Mother Earth Living

The Basics of Passive Solar Home Design

Passive solar is the most cost-effective solar technology available. Take advantage of the sun’s heat and make your home a more comfortable place to live.
By Dan Chiras
January/February 2011
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A cat lying in the sun is an example of instinctual use of passive solar heating.
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Passive solar design allows homeowners to heat their homes naturally, dramatically reducing dependence on costly heating systems and environmentally damaging fossil fuels. The most cost-effective solar technology on the market today, passive solar design could save you tens (if not hundreds) of thousands of dollars over your home’s lifetime—without a large investment.

Homeowners have three passive solar design options:

Direct-gain design, the simplest and least expensive option, incorporates lots of windows on a home’s south side, allowing the low-angled winter sun to enter, turning the home into a huge solar collector on the coldest days.

Indirect-gain design captures the sun’s heat through south-facing glass and includes a thick wall of brick, concrete or cement blocks (known as thermal mass) immediately behind the glass. During the day, sunlight streams through the glass, heating the thermal storage wall (also referred to as a Trombe wall after the French engineer who developed the technology), then radiates into adjacent rooms, gently heating them long after the sun has set. Installing windows or vents in the Trombe wall accommodates direct daytime heating. Solar radiation warms the air, which expands and rises through the top vents to heat adjoining rooms. Cool room air enters through lower vents and is heated, creating a thermal convection loop.

An attached Sunspace­—or solar greenhouse—moves solar-heated air into nearby rooms with help from a quiet thermostatically controlled fan. Poorly designed sunspaces can overheat in summer and become extremely cold at night in winter. Consult a knowledgeable solar designer before you add one to your home.

Making passive solar work 

The best solar homes are rectangular in design, allowing the largest possible south-facing surface. A solar designer or an architect with experience in passive solar design can help calculate how much south-facing glass to incorporate, based on your climate. Most homes in colder climates require south-facing glass to make up 15 to 18 percent of total square footage. Warmer climates require less heating, and therefore less south-facing glass.

Rooms that require the most heat, such as living rooms and home offices, are typically located on the home’s south side. Rooms that require less heat, such as bedrooms and utility rooms, or those that generate their own heat, such as kitchens, are typically located on the north side.

To prevent overheating and ensure more even temperatures day and night, install adequate thermal mass throughout the house, which includes masonry-type materials that absorb solar heat during the day and release it into your home’s interior at night. Adequate eaves or overhangs will prevent the high-angled summer sun from entering during summer months.

Design Options 

Direct-gain 

Pros: Least expensive; easy to incorporate in new building designs; attractive, open designs; creates bright and cheery interior; allows plenty of natural day lighting
Cons: Home can overheat if thermal mass or overhangs (eaves) are inadequate; challenging to incorporate in existing buildings; requires use of window shades to reduce heat loss at night; can result in glare on TV and computer screens and extremely hot, uncomfortable areas in a home

Indirect-gain (thermal storage wall) 

Pros: Great for day and night heating; produces even heat; works well in all climates; great for home offices, entertainment areas (less glare) and bedrooms; attractive if mass wall’s interior is painted or plastered
Cons: Requires costly foundation to support mass wall; most expensive option; uses the most resources; difficult or impossible to
incorporate in existing homes

Attached sunspaces 

Pros: Easy to install; fairly inexpensive; attractive; suitable for retrofits and new homes; great for growing plants year-round, if designed correctly
Cons: May overheat or freeze; may get too cold at night; difficult to move air inside sunspace throughout home without a fan

Resources 

The Evergreen Institute 
(303) 883-8290
passive solar design classes

Further Reading

The Homeowner’s Guide to Renewable Energy by Dan Chiras

The Passive Solar House by James Kachadorian

The Solar House: Passive Heating and Cooling by Dan Chiras

Dan Chiras is the president of Sustainable Systems Design in Evergreen, Colorado, and the founder and director of The Evergreen Institute’s Center for Renewable Energy and Green Building in Gerald, Missouri. He is the author of more than two dozen books, including The Solar House: Passive Heating and Cooling. 


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