Using Attached Sunspaces in Passive Solar Design

Increase home heating and cooling performance and boost savings quickly by adding attached sunspaces to existing structures.
By Dan Chiras
April 14, 2014
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Fig. 4-6: Attached sunspaces are an easy way to retrofit a home for passive solar, although they often don’t perform well. This all-glass attached sunspace must be covered during the late spring, summer, and fall to prevent overheating. All-glass attached sunspaces also tend to lose lots of heat at night and therefore must be isolated from living spaces by doors.
Photo by Dan Chiras
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Homeowners scramble to cut utility bills and find alternatives to fossil fuels as the prices of oil and natural gas continue to rise. The Homeowner’s Guide to Renewable Energy (New Society Publishers, 2011), by Dan Chiras, offers excellent ways to improve energy efficiency by making the switch from fossil fuels to clean, affordable, renewable energy. The following excerpt from “Free Heat” discusses the use of attached sunspaces in passive solar design.

You can purchase this book from the MOTHER EARTH NEWS store: The Homeowner’s Guide to Renewable Energy.

Types of Attached Sunspaces

The final option for passive solar design is the isolated gain system, more commonly referred to as the attached sunspace or solar greenhouse (Figure 4-4c). Attached sunspaces are passive solar heat collectors built onto the side of buildings. They are heated by the sun; the heat they generate is then transferred to adjoining rooms. Hence the term, isolated gain. (Heat is gained in an isolated space.)

Attached sunspaces are relatively easy to build onto many homes, provided there’s adequate solar exposure. All-glass attached sunspaces are available in kits, and there is no shortage of installers who can put one in for you. Unfortunately, this design is fraught with problems.

All-glass designs — that is, attached sunspaces with glass walls and glass roofs — tend to overheat in the summer and fall, causing severe discomfort in the home. They may even overheat in the winter. But isn’t this structure designed to collect heat and transfer it to the house in the winter?

Absolutely, but don’t expect to be able to use the space for much else. Overheating renders the sunspace much too hot to enjoy during daylight hours in the winter. Moreover, all but the hardiest of plants (cacti and succulents) find the intense heat oppressive. Most plants need to be kept below 85°F (29°C) for optimal growth; photosynthesis grinds to a halt at 100°F (37°C).

Despite the fancy brochures and advertisements you receive in the mail touting the value of all-glass attached sunspaces, you’ll very likely be disappointed if you go this route. I recommend that you think long and hard about the downsides of this retrofit. You’re basically buying a solar oven. To prevent overheating, you’ll need to cover the glass much of the year in most climate zones. The results can be quite hideous, as Figure 4-6 demonstrates.

A more effective attached sunspace design is shown in Figure 4-7. This design has a solid roof, which permits solar gain during the late fall, winter, and early spring, when the sun is in an intermediate and low position in the southern sky. Sunlight penetrating the south-facing windows warms the interior of the attached sunspace; the warm air created in the sunspace can then be transferred to adjacent rooms. During the summer, the solid roof all but eliminates unwanted solar gain.

While attached sunspaces of this nature are the best option in new and existing homes, you still need to design them very carefully to ensure optimal performance, especially in terms of maximum heat transfer into the house; otherwise, they’ll basically heat themselves and provide very little, if any, additional heat to your home.

Making the Most of Attached Sunspaces

Here’s what you need to do to make an attached sunspace work optimally: First, install openings in the wall between the sunspace and the adjoining rooms to permit warm air to flow into the house. To promote passive air movement, install openings near the floor and the ceiling of the wall between the sunspace and the adjoining room. This helps to create a convection current that circulates warm air into the house. (Note that a door opening into the space rarely suffices for heat transfer.)

To improve heat transfer from the sunspace to the house, I recommend installing a thermostatically controlled fan in the window or in an opening between the sunspace and the house. (Be sure to install a quiet one. Look for a low-sone fan with a rating of around 1.0.) DC fans can be powered by a 20- to 50-watt photovoltaic module. When struck by sunlight, the module generates DC electricity, powering the fan. When the sun sets or is covered with clouds, the fan shuts off. You won’t need any fancy switches or controllers of any sort with this sort of system.

Second, be sure that the openings can be closed off at night to prevent heat from escaping into the cooler sunspace.

Third, be sure the wall between the attached sunspace and your home is insulated to prevent heat inside the house from escaping at night into the cold interior of the sunspace. You may also want to consider building a mass wall between the two, as I did on a home office I retrofitted for passive solar (Figure 4-7).

Fourth, be sure the ceiling and foundation of the attached sunspace are well insulated. Ceiling insulation should be in the range of R-50 to R-60 in most areas. To make the floor, I’d recommend pouring a four- to six-inch concrete slab or excavating and backfilling with crushed rock. Insulate around the perimeter of the foundation and under the slab to retain heat. Use rigid foam insulation rated for underground applications. Two to four inches of blueboard or pink board (extruded polystyrene) will generally suffice.

