Sunspaces have long been appreciated as a way to provide some extra living space with sunny appeal. They can also be used as supplementary heaters, collecting free solar energy that can be channeled to warm interior rooms on sunny winter days.
This article describes a low thermal mass sunspace (LTMS), which can be thought of as just a big “walk-in” hot air collector. This design is optimized to provide significant heating for the house.
The features that characterize a LTMS are:
A sunspace designed to these parameters will be quite efficient as a space heater for the attached house. It will also be comfortable during the day for other activities, since the excess heat that would make the sunspace uncomfortably warm is being distributed to the main house. A sunspace can start providing heated air to the house as soon as the morning sun shines on it, since the low thermal mass allows it to heat quickly and little heat is stored in the space itself.
The big advantage for the LTMS is that it provides space heating equivalent to what active solar thermal collectors provide while allowing the space to be used for many activities, including lounging, four-season clothes drying, wood drying, or a well-lit shop. A downside of the LTMS is that once the sun sets, the space will cool to outside temperatures quickly—it is not a good space to hang out on cold evenings. In cold climates, it is also not suitable for growing plants through the winter.
In contrast to a high-mass sunspace, the LTMS does not provide any storage for the heat it produces. This is usually not a problem since the house’s heating demands plus the ability of the house’s thermal mass to store heat are enough to absorb the heat that the sunspace can produce. Solar gain glazing (including sunspaces) can be up to about 10% of the floor area of the house without requiring storage.
If the house has low heat loss and/or the sunspace has a large amount of glazing compared to the house size, then sunspace heat can be stored. For example, one strategy in the past, which has fallen somewhat out of favor, was to duct the heat to rock-bin heat storage; an air-to-water heat exchanger also can extract and store some of the heat in a water tank. It is best not to store heat in thermal mass inside the sunspace since most of that stored heat will be lost through the sunspace glazing after sunset.