Frequently Asked Questions about Passive Solar Design

How can passive solar design be warm in the winter and cool in the summer?

If properly designed and built, a passive solar building will take full advantage of the sun through the seasons.

Diagram of how the High Summer Sun travels a different path than the winter sun.

Courtesy – U.S. DOE

The building needs to be designed to allow the low winter sun to shine on the building’s thermal mass, allowing it to store the sun’s heat energy. The stored heat energy will later radiate out and warm the building throughout the cooler evening and night.

In the summer, when the sun travels high in the sky, the dwelling and its thermal mass is shaded from the heat of the sun, thus, the dwelling stays cool.

To illustrate how this works, think of how cool, shaded tile or marble can feel on bare feet on a hot summer day. As long as the tile (or other object with thermal mass) is shielded from heat sources (mainly the sun), it will stay cool because of its density. This density, along with the object’s capacity to absorb heat is often referred to as its thermal mass. When a material has a lot of thermal mass, it resists quick temperature fluctuations.  If the sun shines all day on tile or marble, it absorbs its heat and as a result will be warm, then will slowly release the heat. If the sun is shielded from reaching the thermal mass, it will stay cool.

This key to understanding the concept of passive solar design is that it is based on the sun’s movements and its ability to provide heat energy, combined with the potential energy of the materials that are used that help to create a building that is situated and built to maintain a stable temperature.

See pictures of the sun on the winter and summer solstices


What is the difference between passive solar and active solar?

A passive solar building will simply, passively sit where it located and be efficient because of its design. The design is created based on the movements and available energy of the sun, as well as the materials used to construct it. Passive solar design utilizes the sun’s energy in the forms of heat and light to warm and cool a building without additional mechanical or electrical input.

Active solar heating and electrical systems also use the sun’s energy but with additional mechanical means used in the process of harnessing its energy. Photovoltaic (PV) panels, pumps, thermostats and fans are objects that are commonly referred to as being active solar because they need some electrical input or conversion for their systems to function.


What is a Trombe Wall?

Trombe Wall diagram

Courtesy – U.S. DOE

A Trombe Wall is made out of a dense material that has thermal mass.  Typically cinder blocks are used to construct Trombe Walls.  When used in passive solar building, they are located directly behind a window or glass wall, perfectly situated to absorb the sun’s heat.  It transfers the heat into the main living area.

In passive solar design, a Trombe Wall works together with an overhang designed to warm and cool the building through the seasons by allowing the winter sun to warm the building, while it blocks the summer sun and keeps the living area cool.

The diagram to the right show a Trombe Wall transferring or radiating the winter sun’s heat into the main living area.

This style of building is named after Felix Trombe, a French engineer who is popularly known for advancing this indirect method of solar heating.  Here is a house built with two Trombe Walls and another modified Trombe Wall that was entered in the 2011 Solar Decathlon.

Which direction should a passive/active solar house face if located above the equator in the northern hemisphere?  Is it any different for solar buildings built in the southern hemisphere?

When building a solar house, it is important to face the active solar implements and most of its windows toward the direction that faces the sun most of the day, as long as the building is located in a place that has cold winters where winter heat is desired.

The northern hemisphere is highlighted in yellow. The southern hemisphere is white.

This direction that a solar house should face and be designed and built for is different depending on where the building is being built.

If the location is above the equator, in the northern hemisphere, the general direction for a building or solar panels to receive sun throughout the day would be to face them to the south.

If the building’s location is below the equator, within the southern hemisphere, the general direction to receive sun throughout the day would be to face the solar panels or windows of the structure to the north.

The equator is the imaginary line located in the horizontal center of Earth.  The area above the equator is designated as the northern hemisphere and is shaded in yellow, while the area below the equator is designated as the southern hemisphere and does not have any shading in the diagram above.  As one travels further from the equator, typically there is a greater need to heat in the winter.  The climate around the equator is typically temperate, although altitude and local weather conditions also affect the climate.   It’s always smart to take into consideration local climate conditions like prevailing wind patterns.

Here is an article about a passive/active solar house built in New Zealand for the 2011 Solar Decathlon.  While in New Zealand, they build the house orientated to the north. When they traveled to the U.S. to compete in the Solar Decathlon, they simply flipped the house from facing the north to facing the south. They did really well, getting first place in the Energy Balance and Engineering contests, going on to place 3rd overall.

Can I make my own windows?

The original question posted on Facebook – but please ask questions here (so they don’t have to be reposted).

I was wondering if you could tell me where I could find windows for a passive solar home. I want just the glass part, so that I can build the frames myself. Do you know of any companies that manufacture glass for windows for passive solar homes?

If you want to get windows and build the frames yourself, then see this Off Grid Passive Solar Earthship or this Passive Solar Quonset Hut Retrofit

In the first post, they made all their windows by purchasing old sliding glass doors (the glass is tempered and uniform in size) tore them apart and made their own windows.  They went to a local deconstruction store, similar to a Habitat for Humanity ReStore to purchase them. With their project it turned out to be 10 dollars per pane.  A positive aspect is that this way is inexpensive. Negatives are that they might not be as efficient, allowing heat to escape out of the house and they are a lot more work.

Regarding the second post, he built his own windows too, but custom ordered the glass.  Depending on where you are located, just contact a local glass shop, give them your dimensions and receive a quote.


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