What’s the Difference Between Insulation and Thermal Mass?

If you’re new to passive solar principles, and sometimes even if you’ve been dealing with them for years, you may be confused or uncertain about insulation vs. thermal mass. Specifically, here are the questions we had about them when we were new to this:

  • Is there any difference between insulation and thermal mass, or are they really just two terms for the same thing?
  • When should you use insulation, and when should you use thermal mass?
  • What makes a material a good insulator, and what makes it good thermal mass?

Is there any difference between insulation and thermal mass, or are they really just two terms for the same thing?

Yes, absolutely, insulation and thermal mass are different, and no, they are not just two terms for the same thing. Insulation and thermal mass are similar in that they both slow down the movement of heat between exterior and interior spaces, but they are different in lots of ways. One of the mistakes people make most often with passive solar houses is using insulation when they should be using thermal mass, and vice versa.

When should you use insulation, and when should you use thermal mass?

You should use insulation when you want to maintain a temperature differential between indoor and outdoor space. You should use thermal mass when you want a substance that will slowly take on heat and then slowly release it. Let’s illustrate with an extreme example.

Assume that you want to maintain an indoor temperature of 74 degrees F, or as close to it as possible. And assume that the outdoor temperature is consistently 110 degrees F. Do you want insulation or thermal mass? You want insulation, because you want to maintain a differential between the 74 degrees inside and the 110 degrees outside. You want something to stop that heat, or to let as little of it as possible penetrate your interior space.

What would happen if you used thermal mass in or on a wall instead of insulation? Well at first, the thermal mass would take on heat very slowly; that’s what thermal mass does. And eventually, thermal mass being thermal mass, it will reach a temperature at or near that of the outside (remember: thermal mass always takes on heat). Once it has done so, it will act as a gigantic wall heater for your interior space, generating extra load for your air conditioning equipment and keeping you and those you love uncomfortably warm.

Now let’s use another example. Let’s say you want to maintain the same 74 degrees F, but change the outside temperature. Instead of a steady 110 degrees F, it heats up to 110 degrees F during the day and cools down to 40 degrees F at night. Do you want insulation or thermal mass? Here, what you use will depend on your comfort strategy. If you simply want to isolate the interior space from the exterior, insulation is probably your best bet. As the daytime temperature climbs, the insulation will maintain a differential between the uncomfortably hot exterior and (you hope) the comfortably temperate interior. Then at night when the mercury dips, the insulation will maintain the same differential, only now it will be between the uncomfortably cool exterior and the temperate interior. You could also use thermal mass in the exterior wall, because it would act to “smooth” the daily swings in temperature, slowly taking on heat during the day and slowly releasing it at night.

In this “swing climate,” characterized by daily changes in the outdoor temperature of 50 degrees or more, thermal mass is a great addition to the interior space, particularly where it is in the direct path of the sun’s rays. During the day, it blocks the harshest direct rays of the sun and slowly takes on heat. By doing so, it slows down the rate at which the interior heats up. Then at night, when the solar heating is no longer present, the thermal mass slowly gives up heat. Again, it acts to slow down the rate at which the interior cools down. Thermal mass acts to moderate changes in temperature, making increases and decreases less dramatic.

In general, thermal mass makes the most sense when the outside temperature is both above and below the desired indoor temperature during a normal 24-hour cycle. If it stays warmer or stays colder than you want the indoor temperature to be, you’re better off with insulation.

What makes a material a good insulator, and what makes it good thermal mass?

Given that the purpose of insulation is simple, to maintain a temperature difference, the principle criteria for selecting an insulation material is its resistance to the passage of heat. The technical term is “thermal conductivity.” You probably are familiar with R-values, which are a function of the thermal conductivity of a material and its thickness.

If the only thing insulation needed to do was to retard the passage of heat, then you could shop solely on that basis and just look for the most heat resistance (or highest R-value) for the money. Unfortunately, it’s not that simple. For purposes of this page, however, it’s sufficient to understand that what makes good insulating material is low thermal conductivity and enough thickness to make a difference, usually expressed in terms of an R-value.

Examples of material that’s good for insulation are fiberglass, rock wool, cellulose, and polystyrene or polyurethane foam.

What makes material good thermal mass is a little more complicated. In general, a material needs to blend three things to be good thermal mass. It must hold heat well, it must be dense, and it must have thermal conductivity that is low without being too low. You can read more about what makes good thermal mass.

Examples of good material for thermal mass are rock, water, adobe, earth, mud, and concrete. Perhaps one of the most common examples of thermal mass in everyday use is a stone chimney for a fireplace.