Low Thermal Mass Sunspaces: Page 4 of 4

The Little-Known Gem of Solar Heating

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

Preventing & Dealing with Overheating

For the summer season, some method will be required for positively venting the sunspace on sunny days. Having at least two inlet vents (located low) and two outlet vents (located high) is recommended. To ensure comfortable temperatures during the midday, exhaust fans will probably be needed in the outlet vents. One good choice would be thermostatically controlled greenhouse ventilation fans. Another option would be solar-powered fans, perhaps the type of solar fans sold for attic ventilation. The solar fans would provide more airflow as the solar intensity increases, and would probably not require a separate control.

I have converted the test LTMS sunspace into a solar greenhouse, and for the summer ventilation I’m using a solar-powered attic vent fan in the east peak and one of the 10-inch fans in the west peak. The solar-powered fan varies its speed with sun intensity, and is quiet—even in full sun. Alone, however, it does not provide enough ventilation. The Dayton fan is set to come on when the peak temperature in the sunspaces is 90°F. Two solar-powered fans would probably serve the space’s ventilation needs.

Summer vent inlets could be located low on the east and west walls, or below the south glazing, or opening windows in the south glazing. My vent doors are in all three locations, with the south ones being below the glazing. Some LTMS designs have entry doors from the outside that also serve as vents.

Other design features for summer temperature control:

  • At mid-latitudes, vertical glazing receives only about half as much solar radiation in midsummer as in midwinter, so it’s an automatic way to reduce summer overheating.
  • An overhang above the south glazing that is sized to shade all or part of the glazing in midsummer but admit full sun during the heating season will help summer overheating (see the overhang design tool in Access).
  • Shade cloth can be used over all or part of the glazing during the summer. Choose a shade cloth density that admits the amount of sun you want.

Heating on a Budget

LTMS additions can be built on very low budgets. One of the most inexpensive sunspace strategies is to erect a half hoop house (often used for greenhouses) against the south wall of the house. Glazing can be two layers of greenhouse poly with a small blower (30 cfm for a modest-sized sunspace) to inflate the space between the poly layers. This could be accomplished with a small DC blower wired to a single PV module. The floor should be covered with an insulating material—perhaps mulch or garden bark. Vents will need to be cut into the side of the house, or windows or doors can be used, as long as there’s an inlet duct going down and an outlet duct to the peak of the sunspace. This simple sunspace can have a payback of less than one heating season.

More permanent sunspaces will cost more, but will have better wintertime performance depending on air-sealing, insulation, and glazing. General payback can be difficult to figure, as sunspaces need to be customized to their application. For example, Mike Pelletier’s 221-square-foot sunspace in Colorado (see “Inspiring Sunspaces” sidebar) cost $4,500. According to PVWatts version 2, about 4.7 kWh per square meter per day fall on a vertical collector in Gunnison, Colorado, on an average winter day. Pelletier’s sunspace, then, could theoretically produce 118 kWh per day [(270 ft.2 ÷ 10.76 ft.2/m2)(4.7 kWh/m2)]. Even if only 40% of that energy is harvested, that would be equivalent to about 47 kWh per day. If the heating season is 4.5 months, that translates to more than 6,300 kWh per year—a savings of $630 at $0.10 per kWh, for a 7.1-year payback ($4,500 ÷ $630), plus all the other benefits of having the sunspace. Your savings will vary depending on the sunspace’s efficiency, local fuel costs, heating requirements, and solar insolation.


Gary Reysa is a retired airplane engineer living in southwestern Montana who spends way too much time on solar projects.

Build It Solar LTMS section • bit.ly/BldItSolarLTMS

Inflation blower kit for greenhouses • bit.ly/InflatFanKit

Overhang sizing tool • susdesign.com/tools.php

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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