Tapping into the Sun: Passive Solar Heating for Your Home

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Planters and tile floors are effective examples of thermal mass necessary for passive solar performance.
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Houses with windows maximized on the south side provide optimal results.

Our lives are made easier–or so we’re told–by an assortment of gadgets and gizmos, from computers to blenders to microwave ovens. Useful as these products may be, many of them live short and rather unproductive lives. There is one technology, however, that breaks the mold. It has only one moving part–solar radiation–and a lengthy warranty. It is called passive solar heating.

Passive solar heating can provide safe, comfortable heat and trouble-free service for as long as the sun continues to shine. This technology has been practiced for centuries–first by the ancient Greeks and later by the Anasazi Indians of the desert Southwest. Passive solar can be easily incorporated into new construction and can also be added to existing homes. It’s amenable to a wide variety of architectural styles and climates.

If designed and built right, passive solar homes stay warm in the winter and cool in the summer with little, if any, need for environmentally damaging fossil fuels and noisy furnaces or air conditioners. And passive solar design and construction reap huge economic benefits at little, if any, additional cost. According to Ron Judkoff, director of the Buildings and Thermal Systems Center at the National Renewable Energy Laboratory in Golden, Colorado, passive solar homes may cost up to 3 percent more than conventional homes, but they can save homeowners tens of thousands of dollars over time.

And passive solar homes can be exquisitely beautiful. Generous south-facing glass and open interior designs create an inviting interior bathed in invigorating sunlight. Successful passive solar home building does, however, require knowledge, skill, and strict adherence to a set of guidelines.

Seek solar exposure

Passive solar homes require plenty of sunlight during the colder months. Ideal sites provide sunlight on the south side of a home from 9 a.m. to 3 p.m. If a site is shaded by trees, other buildings, or hills, look elsewhere. Solar gain will be dramatically reduced. Don’t be deterred, however, if you are building in a less favorable solar climate. Even in such places as Buffalo, New York, where sunlight in the dead of winter is a rarity, solar energy can provide up to half of a home’s annual heat requirement. Why? Although the sun shines infrequently during the winter, there is plenty of sunshine for comfort during the fall and spring. In the winter, solar design can tap into solar heat while a furnace or wood heater provides the bulk of the warmth.

Orient to the south

For optimal year-round performance, orient the long axis of a home to within 15 degrees east or west of true south, a line that runs from the North Pole to the South Pole. Remember: True south often varies from magnetic south, which is determined by iron-bearing minerals in the earth’s crust. Orienting to true south ensures the greatest solar exposure on the south side of the house and the greatest solar gain during the winter.

Deviating from this orientation to align a home with a street or a picturesque view is possible and often desirable but solar gain will be decreased in winter, when heat is needed, and increased during the summer, when heat is unwelcome.

Place windows on the south side

South-facing windows are the solar collectors of a passive solar home. Be careful not to install too much glass, though. Generally, the surface area of south-facing windows should constitute no more than 7 to 12 percent of the total heated floor space. The colder the climate, the greater the window allotment. While skylights on south-facing roofs can be used to gather sunlight, they should be used sparingly. They frequently emit too much sunlight during the summer, and they may also result in excess heat loss in the winter.

In a passive solar home, north-, east-, and west-facing glass must also be carefully specified. North- and east-facing glass should make up no more than 4 percent of the total floor space, and west-facing glass should be held to 2 percent.

Design an open interior

For optimal performance, houses should be slightly longer than they are wide. This allows for maximum solar gain in each room, with each room heated independently, and minimizes the need to move warm air to colder rooms. If you prefer a less rectangular design, locate rooms that require less heat, such as bedrooms and utility rooms, on the north side. An open floor plan also results in a more uniformly heated home.

Balance thermal mass and glass

The key to success in a passive solar home lies in balancing glass with thermal mass. Thermal mass is solid objects and materials within a home that absorb solar heat, preventing daytime overheating and stabilizing internal temperatures.

