ASK THE EXPERTS: Sunspace Thermal Mass

A sunspace
The amount of thermal mass in a sunspace affects temperature extremes within the space and the heat available to the adjacent living space.
A sunspace

Would you please compare the advantages/disadvantages of a low versus high thermal mass sunspace? It seems to me that a sunspace with high thermal mass would have the advantage. It would heat up during absorption, and then more evenly and over a longer time release this heat back into the house.

Ken Kardong • via email

Deciding on whether it’s more advantageous to incorporate a lot of thermal mass into your sunspace depends on what you want to use the sunspace for. If the sunspace is intended for growing plants year-round or if you want the sunspace to be comfortably warm into the evening, then having thermal mass in the sunspace is beneficial. The mass absorbs solar energy during the day, and releases it after sunset, keeping temperatures high enough for plants or people. The mass also reduces the need for backup heating in the sunspace to keep plants from freezing. In addition, a sunspace with mass requires less ventilation, used to prevent overheating during summer days.

If the sunspace is intended primarily for space heating for the house, a low-mass sunspace would be better. The solar energy needs to heat air—instead of mass—which is blown into the house to provide space heating. If you put in a lot of thermal mass, much of the solar radiation will heat the mass and significantly less will heat air for the house. The heat that goes into the sunspace mass is lost very quickly out the sunspace glazing after sunset, and is wasted. If the sunspace has dark-colored, well-insulated, but low-mass surfaces, the sun-heated surfaces quickly get warm, which circulating air then transfers into the house.

The disadvantage of a low-mass sunspace is that it loses heat quickly after sunset—it’s not a place you’ll want to spend your off-summer evenings, and won’t provide much freeze protection for plants. In exchange for much more efficient space heating, you’ll give up evening use and winter plant growing.

There are pros and cons to both approaches, and it depends on what you want to accomplish. You can also take a middle path and get some of the benefits of each approach.

Gary Reysa •

Comments (1)

zap101's picture

Thank you for this comparison. But instead of a perspective about growing plants, How about human comfort while being in a high mass or low mass sunroom an s definition of low mass verses high mass. Thanks Tim

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