Passive solar design is a great strategy for those who want to build a new home that heats and cools itself naturally— without costly mechanical equipment and fossil fuels. But some passive solar techniques can also be incorporated into existing buildings.
This article will help you benefit from this clean, low-tech, cost-effective, and environmentally friendly form of space heating. We will explore ways to open up the south-facing facades to permit the low-angled winter sun to enter, as well as ideas on building passive solar additions and attached sunspaces.
Retrofitting an existing home or business for passive solar requires that the building have room for windows in a south-facing wall with unfettered access to the sun, all winter long. No trees or buildings should shade the wall during daylight hours, and solar access should be available from at least 9 a.m. until 4 p.m.
Newly built passive solar homes are typically rectangular buildings with the long axis running from east to west, for maximum south-facing windows—to get the best solar gain. In a passive solar retrofit, the south-facing wall doesn’t need to run the length of the home or business, although that’s ideal. Any south-facing wall will work, although the more windows, the better. In passive solar homes, the amount of south-facing glass varies by climate and solar heating goals. Typically, passive solar designers aim for a south-facing glass area between 7 and 18% of the square footage of a building with traditional 8- to 9-foot ceilings. The colder the climate and the more solar heat needed, the higher the percentage.
Install Energy-Efficient Windows
After you have weatherized your building (see “First Things First”), it’s time to install windows on the sunny south side of the building to increase direct solar gain. In most retrofits, there will only be room for a couple of windows, so your space is unlikely to experience overheating.
Simple as it sounds, adding openings in a home can be tricky. It will, for instance, require some framing to create a rough opening into which the window will be installed. As a result, it is a job generally best left to professional builders—or, better yet, to professional window installers.
To add a window, an installer will cut a large hole in the wall through the interior drywall and exterior sheathing and siding. The installer then removes the wood framing in the wall and installs headers—horizontal load-bearing framing over the top of the opening. The header supports the weight of the wall above it and transfers that weight to the framing members installed along the sides of the windows. This arrangement transfers the load from the roof through the headers and framing members, instead of to the window, which would break under the weight.
After an opening has been framed, it is time to install the new window. Be sure to follow directions very carefully. Windows must be sized about a half an inch smaller than the rough opening.
Window frames attach to the rough opening. The window must be leveled, then attached on the sheathing on the outside by nailing its bottom metal or plastic flange. Then the window is squared and nailed the rest of the way around. Any gaps between the window and the wooden framing members are filled from the inside with foam insulation such as backer rod or foam tape, or expanding spray foam insulation designed specifically for window and door installation. Using spray foam that expands too much—like a product designed to seal large gaps and cracks—can damage the window and void the window manufacturer’s warranty, so read instructions carefully.
After the window is secured, be sure to apply adhesive sill flashing, a tape-like product that seals the window opening from the outside to prevent air and moisture from penetrating. Once the window is in place, install the trim and caulk around it to provide an additional airtight seal.
For best results in a passive solar retrofit, new south-facing windows should be shaded by eaves to prevent summertime overheating. You may need to construct eaves or install retractable awnings over these windows if there is no overhang on the south side of the building.
If you dramatically increase the amount of south-facing glass, you may need to add thermal mass inside. Thermal mass is any material that absorbs solar energy during the day and releases it at night or on cold days, preventing overheating and helping to maintain a more constant internal temperature. Thermal mass materials include concrete, brick, stone, and tile.
As a general rule, no additional thermal mass is required if the south-facing glass is less than 7% of the sunlit floor area. Incidental mass—that is, mass in the building such as drywall and framing materials—is sufficient to accommodate the solar gain. If south-facing glass exceeds the 7% mark, additional thermal mass is required. Generally, for each square foot of south-facing glass over the 7% limit, you need to add 5.5 square feet of 4-inch-thick floor mass—that is, mass that sunlight will strike. For walls used as thermal mass, you need 8.3 square feet of 4-inch-thick thermal mass in walls. Floor mass not struck directly by sunlight is virtually useless.
Adding 4 inches of thermal mass to walls and floors is difficult. In most buildings, smaller amounts are added by installing tile on the floor, or adding a second layer of drywall or a brick facing to sun-bathed walls. All three strategies add thermal mass and help prevent overheating when solar glazing exceeds the 7% mark.
Mass should be dark to increase absorption of sunlight, but not too dark. Clay-colored tile, for instance, is a good compromise for aesthetics and performance. If the mass is too dark, it can create hot spots in your home.
