Thermal Mass—does it really matter? Part 1

Mar 28, 2016 | Uncategorized

Proponents of Insulating Concrete Forms will cite several reasons for their claim that ICF homes outperform wood framing. One of the major ones mentioned is thermal mass. They will say wood framed homes do not have it, but ICF homes do.

However, many in the high performance industry dismiss that thermal mass can make a significant contribution to the performance of a home, particularly in a heating dominant climate such as ours.

Who is right?

What is thermal mass?

Every material we build our home from has a mass, some more dense than others. For instance, insulation has a lower mass per cubic foot than stone. In general, the higher the density of a material, the greater amount of energy it can store at a given temperature. If you drop a ball bearing heated to 500 degrees in a cup of cold water, that water will get a LOT warmer than if you dropped in a 500 degree piece of foam of equivalent size.

More thermal mass = more energy storage capacity.

Stone Thermal Mass

Stone has a high density and is a commonly used mass material

Straw Homes

(Straw has a low conductivity, making it a good insulating material)

Every material also has a conductivity rating. Again, insulation by definition has very low thermal conductivity, while metals have a high conductivity. This is the reason wood floors feel warmer than ceramic tile, even though they are both at 70 degrees. The tile has a higher thermal conductivity, and thus removes the heat from your bare feet much faster. This ability to conduct heat is necessary in a mass material, but high conduction can be a problem too.

The ideal thermal mass material strikes a balance between storage capacity and conduction.

Such materials run a spectrum between wood, brick, concrete, and stone. You may already be familiar with the concept if you have ever traveled to the Southwest and visited an adobe home. During the day these homes are absorbing the sun’s heat, keeping the interior cool; but at night they reverse—releasing the sun’s heat to the interior to keep occupants warm.

Thermal mass can be used in the exterior walls, interior walls, the floor, or even the roof. If using high thermal mass in your home were cheap, easy, or both; everyone would be doing it. But it is neither cheap nor easy.

The natural question becomes: is it worth it?

I alluded to the charge some builders bring against using mass: That in our climate mass has little effect, since the outdoor temperature doesn’t swing above and below the interior temperature in a given 24 hour cycle. I submit to you that such people have not lived in a high-mass home. EVEN IF there were ZERO energy benefit, the increased comfort alone is enough to justify widespread use in today’s homes.

  • High mass walls shrug off a night of sub-zero temps without relying on the HVAC to run frequently.
  • High mass floors maintain an incredibly consistent temperature all the time.
  • High mass materials recover the interior temperature quickly after your kids left the door open while they brought groceries in for 10 minutes.
  • High mass materials promote even temperatures throughout the home.

Such critics have probably never lived in such a home, yet they have a point. In a cold climate, high mass (alone) would tend to make the house cold all the time. With that cold would come clammy air. But therein is what we do differently: We insulate the exterior of the mass. Doing this allows that mass to warm up, and benefit the homeowners immensely, with consistent comfort, and real energy savings.

How do I know? I’ve lived it. How does it work? Stay tuned for part 2!

Jake Vierzen

Adobe Homes
(Adobe homes of the Southwest stay comfortable without insulation)

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