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 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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18 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.

  7. Steven October 28, 2014 at 5:55 pm

    I would like to build a passive solar off grid house. The location is near to the equator and the climate is moderate. There is not much of any change during the seasons: the weather is 7-15 degrees C each night and 20-27 C during the day, year round. And the sun does not vary much in its orientation overhead during the day or between the seasons, as the location is so near to the equator.

    My question is, given the above, what is the optimal design for my house given my desire to be off grid 100%?

    Thank you

  8. Jean-Roch November 17, 2014 at 12:08 pm

    Hello,

    Very interesting.. thank you for all this work.

    I’m just starting and would like to leverage the South face of my house. Yesterday I cut lots of spruce branches in the middle section to let pass the sun through! Well it’s.. less beautiful than before (ugly!) but at least I’ve got the pre-requisites now!

    Questions:

    1) In the case of these external rectangular boxes with bottom input vent and upper output vent, I’m considering some used storm doors material, or used patio doors on craigslist. But most are insulated “low e” double glass. They insulate not only from the cold but to some degree (!) from the sun too, although not as much? If I knew I’ll loose 10-20 % of the heat I think I’d be OK.. Ideally a shower tempered glass?

    2) On youtube demos, people use high-temperatures black paint, the sort you use on exhaust pipes (motorcycles etc..). But for indoor use now, since the air that’s been exposed to it in a confined space is pumped in your room, isn’t that a bit concerning? I can imagine the toxicity of these paints..

    Thanks!

  9. Keya Lea November 30, 2014 at 3:43 pm

    Air quality is an important concern within living areas. Some projects that I’ve seen on youtube incorporate plexiglass, a transparent plastic product as a method of capturing solar warmth, but glass can heat up while not off-gassing as plastic products can. Many builders I’ve talked to will only use glass for this reason.

    Similarly, paint, depending on the type, can also off-gas when heated. It is important to consider the air quality of living spaces.

    Perhaps a solution would be to build a Trombe wall?

    Best of luck. (Sorry for taking so long to respond.)

  10. Curtois March 25, 2015 at 1:59 am

    Hi, i am a student studying Passive Solar for a research project and i am living in Australia, could someone please tell me the considerations associated with such a hot climate?

  11. Keya Lea March 30, 2015 at 1:09 pm

    If you’re building a house for a hot climate, the basic principles of passive solar are still the same: use the sun’s predictable movement throughout the day and seasons to keep it cool in the hot months by using specific materials in specific places.

    For example, if someone is building a house in a hot desert climate, instead of placing windows in the south (north if you’re in Australia) to gather sun in the winter, perhaps it would be better to place the windows in the north (south for Down Under the Equator) so that no direct sunlight (and heat) enters the house.

    A person could also use thermal mass to cool the house. Dense materials like stone, concrete and water, will stay cool if there’s no heat source on them. Remember that materials like thermal mass can help a building maintain its temperature and can be used to cool, as well as heat a building. Best of luck!

  12. Jen June 30, 2015 at 5:41 pm

    We own a passive solar house built in 1969. We want to replace windows due to leaking, etc. but want to keep our house heated.

    We are nervous that if we purchase new windows we won’t benefit from the solar warmth that we get in the winter. The downside is that at nighttime the warmth of the house is taken away (our house is elevated). We would like to trap the warm air in during the winter. Any ideas?

    We never have to light the house during the day and last year only needed to turn on the heater during the winter twice! It really works! We live in the SF Bay Area. Basically, need help figuring out what to replace our aluminum windows with. Thanks!

  13. Keya Lea June 30, 2015 at 8:35 pm

    Hi Jen,

    The necessity of window replacement is something that happens in older homes. In the passive solar homes that I’ve visited for this site, some owners specifically request and build with uncoated glass (the purists), while others opt for double or even triple-paned windows (on the west side of the home) to allow solar heat in, while keeping helping the building to retain as much heat as possible through the cooler evening and night.

    A lot of passive solar homes, will use insulated curtains on the windows to help with heat retention in the winter.

    This passive solar home with a Trombe wall. The owner-builders used low-e (low emissivity) windows and they have been really happy with them. (They said that they wished that they had extended the Trombe wall throughout the home, instead of in sections that they did.)

    Another builder purchased windows from a Craigslist surplus sale and after building with them (concrete modernist home) found that the different coatings on the windows impacted the amount of solar heat that was allowed into the home. (Lesson learned: Window coatings matter.)

    There are different types of window coatings (some are reflective, with a metallic film on the inner facing side of the window) while other windows can have gas in between the panes. These different types of windows can be used strategically throughout the home.

    I’d recommend going to a window store and asking for a type of window that will allow the most solar heat into the home while allowing the least out of the home. They usually have charts that show U-factor, spacing, film-type, etc. that are handy to help determine the type of window that you’d like to get.

    Here’s a link to Wikipedia that goes through different energy efficient window types.

    Best of luck and let us know how it goes.