Fifth, for best performance, use low-e double-paned glass. Be sure the glass has a high solar heat gain coefficient, which means that it permits lots of sunlight to enter the structure. Ratings of around 0.5 or higher are ideal for many locations. In southern regions, like Florida, solar heat gain coefficient should be much lower, around 0.35 or so. A knowledgeable glass supplier will know what you mean when you tell him this and should be able to help you select the best glass. If he or she doesn’t know what you are talking about, call someone else. You’d be amazed at how many window installers don’t understand their product.

Sixth, for the absolute best performance, insulate the glass at night. I recommend that you use rigid foam insulation panels placed against the glass between the framing members to reduce heat loss. Or you may want to install insulated shades. (Of the two, rigid foam panels are better; they have a higher R-value.) Rigid foam insulation panels will require additional work on your part each day; you’ll have to take them out in the morning and put them back at night. Raising and lowering shades will also require some extra effort, but it’s less work than inserting foam insulation panels. Either way, the additional labor on your part is a small price to pay to keep the interior of an attached sunspace warm at night. The sunspace will also warm up much more quickly the next day, resulting in more heat gain in your home — where you want the heat!

Seventh, be sure to install some operable windows in the attached sunspace to bleed off hot air during the summer and fall. To ventilate naturally, that is, without fans, you’ll want to create a convection current by installing a couple of opening windows low in the structure and a couple opening windows up high — or include a roof vent. Because warm air rises, it will escape through the upper windows or roof vent, drawing cooler air in through the lower windows. You can purchase openers that operate automatically — opening a vent when the temperature reaches a certain level.

Pros and Cons of Attached Sunspaces

As noted above, attached sunspaces are the “easy option” for solar retrofits. They’re pretty easy to integrate into an existing home and relatively easy to build. They’re not very expensive, either, and they go up quickly, providing instant savings and comfort.

Attached sunspaces also provide a space for a solar cooker like the one shown in Figure 4-8. A solar cooker is a remarkably simple device consisting of a box with reflectors that concentrate the sunlight striking the device, a dark interior to convert sunlight to heat, a glass lid to hold in the heat, and a shelf to hold cooking pots or cookie sheets. You can purchase one online through Gaiam Real Goods or make your own. Numerous plans to build your own are available online.

Attached sunspaces of the type I’m recommending generally perform much better than the all-glass variety. They stay cooler in the summer, provide more usable space, and provide adequate levels of heat during the winter. Still, they do have some downsides.

For one, they make lousy growing spaces. If you are hoping to grow vegetables year round in an attached sunspace with a roof, forget it. You’ll be disappointed by the results. That’s because many vegetables do best when the sunlight comes from above. A lack of summer sunlight often causes plants like tomatoes to become tall and thin — we say they become “spindly.” Spinach and lettuce like overhead light, too. Although such plants may thrive in the sunspace during the winter, especially if you can keep the temperature up at night via the insulation strategies I’ve mentioned, they’ll languish in the shade inside the sunspace in the summer, because the sun is high in the sky, beating down on the roof and overhangs. As a result, very little sunlight will penetrate the structure.

To offset this problem, you can install a few standard skylights in the roof. Unfortunately, this could lead to summertime overheating. In addition, skylights lose huge amounts of heat on cold winter nights, causing the space to chill down, thwarting plant growth. Another option for providing overhead light in the summer is the solar tube skylight, shown in Figure 4-9. Solar tube skylights consist of a small glass or plastic dome-shaped lens mounted on the roof that collects sun and directs it into a polished aluminum tube that extends from the roof to the ceiling. (The polished aluminum ensures maximum light transmission.) Light enters the room through a ceiling fixture, a diffuser that disperses light.

Solar tube skylights let in lots of light from a rather small opening — much smaller than standard skylights; as a result, they minimize unwanted heat gain caused by conventional skylights. This in turn considerably reduces the threat of summertime overheating. Because they utilize a small opening, tubular skylights also minimize wintertime heat loss at night, a huge problem with conventional skylights. (See Slide Show for before and after.)

Another problem that you may find with an attached sunspace is that it won’t be habitable during the winter, except perhaps during the morning before the sun has warmed it. Once the sun begins beating in, however, you’ll fry.

Like other forms of solar retrofitting, you will have to obtain a permit from the building department for an attached sunspace. They’ll inspect the project at various stages and upon completion of the project.

For more ways to use solar energy, read How to Size Solar Hot Water Systems.


This excerpt has been reprinted with permission from The Homeowner’s Guide to Renewable Energy: Achieving Energy Independence Through Solar, Wind, Biomass and Hydropower by Dan Chiras and published by New Society Publishers, 2011. Purchase this book from our store: The Homeowner’s Guide to Renewable Energy.


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