Passive solar

Passive Solar is a set of building protocols that, properly implemented, produce structures that are warm and sunny in winter, and cool and shady in summer. No mechanical systems are required. (Mechanical systems make a building "active solar"; solar hot water and photo-voltaic systems are examples.)

The classic passive solar structure has a long east/west axis with most of its glass on the south side. In the northern hemisphere, the winter arc of the sun is low in the southern sky causing sunlight (solar energy) to enter these south windows. In summer, the sun's arc is high in the sky, and little or no sunlight enters on the south side.

classic passive solar structure

A thermal mass is required to store solar energy and stabilize internal temperatures. This mass can take the form of a floor or internal walls made with concrete, stone, bricks, etc.

Without a thermal mass, large south windows will cause intense temperature swings and overheating. (AAC and lightweight concrete are not suitable for creating internal thermal mass: they are too light.)

Windows must be "Thermapane" and properly sized. We specify "low-e" (low emissivity) glass in all windows except those on the south.

A passive solar house requires summer shading on the south, east, and west. This is accomplished with properly sized overhangs, foliage and placement of trellising vine supports and porches. In many areas of the country, cooling is as important, or more important, than heating. Criticisms of passive solar technique generally originate in improper cooling design, and rightly so: what good is it to save money on heating only to spend it on cooling? A properly designed passive solar house will both heat and cool itself.

A passive solar house doubles as a sunroom in winter and a screened porch in summer. Accurate solar orientation of a house varies from the east coast to the west. We supply instructions for siting your house in your area of the United States.

Site suitability

You can determine if a site is suitable for a passive solar house by first finding south with a compass, and deciding approximately where you want to locate your south outer wall; you must have no structures or evergreens that will cast a shadow on this prospective south wall at winter solstice, when objects cast shadows 1.7 times their height. In other words, an object 10 feet tall casts a shadow 17 feet long – so an object that is 10 feet tall must be at least 17 feet from your prospective south wall.

At the east and west corners of this wall, objects along a 45-degree imaginary line will cast a shadow 3.5 times their height. In other words, an object that is 10 feet tall casts a shadow that is 35 feet long.

This area on the south side of your house should be clear of shading obstructions for six hours/day at solstice. Deciduous trees will reduce optimal winter solar gain, but for many homeowners in hot climates, that is an acceptable tradeoff.

A lot that is flat, or drops away to the south, reduces the amount of clearing for the solar access area. For those building outside municipalities, this can be a good place for a septic field - which must be lower than the house to avoid pumping.

A lot that rises to the north can be used to great gardening advantage, but will require more cleared space in areas with tall trees.

Small lots present greater challenges because the owner must retain control of solar access on the house's south face; close neighbors can build a multi-story building or plant tall growing evergreens that block the sun. Beware of easements and covenants that may impact solar access. A good city or suburban lot requires a careful search; look for roads that run east/west and a lot that is deep from north to south; a house with a solar front will work on the north side of the street and a house with a solar back, on the south side.

arcs of the sun during summer and winter

Costs

It can cost no more to build a passive solar house than a similar conventional structure. Energy bills are commonly reduced by at least 75% for the life of the house, a veritable cash windfall for most people.

In North Carolina, our houses have a typical seasonal indoor range of about 25 degrees, without operating mechanical heating or cooling systems. This means that unheated, our houses drop to about 55 degrees when outdoor temperatures go to zero; un-cooled, they climb to 78 degrees when outdoor temperatures reach 100 degrees. This extremely narrow range of indoor temperatures translates into very modest annual energy costs. Passive solar design makes irrefutable economic sense. These economic facts are measurable – unlike the immeasurable pleasure of living in a house that is warm and sunny in winter and cool and shady in summer.

Solar tax credits

Good old Jimmy Carter. President Carter instituted national solar tax credits that were rescinded by the next administration. So, it was left to the states to decide whether to encourage energy efficiency and independence; some did and some didn't. Some may yet with sufficient lobbying. These states could use North Carolina as a model; our state now proudly offers the biggest tax credit for solar features in the US – and this from a state with no bottle law.

These tax credits are significant inducements to use solar technology.

Ethics

Americans comprise 5% of the world's population and gobble 25% of its resources. Because passive solar houses costs no more to build than conventional houses, but use so much less energy, they offer a painless and "passive" means of reducing our resource demand. Just build the house, and you have made a small improvement to our collective ethical profile – and to our general wellbeing.