Insulation vs. Thermal Mass

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Keeping a home at a comfortable temperature is an important design goal for any house including those built with natural materials. There are two ways of doing this; in building science terms we call them insulation and thermal mass. These two properties are very different and the difference can be difficult to understand at first. In most climates it’s best to have both insulation and thermal mass to keep your home at a comfortable temperature. Let’s discuss the details of each of these.

Defining our terms:

Insulation: A material with the property of slowing heat transfer. Conventional construction examples: fiberglass insulation batts, foam, cellulose. Natural examples: straw, sheep’s wool, pumice.


Thermal Mass: A material with the property of absorbing or releasing large quantities of energy when changing temperature. Conventional examples: concrete, brick, steel. Natural examples: adobe, stone, cob, rammed earth.


Thermal Energy: Energy in the form of heat.

An analogy between extreme sports and buildings:

According to science, heat always transfers from a warmer place to a cooler one (second and third laws of thermodynamics) much like gravity always pulls things down. In certain extreme sports people defy gravity similarly to how we want our homes to defy heat transfer.

skydiver bungee jumper insulation thermal mass

Insulation is like a parachute; it slows heat down in a similar way to how the parachute slows down a skydiver. It won’t stop heat from getting through your wall, ceiling or floor just like a parachute doesn’t stop a skydiver from getting to the ground. In the construction industry we quantify how much the heat is slowed with R-value. The “R” is for resistance to heat transfer–the higher the value the more it slows down the heat. The scientific community uses a different term: thermal conductivity. Low thermal conductivity would correspond to high R-value.

Thermal mass is more like a bungee cord because it absorbs thermal energy (heat) like a bungee cord absorbs energy. After a bungee cord absorbs the energy it releases it back causing the bungee jumper to bounce back up. Similarly thermal mass will release the energy back in giving off heat that was previously absorbed. The scientific community refers to the amount of heat a material can hold per pound of material as thermal capacity. The construction industry normally doesn’t provide information on a material’s thermal mass and building codes don’t factor it in; however, it does affect how buildings change temperature.

An example from the kitchen:

Many people already understand thermal mass and insulation from their experience with food–they just don’t know it. Many people have used a cast iron skillet, but don’t know it has a lot of thermal mass. A cast iron skillet takes a long time to heat up compared to a thin steel frying pan, and a lot longer to cool down. In fact, they hold heat so well you could turn off the burner under a cast iron pan when finished cooking pancakes and still fry an egg on it. This idea of “holding heat” is thermal mass. However it will cool down after several minutes so you can’t use the heat from cooking breakfast to keep your soup warm until lunch.

cast iron and thermos

If you want to eat hot soup for lunch and can only heat it at breakfast time you want a thermos. A thermos has a vacuum between two layers of steel. A vacuum makes for wonderful insulation. With the cast iron pan you put a lot of energy in and it gives off heat when the energy stops going in, but only for a while. That’s how thermal mass works. But with the thermos you put in hot soup without adding any additional energy, and the heat doesn’t transfer out of the soup into the surrounding environment. This is because it is well insulated.

How do insulation and thermal mass effect building performance?

Having lots of insulation will cause your home to change temperature more slowly. And guess what having lots of thermal mass will do? It also makes your home change temperature more slowly. But there’s a big difference in how each of these properties do this. Insulation causes the temperature to change more slowly by reducing how much energy is transferred. Thermal mass causes the temperature to change more slowly because a really large amount of energy transfer needs to occur for the thermally massive material to change temperature.

thermal mass vs insulation graph
This graph shows what insulation and thermal mass do for a building. (Note: insulation does not resist temperature change in itself, but a well insulated building will be resistant to temperature change.)

This means that if you’re using a heater, furnace, boiler or air conditioner to control the temperature in your home you need insulation, because you don’t want to use a lot of energy. The more extreme the outdoor temperature, the more insulation you will want to slow down the transfer of heat (thermal energy). Thermal mass is great in many applications, but pairs especially well with passive solar heating. In this case you need a high thermal mass material where direct sunlight will hit allowing absorption of free solar energy. The mass absorbs a lot of energy when the sun is out and releases it after sunset, like the bungee cord bounce. In most climates you want both thermal mass and insulation in order to keep your home comfortable without using a lot of energy.

Also remember that insulation and thermal mass are not the only things that affect a building staying at a comfortable temperature. One of the biggest factors in a comfortable, energy-efficient house is air tightness–but that’s a whole other topic. What you need to know about insulation and thermal mass is they among the biggest factors in both comfort and efficiency.

What about natural buildings:

Many natural building techniques have a lot of built-in thermal mass based on materials used in construction (sand, clay, stone, etc.) and a few have plenty of built-in insulation (strawbale, cordwood, etc.). Just how much thermal mass and insulation a home has will affect how quickly it changes temperature. Make sure you design your home with climate appropriate amounts of insulation in order to keep it comfortable without using a lot of energy. There are natural materials that have insulative properties and others which have thermal mass properties. Use them both, use them to you advantage.

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  1. Nancy
    July 11, 2020

    If there is no 4″ slab of concrete to worry about shading the absorption of solar heat of large southern facing windows in the winter, can a very large collection of aptly placed cast iron pans help to warm a house at night a tiny bit? Maybe by one degree for four hours? Is there a simple rule of thumb or uncomplicated math formula for quarter inch thick cast iron thermal mass over time release?

    Can placing a sky high pile of cast iron pans onto a cast iron or or rocket mass heater help retain heat in a well insulated house a degree or two for an hour or two?

    It has got to be a better thermal mass store and slow release and than a collection of origami.

    If one is already a collector of any type of thermal mass in a passive solar house does every little bit help over time or does it not amount to anything.

    Would it help a tiny bit in the summer with absorbing the peek off of the heat?

    A 4″ by 10′ by 10′ in the sun slab of concrete vs maybe 100 12″ cast iron frying pans at optimal sun catching angle or in random in the sun piles.

    • September 20, 2020

      The specific heat of cast iron is about .11 Btu/lb*F. That means that one pound of cast iron will release .11 Btus of energy for each degree (F) that it cools. how long that will keep your house warm depends on how well the house is insulated and the temperature difference between inside and outside.

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