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Passive Solar Design for Tiny Houses

Many people are aware of the concept of passive solar design, but it’s such a big and potentially complex subject that it’s easy to get overwhelmed or lost in a few details that are only part of the whole picture.

In this article, I’ll walk through some basic steps for applying passive heating and cooling principles to a tiny house design. As always, you’ll get the best results by doing as much research as possible and/or working with an architect or designer familiar with the principles of passive solar design (all architects should be, because passive solar principles can and should be incorporated into every building built!)

Let’s define what we’re talking about: Passive solar building design involves using windows, walls, and floors to collect solar heat energy when it is needed (usually in winter) and reject it when it is not needed (usually in summer).

Here are the basic steps to take when thinking about passive solar design:

Know your location

Step 1 is already difficult for some tiny houses—what if you decide you pick up and move your tiny house a thousand miles away? To design your tiny house, you’ll need to pick a location to design for, and know a couple basic things: your latitude (which determines the sun angles) and your climate region (which determines your passive solar design priorities).

Luckily, there is some good information on climate available online. The map below comes from the U.S. Department of Energy’s Building America Best Practices program, which also offers climate-specific building advice.

Image credit: U.S. Department of Energy

Know your site and orientation

A lot of passive design depends on specific details about the site and orientation of the building. Of course, this might be a challenge if you don’t have a site and orientation in mind for your tiny house, or if you plan to move your tiny house to multiple sites.

If you have a site in mind, find out which direction is south. It’s best to orient one of the long sides of your house to the south. Are there any trees or hills blocking the sun? If so, when—morning, noon, or afternoon? Shade is good in the summer, but it can be a problem in the winter.

If you don’t have a site in mind, I recommend picking one of the long sides of your tiny house to be the “south” side for the purposes of passive solar design. When you park your tiny house, you’ll want to orient this side as close to south as possible.

Determine your priorities

Your design priorities will depend on the climate in your specific location. I’ll talk about some general passive heating and cooling priorities for the major climate regions based on the map above. However, any specific information about your location and site should be taken into account.

Cold / Very Cold / Marine climates

The two priorities below can be applied to the design of a tiny house. A third—using trees or other elements to block the cold winter wind—may help determine where to place the tiny house on its site.

  • Insulate well. Insulation is priority numero uno in most climates, but especially in ones with cold winters. If you don’t insulate well, you’ll lose any heat you might gain from the sun or from your heating system. Look into advanced framing, insulating sheathing, SIPs, and other ways to improve your insulation.
  • Let the winter sun in. Know your solar geometry: The winter sun is at a lower angle than the summer sun. (See the basic solar geometry review below). So, orient lots of windows to the south (or slightly southeast, or southwest) and make sure nothing’s blocking the sun in the winter.

Mixed-Humid / Hot-Humid climate

In these climate areas, the previous two priorities (Insulate well and Let the winter sun in) still apply. In addition, two more priorities are:

  • Shade the summer sun. In climates with hot summers, the sun is not always your friend. Design a shading system for the south-facing windows that blocks the high summer sun without blocking the low winter sun. (See the previous article on daylighting for a few ideas). Consider retractable shades or awnings for more flexibility. Avoid having lots of windows on the east and west walls if possible; it’s hard to shade the low morning and afternoon sun, and overheating is likely to occur.
  • Allow natural ventilation. In temperate climates, natural ventilation may be all the cooling you need for much of the year. Make sure you have windows that can open. Install insect screens if necessary (be aware they cut down a little on airflow). Put windows on two or more walls to give air a path to flow through your house.

Hot-Dry / Mixed-Dry climates

In these climate areas, the previous four priorities (Insulate well, Let the winter sun in, Shade the summer sun, and Allow natural ventilation) still apply. In addition, two more priorities are:

  • Use thermal mass to stabilize temperature swings. In some climates, especially American Southwest, the temperature swings significantly during the day—it’s too hot during the day and too cold at night. Thermal mass takes advantage of this fact by storing heat during the day and releasing it slowly. Use massive materials like concrete, stone, or water-filled containers. (If weight is a concern for your tiny house, a water-filled container that can be emptied for transport might be a good solution. Water is one of the best materials for storing heat.)
  • Take advantage of evaporative cooling. In dry climates, adding a small amount of water vapor to the air can make the house significantly more comfortable. A basic evaporative cooler (or swamp cooler) is not technically a “passive” technique, but it’s inexpensive and uses very little energy. You can even build your own.

With these passive heating and cooling priorities, you should be able to think carefully about your window placement and other elements of the design. For a full run-down of the various passive solar techniques, here are some resources to look into:

Window placement

Because the sun travels mostly across the southern sky (see below), it’s fairly easy to design a shading system for the south-facing windows that can protect from the high summer sun and allow some of the low winter sun to get through.

On the other hand, it’s much harder to control the low morning and afternoon sun coming in the east and west windows. For this reason, passive solar designers strongly encourage limiting windows on the east and west walls, and including blinds or some other shading system for east or west windows.

