Low Thermal Mass Sunpaces

The Little-Known Gem of Solar Heating
Intermediate

Inside this Article

Low Thermal Mass Sunpaces
Low Thermal Mass Sunpaces
The author’s “low-mass” sunspace
The author’s “low-mass” sunspace is actually a high-mass greenhouse that was tested for thermal performance—before the mass and plants were moved in.
The greenhouse used for sunspace testing
The greenhouse used for sunspace testing. The long ducting here was used to measure airflow. Make sure to size the fan to deliver 3 cfm per square foot of glazing with the duct system that you have chosen.
10 mm double-wall polycarbonate
The glazing used in the test LTMS is 10 mm double-wall polycarbonate with an R-value of 1.8. At about $2 per square foot, it is less expensive and easier to work with than two panes of glass, although its lifespan is shorter.
Ranco ETC thermostat
The Ranco ETC thermostat turns the fans on and off based on the sunspace’s temperature. This is a temporary installation for testing; permanent installation would be made more cleanly.
10-inch Dayton fan and R-21 fiberglass insulation
The west end of the sunspace shows the 10-inch Dayton fan and R-21 fiberglass insulation (to be covered with Rboard). The Rboard on the north wall is painted black for improved solar energy absorbance.
The east-end ducting and blower fan
The east-end ducting and blower fan—a Dayton 10-inch, which provides 665 cfm of free air delivery. The shade cloth was an experiment at preventing overheating in the space.
Low Thermal Mass Sunpaces
The author’s “low-mass” sunspace
The greenhouse used for sunspace testing
10 mm double-wall polycarbonate
Ranco ETC thermostat
10-inch Dayton fan and R-21 fiberglass insulation
The east-end ducting and blower fan

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.

Characteristics

The features that characterize a LTMS are:

  • A large, south-facing glazed area that is steeply tilted or vertical for excellent winter sun collection. Double-glazing should be used in cold climates to decrease heat loss.
  • Nonglazed surfaces (walls, roof, and floor) that are insulated and sealed to reduce heat loss.
  • Sunlit surfaces that are dark in color for good heat absorption, and are low in mass so the solar radiation rapidly heats these surfaces, and the surfaces transfer that heat to the air. (This is in contrast to high-mass sunspaces, in which the solar energy goes primarily into heating the sunspace mass.)
  • A high-capacity fan and duct system that transfer heated air in the peak of the sunspace into the house, and a return air system that returns cool house air near the floor of the sunspace.
  • A high-capacity vent system allows excess heat to be vented outside when space heating is not needed.
  • These sunspaces are normally attached to or integrated with the home’s south face.

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.

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Comments (1)

Dan Rhodes's picture

Very good article on Solar rooms. I have a 1250 sf off-grid home near Flagstaff, AZ. My home also has a 270 sf solar room with insulated slab that helps provide about 70% of my heating needs. Highly recommend this type of heating.

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