FAQ

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 more of a 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.

Here is an article about a passive/active solar house built in New Zealand for the 2011 Solar Decathlon.  They simply flipped the house from facing the north to facing the south.

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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6 Responses to FAQ

  1. Luis Alvarez December 7, 2011 at 11:18 am

    I appreciate the great education you provide towards helping to create a more sustainable environment. I have a question: In South America, Colombia for example,in building a passive house, do you build north facing windows?
    Thanks
    Luis

  2. Keya Lea December 7, 2011 at 4:52 pm

    Hi Luis,

    Thanks for your question. When building a passive solar house, it is important to face the house and most of its windows toward the direction that faces the sun most of the day. This direction is different depending on where you live. If your location is above the equator, the general direction to receive sun throughout the day would be to face the windows of the structure to the south.

    If your location is below the equator, the general direction to receive sun throughout the day would be to face the windows of the structure to the north.

    Also keep in mind that there are a few other aspects to having a passive solar house. You must also consider the climate and elevation that you live in. Columbia is located just about the equator, so a passive solar house would face the south, but do you have more days that you need to heat or cool your house? If you are located in a hot area and want your house to stay cool, you may want to minimize windows on the southern side because they let in a lot of solar heat. The placement of windows is important depending on what you are trying to achieve.

    I’ll post a short write-up within the next couple of days with diagram that shows the differentiation between the northern and southern hemispheres.

    Your question, however, asks about a passive house, which is different from a passive solar house. A passive house is one that is tightly sealed, well insulated and air flow is controlled. Is that what you are asking about?

    The answer to your question largely depends upon exactly where you live and what ideal temperature you want to achieve. Because passive solar homes are largely efficient based upon the home, the materials (windows, thermal mass) it is made out of and how it interacts with the sun.

  3. Pat January 3, 2012 at 6:50 am

    I would like to follow up on the question on Colombia. I am currently trying to design a house at 2500m above sea level near Medellin Colombia at aprox 4 degrees north. The temperature during the day varies between 19 – 25 degrees celcius and falls to 6-8 degrees celcius at night.

    I stayed in a house near the site which used a type of opaque roof light made of glass planks. The idea was that this would distribute the heat from the sun falling on the rooflight during the day and stores it throughout the structure of the house during the day releasing it at night. The house was not very air tight so the heat was probably sucked out of the house quite quickly. The walls under the roof light were painted a light colour possibly designed to distribute the heat rather than using direct solar gain. A dark coulour would have aborbed more heat directly into the walls underneath.

    The owners claimed this worked, and it did seem to store the heat for several hours. It was pretty chilly later in the night. Are there any documented case studes for successful high level Andean solar homes? I would not have thought trombe walls will work at this latitude as any overhang of the roof will cut out any sun on vertical walls?

  4. Keya Lea January 3, 2012 at 10:40 pm

    Pat,

    I now see where you are located and understand a little better about what the climate is like there. I had to do some conversions (in the U.S. things are different from the rest of the world) and see that your temperature seems to vary between 66 – 77 degrees Farenheit during the day and falls to 42 – 46.4 degrees Farenheit during the night.

    Because you are located near the equator, the location would not have that much of a variance between the winter and summer solstices and how the sun ‘travels’ as it does for people located further away from the equator. Does the sun ‘travel’ a fairly high arc through the sky? The house that you visited had some type of clear glass on the roof that allowed the sun to shine it’s heat and light energy into the home through the day. The wall painted a lighter color would reflect light better, but the color itself does not absorb heat, but the material that it is painted on – potentially can.

    When building a passive solar house, materials with thermal mass are placed in areas where they can absorb solar heat. Dense materials with thermal mass like tile, concrete, or masonry have the ability to absorb, then slowly release heat.

    A Trombe wall works in a similar way. It absorbs heat during the day, and radiates it out in the colder evening and night. A Trombe wall probably would not work where you are located, but similar priniples of storing heat would work. In your area, if you allowed the sun to heat an area with thermal mass – say the floor, if you also used some type of glass in the ceiling, it would allow solar heat to be stored and heat the house later, after the sun goes down.

    Do you remember what the house was made of that you visited? You are correct in that a house that is not well insulated will loose it’s heat (or coolness, whatever you are trying to achieve) to the cooler outer elements on a cold night, so it’s important to remember that the better insulated a house is, the longer it will stay a constant temperature. Also remember that a good house is an amazing system that utilizes different principles of physics to work well. Every building site and home design is different, but if it is well thought-out, it can be a beautiful and comfortable house with little or no utilities. I’d love to see your project as it takes shape. I don’t know of other Andean houses, but it’s a great idea to talk to others in the area, see what is working for them, ask what they would have done differently, and remember the passive solar principles and modify them to your specific location. Best of luck!

  5. Joe the Stack January 23, 2013 at 4:16 am

    I have a theoretical question:

    What if one were to build a passive solar house using any available techniques, designs, and construction methods except for one constraint – that one start with an existing conventional house and not demolish, rebuild, or replace it? It may sound silly, but what I mean is if there were no other limitations, what sort of things might one build around, adjacent to, under, or over the existing house? For example, freestanding sun screens outside the east and west walls of the house, awnings over all south-facing windows that block the summer sun but allow the winter sun, heat storage tanks under the house fed by solar collectors, etc? Repurposing certain rooms? Digging a basement? Enclose the entire house and yard in a geodesic dome? I know of several houses that might benefit from such things and from the principles you describe.
    Thanks!

  6. Keya Lea January 24, 2013 at 8:58 am

    Hi Joe!

    Your question is a great one, and one that many people interested in efficient homes have grappled with. There are many cool, beautiful and efficient outcomes. There’s a retrofit category on the website as well.

    All passive solar retrofits will be different, since the home that is being build onto is different. Here’s one that was once a small farmhouse that was retrofitted to be a modern passive solar home. They also integrated passive air movement to both heat (winter) and cool (summer) the house. In the process, they also tripled the size of the house.

    Here’s a retrofit where the owner-builder raised a Quonset hut, turned it to face true south, then built an adobe foundation and southern facing windows.

    Here’s a retrofit that added solar-facing space to the south and north side of the house.

    I think with a little ingenuity, any existing structure can be converted to passive solar.

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