REW House & Home: Passive Solar

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Passive solar design of houses attempts to capture sunlight in winter and reject it in summer. The idea is to use solar energy to reduce heating and lighting costs, and to use natural ventilation for cooling.

 
“In Virginia, the climate imposes roughly equal loads for both heating and cooling,” says Charlottesville architect Andrew Thomas, “with a slightly greater need for heating. In both conditions, the structure must be well-insulated and sealed, with control of moisture.” 
 
Thomas built a group of six houses on Druid Avenue in 2007, a “pocket neighborhood” with attention to passive solar collection, interior air quality, and rainwater management. Sited on the brow of a hill, the houses also offer sweeping views to the south.
 
In the realm of technology, passive solar is as old as the art of building, and as up-to-date as computer modeling. It avoids mechanical and electrical devices. It relies instead on orientation, choice of materials, and a certain amount of adjusting by hand. What are some typical examples of passive solar design? Can you apply them to your home?
 
The first principle is to know the local climate, including the latitude, which determines sun angle. What is the average temperature range, and what are the extremes? How many days of sunshine does your site receive in a year? How many inches of rainfall? What direction is the prevailing wind? Standard design books contain reference charts and climate maps, and the United States government provides weather and climate data online. You can also learn a good deal by asking long-time residents, and by observing local building customs. Here, for example, the wind blows mainly from the west. Generous porches and roof overhangs hint at strong sun and storms.
 
Since we are in the northern hemisphere, we orient glass area and whole buildings toward the south to collect sunlight. In choosing a site, be sure it is not shaded by adjacent properties, and that it allows for living areas to be located on the south side. East and west exposures receive morning and evening sun respectively, but they are less significant. Deciduous trees are ideal for passive solar, since they shade the summer sun and allow the winter sun to pass through bare branches.
 
Once sunlight enters the structure, it must be absorbed to generate heat. Walls and floors are designed to have “thermal mass,” such as concrete or stone, which stores then radiates the heat. In desert climates with wide temperature swings from day to night, the thermal mass of adobe, for example, takes in heat during the day and releases it during the night. In Virginia, a well-insulated “envelope” or exterior is combined with carefully positioned glass. Shading devices can include roof overhangs, louvers above windows, and old-fashioned wood shutters and fabric awnings. The point is to shade the sun outside, not with blinds in the living space.
 
Insulation and sealing, which reduces air leaks, go a long way to reducing the need for heating and cooling. A “superinsulated” house uses so little energy that most of it can be supplied by the sun, appliances, a stove and the inhabitants. The German Passive House Institute was founded in 1990 in Darmstadt to promote this type of design.
 
Barbara Gehrung is a Charlottesville architect trained in solar and sustainable design. A native of Stuttgart, she grew up in a family that saved resources and helped found the German Green Party. As a child, she remembers the Chernobyl disaster of 1986, which spurred the drive for renewable energy. She was on German and American teams in the Solar Decathlon in 2005 and 2007. In 2010, she was certified as a passive house consultant. 
 
“The standards grew from superinsulated houses in Canada in the 1970s,” she says. “First, invest construction money in the building envelope, of which windows are the most expensive part. Smaller house size and a low ratio of surface to volume help energy performance too.”
 
Passive solar also has roots in Scandinavia. In a recent project at Lake Monticello, John and Patricia Platt added a sun room to a Trelleborg house. Trelleborg is a Danish company founded 40 years ago to produce superinsulated wood frame houses. Built in 1985, the Platt house was imported from Denmark. The original house, with a tile roof and dormers, looks like a cottage and uses very little energy. Designed by the author, the new sun room is a double-height space with large south windows, which overlook the lake. The builder was Dennis Kidd, of DAK Construction in Palmyra.
 
Retrofit in the form of an addition is the most practical way to add passive solar. Contrary to what you might think, skylights are not recommended. They lead to overheating, and the brightness is difficult to control. A high, north-facing window is better for daylight and whole-house ventilation. 
 
Robert Boucheron is an architect in Charlottesville. He is drawing a passive solar house for a lot in the city.
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