  14. Jen June 30, 2015 at 9:05 pm

    Our house has four large (60×60) plus angled windows above facing south, the windows east/n/w are small and we will replace w/ standard double pane if we haven’t already replaced them. We just need to replace the south facing windows…thanks for the tips, will consult with a window store!

    Glad we found your site. We are always telling people how amazingly well our house is designed, our gas/electric bill is only $30 a month! Never knew the term for it!

    Thank you

  15. Ashleigh July 19, 2015 at 2:13 am

    Can you use shipping crates in a passive solar house?

  16. Keya Lea July 20, 2015 at 6:13 am

    Shipping crates (or containers) can definitely be integrated into a passive solar build.

    To ensure that it is a passive solar build, include building characteristics such as southern facing windows, thermal mass, insulation, an overhang, and modify all of the above for an ideal living space within your local climate.

  17. Anette August 18, 2015 at 12:30 pm

    Thanks for great information, and hope you can help! :) Sorry for the essay, but the situation is rather unique…

    We are planning to build a passive solar home with a few exciting challenges and opportunities – so have some areas that we are uncertain about.

    First, what we DO know/can tell…

    About the area

    1. We are in the Southern hemisphere, so will face North.
    2. The small holiday cabin is to be on top of a mountain in a remote area – building is difficult. Double glazing is not even an option.
    3. The entire South side (about a third of the width of the cabin) will be embedded into an existing rock overhang (not quite deep enough to be called a cave, but deep enough to effectively form the roof of a small bedroom area and big bathroom.
    4. The area is cooling dominated, with long, hot summers reaching temps of 90 – 115 deg F.
    5. At the same time, during the short and wet winters, temperatures can drop to around 30 deg F.
    6. Apart from the huge amount of thermal mass in the rock wall, there is also a rock ridge of around 6 x 2 running through the middle of the house. Neither of these will be solar exposed, but we expect them to offer earth-shelter-type tempering of temperatures – we recently measured an air temperature of 43 in our un-built lounge to be, while a probe stuck into a small slit in the rock wall measured 55 F.


    About our plans

    The house is small and extremely open-plan.
    Its length runs from East to West.
    The long N-side will be glazed as far as possible and will house the living, dining and kitchen areas. It will have overhangs to protect the house from any summer sun, but fully allow winter sun
    The East side is protected by trees.
    The West side (unfortunately) offers the best views. Hectic screening planned for that side! :)
    The South side will house the rather open-plan bedroom and bathroom.
    There is one bedroom on the North side, but almost its entire North wall is obstructed by a huge rock.
    We are using travertine on the floors, for direct solar gains.
    There is always a cross draft from East to West, or vice versa, which we plan to exploit (with some high windows and also sliding doors) on those two facades, when we need to cool down the house in summer.
    Because of the unique position of the house, against a hill, the bedroom under the rock overhang (on the West side, effectively) does not get any sun in summer, but bakes in winter! :)

    We will probably look at something like honeycomb blinds to keep heat inside as far as possible in winter.

    So… a lot of really good things automatically going for us. But now we have one main question…

    We suspect that earth coupling will be preferable, because of the huge diurnal temperature variations. And especially in winter we think this is the best idea. However, in the living area (incl kitchen), the sun will be baking on approx 8 x 3 m for most of the day, in winter. We don’t want this heat to simply wick away into the slab at night… would it make sense to put insulation under that part of the slab only, with a thermal skirt, to ‘force’ it to radiate heat upwards into the house at night?

    Phew – a long story to get to the one question… :)

  18. Keya Lea August 19, 2015 at 4:04 pm

    It sounds like a lovely project, with a lot of great passive aspects to work with. I commend you for the site research that you’re doing! I can give my opinion, however, it’s just an opinion, and it’s from the point of view from someone who lives in the mountains and sees a lot of passive solar homes built in areas that have four seasons. Take my opinion with a grain of salt, and talk to others who have built in your area and climate.

    It sounds like for most of the time, you’re wanting to cool the home during the summer, yet for the short, cooler winters, because it sounds like you’ll be using single pane glass, the amount of natural stone mass in the house will be moderating the temperature in the structure.

    The most comfortable homes that I’ve seen, took smart steps in the building process – like insulating the slab, and those things can make a huge difference in the overall comfort of the home. However, those homes are located in areas that can experience a range of temperatures from -20 to 90 degrees Farenheit throughout the year. This home was extremely well-thought out, and did insulate under the slab.

    It looks like you’re looking to insulate under just one section of the home in order to retain the heat in the short winter. Because it’s a small section of the house, yet because it will be pounded with solar heat in the winter, you’re relying on the thermal mass to keep a constant temperature, and you’ll be wanting to keep that heat in the open nature of the home – I think it would be good to insulate under the slab for that small section of the home. It doesn’t sound like it would impact it that much in the summer, and it’s much easier to plan and do before-hand, than to wish you had added something after the house has been built.

    Do explore what other people think – if other people have opinions, they are also free to comment.

    Good luck, and I’d love to see the home after it’s finished!

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