As a general rule, if solar glazing is less than 7 percent of total heated floor space, then incidental mass within a structure–such as drywall and framing lumber–will generally suffice. When glazing falls within the 7 to 12 percent range, additional mass is required.

Thermal mass such as tile, bricks, and concrete can be used in the floors and walls, although planters, mass partition walls, and stairwells also work well. Generally, thermal mass works best if it is evenly distributed throughout a house. The more mass you can place in direct contact with sunlight, however, the better.

Insulate to the max

Proper insulation is vital to successful passive solar design. Wall insulation ranging from R-20 to R-30 and ceiling insulation of R-40 to R-60 is generally required, depending on the climate. Foundation and window insulation are equally important, especially in colder climates. But remember, insulation is only as good as its installation. Gaps left around the edges of batt insulation, for instance, can decrease its efficiency dramatically. In many locations, sheltering a home with earth buffers a home from extreme outside temperatures during the summer and winter.

Reduce air infiltration

Energy efficiency also requires airtight home construction. Most air leakage into and out of houses occurs through cracks around doors and windows and between the foundation and exterior walls, so be sure to seal these trouble spots.

While reducing infiltration and exfiltration can significantly reduce fuel bills and help ensure year-round comfort, sealing a home too tightly can cause health problems if the home contains toxic products. And you may also want to install a mechanical ventilation system.

Create sun-free zones

Although the object of passive solar design is to bring sunlight into a home, it is equally important to create sun-free spaces in your home, so that you can work, read, or relax without being drenched in blinding sunlight. Entryways, hallways, planters, and other design details can be used to intercept the sun.

Provide shade for summertime

Although the sun cuts a high arc across the sky in the summer, and thus barely penetrates south-facing windows, solar designers typically include overhangs over these windows and the adjacent walls to reduce unwanted heat gain. The longer the heating season, the shorter the overhang needs to be.

Interior and exterior window shades fine-tune a passive solar system, controlling unwanted solar gain from early eastern and late-day western sunlight entering the home from the east and west. External shade devices such as adjustable shutters or awnings work better than internal shades but are often less convenient.

Carefully selected trees and bushes provide useful shade in the summer. Conifers are suitable for the north, east, and west sides of a home, but they will block solar gain on the south side during the winter. Deciduous trees are a better option for the south side, but branches and limbs can block a significant amount of desired sunlight during the winter.

Retrofitting for solar gain

It is possible to retrofit an existing home with passive solar technology. The structure must have one outside south-facing wall with unfettered access to winter sun.

Step 1: Insulate to the max, at least to the levels prescribed by the International Energy Code. Most serious solar designers insulate to even higher levels. Insulate the walls, attics, ceilings, and the foundation, and install insulated window shades over all windows.

Step 2: Seal the building envelope with caulk, foam sealant, and weather-stripping. Hire a professional energy expert to run a blower door test, which allows the installer to pinpoint all cracks and crevices in a building envelope.

Step 3: Retrofit, either by adding a solar greenhouse, or by installing more windows on the sunny south side of your home for more direct solar gain.

Many people prefer all-glass solar greenhouses because they’re attractive, inexpensive, available in kits, and are ideal for growing plants year-round. Unfortunately, during the winter they tend to be too sunny during the day and much too cold at night. All-glass designs also tend to overheat in the summer. Far better is an attached sun space with south-facing glass and a roof. This gains less heat and is more comfortable year-round.

It’s helpful to install a thermostatically controlled, low-wattage fan to transport heat into the house. You may also want to close the greenhouse down at night, thermally isolating it from the rest of the home to prevent heat flow from the house into the sun space.

Installing additional windows on a southfacing wall sounds simple, but adding a few windows on the south facing wall can be tricky and may require a substantial amount of reframing. Nonetheless, this measure can greatly increase solar gain. Purchase high-quality low-emissivity (low-E) windows and install insulated shades or shutters to decrease nighttime heat loss. You may also need to add thermal mass inside your home, perhaps by tiling a floor or adding brick facing to nearby walls.

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