If you are planning on adding a new room, consider building it on the south side of the building and designing it for passive solar gain. A solar addition may be heated almost entirely by sunlight and could provide some heat to adjoining spaces, if done correctly. Solar additions are typically designed as direct gain structures—rooms with south-facing windows that allow the sun’s rays to enter, directly heating the space.
Another passive solar option is a Trombe or thermal storage wall. This design incorporates a massive wall behind windows on a south-facing exterior wall. Sunlight streams through the windows, heating up the wall. Heat then migrates by conduction through the wall to the interior, radiating to the space at night. Windows installed in the mass wall allow daylight and some direct solar gain. Vents in the upper and lower reaches of the wall allow room air to circulate between the mass wall and the glass where it is warmed. Cool room air enters at the bottom vents and is warmed in the airspace between the glass and mass wall, then exits through the top vents, providing additional daytime heat.
Thermal storage walls work well in all climates, though they need to be properly designed and built. The glass is typically a low-e double-pane glass with a high SHGC. (Non-low-e or uncoated window glass can also be used in such applications. Uncoated glass has a higher SHGC coefficient than ordinary low-e glass.) That glass is typically placed 3 to 6 inches from the mass wall.
Thermal storage walls can be made from a number of types of thermal mass, poured concrete, cement blocks filled with sand or concrete, brick, adobe, or rammed earth. Walls are typically 8 to 18 inches thick, depending on the material.
Vents need to be sized to ensure adequate airflow. As a rule, you’ll need 2 square feet of vent, divided equally between the top and bottom of the wall, for every 100 square feet of thermal storage wall. Closable vents are vital to prevent warm air from flowing through the airspace at night, losing the heat to the outside. Automatic louvers are more convenient than manually closed vents.
Designing and sizing a Trombe wall requires knowledge and experience. Be sure to consult books on passive solar design or consult with an experienced architect to determine wall thickness, type of material, size of vents and vent closures, spacing between the glass and the mass wall, and other parameters. Be sure not to cover the interior surface of the Trombe wall. Many homeowners make the mistake of furring out the wall, then applying drywall, which effectively insulates the interior surface of the wall, dramatically reducing heat radiation. A coat of paint or plaster makes an effective and attractive finish.
Many people like the idea of all-glass sunspaces attached to their home. They are attractive, fairly inexpensive, and available in kits. They’re often touted as three- or four-season rooms. Unfortunately, since they lack insulation, all-glass attached sunspaces tend to undergo wide temperature swings, overheating in sunny weather and then cooling off to brisk temperatures during cloudy weather or at night. Direct sunlight’s UV radiation is tough on furniture and carpets, and the brightness can also be hard on the eyes, since there’s nothing to temper the glare.
Despite the drawbacks, sunspaces can be used as a giant solar collector to heat adjacent rooms in your home. Installing a quiet, energy-efficient, thermostatically controlled fan in the wall of the sunspace will move the solar-heated air into adjoining rooms. Warmed air can also be ducted to back rooms. (For maximum efficiency, seal and insulate the ducts, and keep duct runs as short as possible.)
At night, close off the sunspace from the main living area to thermally isolate it from the rest of the home. This prevents heat from the house from moving into the sunspace and then to the outside—where it does you no good. You should also consider installing insulated shades to reduce heat loss at night, especially in those attached sunspaces that will serve as additional living space.
Adding solar windows, a solar addition, or an attached sunspace can increase your use of solar energy, a clean, abundant, economical, and renewable energy resource. All three will perform well, if properly designed and constructed. You need to choose which of the options fit your situation.
If south-facing walls in your home offer some room for additional windows, this might be the best option. Direct gain is a great way to heat. If you are planning on adding on to your home, a solar addition on the south side of your home will work well. A thermal storage wall may be ideal for rooms like offices in which you want the solar heat, but don’t want all the extra sunlight. If there’s room for an attached sunspace, and you can deal with their limitations, that may be a good way to go. Or, you can design the attached sunspace to serve as a large solar collector, optimized to heat neighboring rooms. All options can cut your heating costs and very likely add to the value of your home.
Dan Chiras writes about green building and renewable energy, and directs The Evergreen Institute, where he teaches workshops and provides instructor training on passive solar heating and cooling, solar electricity, wind energy, energy efficiency, and green building.
”Sun-Wise Design: Avoiding Passive Solar Design Blunders” by Dan Chiras, HP105
Green Home Improvement by Dan Chiras (RS Means, 2008)
The Solar House by Dan Chiras (Chelsea Green, 2002)