Thermal Mass vs Insulation

One of my New Year’s resolutions is to write more on specific tiny house design considerations. This first post is about two things that often get confused, thermal mass and insulation. Both can help regulate the temperature inside a home throughout the seasons but they work in very different ways. Together they can provide the best performance for keeping your home comfortable.

Thermal Mass – This refers to the ability of a material to store heat energy. Typically in a home this would be materials like masonry like concrete, brick, adobe, rammed earth, and/or earthbags.

A great example of how thermal mass works is an old uninsulated adobe house. During the day an old adobe will soak up the heat of the day and that heat will travel through the walls at a rate of 1 inch per hour. By the end of the day the walls are all charged up with heat which continues to travel into the interior of the house keeping the it warm throughout the night. As the night air cools the house from the exterior at 1 inch per hour the walls continue to keep the interior warm until the morning when the warmth of the previous day is finally exhausted. But now the walls are storing the cool of the night and pour it back into the house all day long. Really amazing simple physics people figured out a very long time ago.

Insulation – This refers to the ability of a material to slow down the transfer of heat energy. Typically in a home this is fiberglass, foam, loose fill, and/or reflective materials.

A good example is a home made of structural insulated panels which are basically big pieces of foam sandwiched between engineered sheathing like oriented strand board (OSB). Since these panels are mostly foam they can create an extremely tight super-insulated shell that keeps the air inside the house sealed from the temperature changes outside.

Best of Both

While an old adobe can naturally regulate it’s temperature you can imagine how cold it might get in the winter after a series of cold sunless days. You might also realize how a super insulated house would rely on artificial heating and cooling systems to control the air temperature.

Mixing the two is the basis of passive solar design. If your goal is to have a home that heats and cools itself naturally without much help from artificial heating and cooling systems then you mix materials that store heat energy and those that slow down heat transfer.

Imagine a structure with a blend of insulated and masonry walls. The masonry walls are heated by the sun or fuels during the winter and protected from absorbing heat in summer by using insulation and shading. Ideally the masonry has layers of external insulation to help lock in the heat it gains from the sun.

We’ve not seen a lot of passive solar design in tiny houses simply because mobility and heavy block walls conflict with each other and tiny insulated spaces are easy to heat with tiny wood stoves or propane heaters. But I think there are some very clever potential designs that would provide little living spaces that virtually maintain their temperature without much help from us. We just need to dream them up and get building.

I’ve posted a few solar house plans and have more in the works. Be sure to see my free tiny house plans and the tiny solar house plans which are available for $9.99.

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