Using Thermal Mass in Passive Solar

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Thermal mass is everything inside a home that absorbs, retains and later emits heat, effectively providing free heating. Like most things in home construction, there are many different approaches to its use.

Direct and indirect exposure

Since thermal mass gathers heat from wherever it can, it is possible to locate it out of the path of direct sunlight. The material will still gather heat from other sources – warm air and so on – but tests show it can take up to four times as much area to gather the same amount of heat as in direct exposure.

Unfortunately, it's often a lot easier to say "put it in direct sunlight" than it is to achieve, because walls, ceilings and other reliable thermal mass do not always naturally sit in the sun's path!

Floors are, therefore, the most obvious first choice for thermal mass installation. A simple concrete slab, stained a dark color, will gather plenty of heat provided there is enough window space to allow the sun's light in. If that's too ugly, you can opt for dark tiles on top of a concrete slab, brick or flagstone laid on top of sand, or even plain adobe if you're feeling organic.

Whatever material you choose, make sure you insulate around the underlying heat storage: there's little point gathering tons of heat if you let it bleed out through the foundations instead of being radiated back into the home!

Internal walls also get a fair bit of incidental sunlight, so smart choices can help there: Trombe walls are particularly good at this, though any interior wall can gather heat. Masonry walls painted a dark color (or plastered in a dark coating) can absorb most of the light that hits them. Alternatively, you can paint them light colors to reflect the light to increase luminosity in your living spaces and have the reflected light energy absorbed by other thermal mass – it's less efficient but still works.

Don't forget that all the other stuff in your house is also thermal mass: planters, ornaments and even furniture placed in direct sunlight will absorb heat that can later be radiated to provide comfort.

Distributed or concentrated?

Another consideration is how to organize your thermal mass: do you want it all in one small space or spread out across the entire home? That depends on your requirements.

Concentrated thermal mass is generally used to heat a small space, whereas distributed mass provides a more even temperature across the whole home. To be honest, it is unlikely that you will choose one or the other as a mix of both works best. It is simply important to remember that you can focus extra heat by concentrating thermal mass.

Dark and light schemes

The difficult part of thermal mass installation is the sober colors. Dark colors absorb significantly more heat energy than their light counterparts, so many people think they live in a home that looks like a dungeon to keep warm!

In reality, a mix of dark and light colors works very well. Light colors often take