Likewise, it’s hard to control the light coming through skylights: they get maximum sunlight in summer (when you don’t want it) and minimum sunlight in winter (when you need it). For this reason, vertical windows are usually a better choice if possible.

Windows on the north side won’t get much direct light, but they can be a nice source of indirect light.

Basic solar geometry review

Photo credit: Notes from Noosphere

In the northern hemisphere, the sun travels mostly across the southern sky. It’s highest at noon on the summer solstice, and lowest on the winter solstice.

At noon on the equinoxes (March and September 21) the sun’s angle from vertical will be equal to your latitude. (For example, if you’re at 33° north, the sun will be 33° below vertical, or (90-33) = 57° above the horizon).

At noon on the summer solstice, the sun’s angle will be 23.5 deg. higher than it was on the equinox. (For example, again at 33° north, the sun will be 9.5° from vertical, or (90-9.5) 80.5° above the horizon.)

At noon on the winter solstice (December 21), the sun’s angle will be 23.5 deg. lower than it was on the equinox. (At 33°. north, the sun will be 56.5° from vertical, or (90-56.5) = 33.5° above the horizon.)

For more information about sun angle calculations, check out these resources:

Have you thought about incorporating passive solar strategies in your tiny house design? Share them in the comments below! In my next post, I’ll talk about advanced framing, SIPs, and other innovative structural systems for tiny houses.

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Vincent Baudoin is a designer and builder with a background in public interest design, sustainability, and integrated design-build. He develops tiny house designs and construction plans -- check them out at Pilothouse Design.
{ 24 comments… add one }
  • Carolyn B February 1, 2013, 11:10 am

    Hello, Vincent. Good article again. Thanks! I’ll be one of those ignoring the directive about limiting east/west windows. I’m very cold-natured so I search for the passive solar heat as much as possible in any season. For awhile I practiced passive solar design in my home. I transformed my southwest bedroom into my living room as I wanted as much of the evening light as I could get.

  • Ken February 2, 2013, 2:56 am

    Thanks for the info on passive solar design Vincent. I like seeing discussions about new technology and old ways coming together to build the right tiny home for each person. Having been a framer and carpenter in So Cal during the 80’s, you learn how to build quickly and efficiently. I wish I knew all this back then and excited to see what the future holds for building science in the next 10-20 years. Good to see people starting to be aware of how they can make their home more energy efficient and sustainable.

  • LaMar Alexander LaMar February 2, 2013, 10:07 am

    One of the biggest mistakes I see in conventional and house on wheels construction is improperly sized and poorly spaced windows.

    Too many large windows in a small house will turn it into an oven in summer and an icebox in winter.

    Correct orientation of a house is also extremely important for both passive and active solar use. In hotter climates windows should be reserved primarily for the north or shaded side. In colder climates the opposite is true.

    Covered proches, blinds and window shading are necessary to protect a house from overheating and a shade proch adds living space to the house and acts as a natural air conditioner cooling the air that passes under the shaded porch before it enters the house.

    The shade porch can als be used as a solarium porch in winter with glazed panels or a simple clear plastic wrap that would greatly increase heat on winter days that can be tranfered into the house via doors or windows and provides a nice mud room for taking off winter clothes before entering the house.

    Thermal mass is anything in the house that will absorb solar radiation including appliances, floor and wall coverings and people.

    Most small houses do not need as much thermal mass but in colder climates a drywall covered wall and wood flooring would add thermal mass to increase heating.

    Air circulation is necessary for al houses and passive roof turbines, solar activated fans, windows and doors should be placed so that air can be tranfered to facilitate heating and cooling. A simple rule is cool air comes in from below and pushes warm air up.

    LaMar

    • Vincent February 2, 2013, 5:14 pm

      Thanks for the great points LaMar. I agree about the windows– I’ve seen a lot of huge windows getting put into houses (6′-0″ x 6′-0″ or bigger) and they’re often completely out of proportion with the space. I would much rather spend the extra money on a better quality window instead. Personally, I like casement windows a lot, too, as you can open 100% of the window.

    • Lisa June 24, 2013, 4:32 pm

      I agree about the windows. How many times I have looked at a tiny house on wheels only to think, “I’d take that window out and put in a Murphy bed, an electric fireplace”… fill in the blank. I’m also concerned about the security factor with big windows. Like Vincent, I love (diamond pane) casement windows but for security reasons, I’d rather have a series of “eyebrow” windows up under the eaves and use the interior wall sace for other things like storage or just blank to deal with feelings of claustrophobia. Great article and comments.

    • Jerry March 14, 2014, 2:22 am

      After a years research on designs for tiny houses, I agree, too much window is not a good thing. So I’ve been cutting down the window space in my designs, and looking for different ideas. I’ve found one company making an interesting solution that would work very well in a tiny house, and I’m sure there are other companies making similar. I call them “pocket” windows, but I don’t know if there is a proper term. The idea is to have the entire window fit between standard studs. This eliminates the need for extra jack and cripple studs, as well as headers and sills (unless you want a sill), freeing up weight. http://www.geometricswindows.com has four basic shapes that fit between the studs, and have flanges and molding that interlock to create the appearance of once larger window if you desire, or use them to put small windows anywhere you like. You can even add them in after the house is built, with each install taking an hour or less. This means you could build your shell, finish the interior and furnish it, and then determine the optimum window placement. They are a bit pricier per sq ft than basic standard windows and trim, but I think they just might be worth it for my needs.

