much mass to simply push it.

Thermal mass is a bit like that. The sun can try to push energy into it, but it doesn’t move, temperature stays the same. It is the ability of a material to store heat without the temperature going up too much. That’s it: the ability to store heat, nothing more.

Water and Sand

Let’s compare two materials: water and sand. Both materials are available at the beach. Now let’s imagine we are at the beach on a blazing hot day in August. Question: what is hotter, the water, or the sand?

thermal-mass-Beach-1024x686That’s an easy one: the sand is hotter. The water has an extremely high thermal mass, i.e. a very high capacity to store energy. All solar energy disappears into the water, but the water temperature goes up only a little. The sun can push and push this water mass, but it won’t move. Compare this to the sand. It has a much lower capacity to store heat, so a little sun and already we are burning our feet. The sand is more like the Fiat, a little pushing and it goes.

This “energy storage system” can go in reverse as well. In the evening the sun disappears, the ambient temperature goes down and guess what: the temperature of the sand quickly follows and goes down, but the water temperature stays the same. The water has a much higher thermal mass and it’s temperature is not so easily influenced. It slowly releases the energy stored during the day.

A Thought Experiment
thermal-mass02

Waterfall at the Catlin Insurance Company Hamilton, Bermuda

What does all this do for the energy efficiency of a house? Let’s do a crazy thought experiment, let’s imagine that we can build our house completely from water, or from sand. So we have a house built completely from water, with water walls and a water roof (impossible, yes, but let’s just pretend), and then compare this house with another impossible house which is completely built from sand.

Now it is August again, and the sun is blazingly hot. Which house is more comfortable during the day? Most likely the water house: fresh and cool. And then in the evening which house is more comfortable? Again the water house, as it’s walls stay warm (alas, warmer than the sand walls) and the water heats the interior of the house.

Now obviously you can’t build a house from water, or from sand. The best you can do is use building materials that have a high thermal mass. The higher the better:

  • Concrete and clay are good, oak is good, wood fiber is good
  • Bricks are … meh …
  • glass wool and polyurethane are not OK.
Thermal Mass In Numbers

Here are some values that give an indication of thermal mass:

Material           J/(kg·K)
================== ========
Water                  4186
Oak wood               2400
Wood fiber             2100
Concrete               2060
Pine wood              1600
Brick                  1360
Rockwook                900
Glasswool               830

Looking at the above table, you start to understand why wood and wood fiber are getting so much attention as building and insulation materials. They store solar energy like no other material (even better than concrete), and at the same time they have excellent lambda-values (meaning: high R-values), very much unlike concrete and brick.

Wood and wood fiber are every eco-builder’s dream material.