  • sunshineandrain February 3, 2013, 3:04 am

    Great guidelines Vincent. I’ll be using them to study and perhaps change my Tiny House design. Thanks.

  • Teri February 3, 2013, 4:20 pm

    Vincent, I am so grateful whenever you and LaMar share such great information. I learn so much! I’ll be moving my house about once a year and won’t be sure what the site will be like each time, so I’m curious about heaters & air conditioners in case I have no control over the passive heating. Any thoughts on this way of controlling our tiny house environments?
    Thanks!

  • Jeff February 4, 2013, 5:44 am

    This seems simple and obvious, but since no one else wrote it, i will. What about making a “summer” side and a “winter” side to the house on a trailer or on wheels? That way the house could be rotated “end for end” with the seasons, solar gaining side toward the low winter sun (south in the N hemisphere) and solar gaining side away from the sun (north in N hemisphere) in the hot summer. A super insulated opposite side wall could be situated, too, letting it block summer sun and cold winter winds.
    To further flesh-out the idea, the solar gain window could turn the “corner” of the roof (where a gutter would be on a pitched roof) and go up the roof a few feet…..this would let the sun penetrate the innards of the home in the heating season but not heat the interior any more in summer (or covered with a seasonal awning).

    • Vincent February 4, 2013, 12:15 pm

      There are a few “normal” houses that rotate in this way, and some even rotate daily to follow the sun. But they require some fairly complex mechanisms to let them do so. Turning a tiny house is quite a bit easier (although it still takes some effort). I think this could open up some really creative passive solar solutions and I would definitely be curious to see this put into practice.

    • Karen B. Jones July 14, 2015, 11:32 am

      I was wondering about precisely this point as well. Just turn it around twice a year.

  • Dawn June 4, 2013, 4:15 pm

    I am building a 484 sq foot home and I live in Canada, where we have hot summers and very cold winters… I’m quite confused now as to where the windows should be placed? I was originally thinking all South facing windows with an overhang and then two small windows on the east side and one on the west side… can someone confirm or deny these plans? And all my windows will be casement windows that will be able to open fully to allow for ventilation and also for safety as I only have one door. Someone please clarify.

  • Will June 15, 2013, 7:18 pm

    I appreciate how you summarized the basics of passive solar design in such a concise way. It is so much easier to understand now. I actually wasn’t looking at tiny houses at all, but this is the clearest, shortest summary of the basic principles, which can be applied to stationary houses as well.

    Another interesting application would be to try to create an ultra-light passive solar camp trailer, perhaps by converting an airstream: Add a water-filled heat mass, some solar panels, an awning, and some strategically placed thermal windows and vents/fans. Just think of the possibilities: Mobile off-grid living, with minimal energy needs.

  • Steve June 24, 2013, 11:53 pm

    Great article. I have a fairly small house and in the summer it stays quite cool by having shade trees on the south side, minimal east and west windows, and a roof vent in the ceiling. The cooler air from the morning flows through the house until late afternoon when it gets quite hot. Then, on the really hot days, I’ll run a window air conditioner for a couple of hours, which does the trick.

    I believe that skylights would be a bad idea on a tiny house because of what you mentioned, and, also, I don’t understand why no one seems to install either ceiling or upper wall exterior fans which would suck the hot air in the summer out and create air flow through the home. This seems like a no brainer. All motorhomes have this and it create a great draft, sucking in the cooler outside air.

  • Nichole June 25, 2013, 1:38 pm

    Awesome article. Thanks for posting!

  • Dan S. March 13, 2014, 6:29 pm

    Thanks for the article Vincent.

    How can one reconcile the apparently contradictory design requirements for tiny houses (lightness, limited weight) vs. passive solar (insulation, thermal mass)?

    For passive solar, insulation is key, requiring high R-values, and thus generating either added weight or cost (light insulation is expensive).

    Likewise, passive solar requires a certain amount / proportion of thermal mass, otherwise the heat you’re accumulating during the day will simply dissipate.

    Have others found / used creative solutions to accomplish passive solar within the limits of a tiny house design?

  • AL-APL March 13, 2014, 6:44 pm

    I have been interested in Passive Solar houses since the 1986 World’s Fair when I saw an octagonal solar house; I thought it was the coolest thing! I also love how Earthships are passive & self-sufficient. I’ve been dreaming about passive houses (tiny or small) for my family for years now: something simple & natural, that uses Nature to our advantage, without hurting it. I really appreciate you writing this article & I will be heading to your website to look at your designs. Thank you for this interesting article, Vincent! (Je ne sais pas si vous parlez français mais si oui, je vous remercie encore mille fois !)

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