Passive Solar Design Basics

solar design south face

In the sunny Front Range of Colorado, we see solar panels on a daily basis. But harnessing the power of the sun for your home is not limited to this high tech solution. People have used the sun in myriad ways in the past, and we can continue to make use of low tech, design focused options for capturing and using the sun’s free power.

Passive Solar Design

Passive solar design is a set of design tools that can be used in any part of the world to create a home that is warm in the winter and cool in the summer while using a minimal amount of energy for either cooling or heating. This means less petroleum products, wood, or other bio-fuels, which in turn means less CO2 emissions and other pollutants that are released in the burning of fuels. Burning of fuel or use of electricity also equates to direct financial burdens on those who use them, so using passive solar design can keep more of your money in your wallet.

Passive vs. Active

You might wonder how passive solar design got its name. Active solar generally refers to systems that require extra items that are attached to a building and are not already integrated into the home’s structural needs. These include solar collectors, solar panels, or rooftop systems for heating water. They often need pumps, timers, sensors, and/or wiring to get and keep them working. Passive solar doesn’t require these extras, but is able to be achieved using only the same materials that you would need to build any house, just rearranged into a design to maximize your ability to gather energy from the sun. Therefore, there is usually no added expense!

Principles of Passive Solar

Know your Location

Passive solar design works with the patterns of nature. The sun’s relative position in the sky is influenced both by the location that you live in and what time of year it is. Ideally, a builder or homeowner would have time to observe the site all year round to notice where the sun rises and sets each time of year, where winds come from, and any natural features that may provide shade, windbreaks, or create microclimates. Obviously, this isn’t always possible, but if you know your latitude, you can at least begin to calculate the angles that the sun will hit the house during different times of year. That being said, there are a variety of apps available for smart phones and online tools that make doing these calculations fast and easy.

Site to the South

Ideally, a passive home in the northern hemisphere should be sited on a south facing slope or in a location that gets plenty of southern sun. Most of the windows and entrances should be located on that side, to allow sunlight into the home where it can create heat. This means that the longest axis of the house will run east to west. Minimizing openings on your North face is important to prevent heat loss.

Don’t forget Outside Spaces

In addition to siting your home facing south, remember that outdoor spaces are best enjoyed with the sun, too. If your home shades your yard, consider moving it so that there’s more usable yard space on the sunny side for things like gardens, patio space, and hammock swinging. Planting deciduous trees on the West face of your home will minimize heat gain during the hot summer months.

Store up Heat

The heat that you are allowing in from the sun will need to be stored using thermal mass. Thermal mass can be any material that holds heat well. Water is one of the best materials for this, but not very practical for inside a home. As natural builders we often use earthen and lime plasters, as well as an earthen floor as our form of thermal mass. Interior adobe or cob walls that get direct wintertime sun are also a great heat sink.

Keep Heat in

Once you’ve got all the sun’s energy stored in your home, you need to keep it in. Use highly insulated walls and ceilings (we recommend natural insulation systems such as straw bale or hempcrete walls and cellulose for ceilings). Make sure there aren’t leaky areas in your home, around windows or doors. Even with a great insulation system, a drafty home will keep you chilly in the winter. Plasters create a monolithic wall coating¬†that are an excellent air barrier.

Not too many Windows…

So if south facing windows are good, maybe a whole south facing wall of glass would be better? Not so, usually! Over-glazing can result in too-hot-homes, any time of year. Also, windows will never be as insulative as walls, so you risk having all the heat escape at night.

Also the Right Windows…

Windows that work, and work for your home, are great! Modern windows have so many features to look for and some will do better on different sides of your house (more to come on that topic!). Vertical windows on the south wall are better for letting in the low, more direct winter sun, and reflecting away higher-angle summer sun.

solar design south face
These south facing windows gather heat from the low raking light of fall, winter, and spring, while the wide eaves of the roof keep direct sun out in the summer.
Cool Down Sometimes

You might think a passive solar house will get too hot in the summer. Opening windows and designing so your house can catch cross breezes is one way to help. So is something as simple as blinds or insulating curtains. Designing wide overhangs on a house can keep the high summer sun out of south facing windows. You can also plant (or keep existing) deciduous trees and shrubs on the south, east, and west sides of the house. That way, your windows will be shaded from summer sun.

Breathe Fresh Air

A house needs to have fresh air coming in. Particularly in modern well-insulated and air sealed homes, indoor pollutants can create unhealthy air conditions! A ventilation system is helpful, or necessary, to keep the home healthy. Using natural materials in and on your walls (straw or hemp insulation and plasters or clay paints) and floors (like earth, tile, or naturally finished wood) is another way to keep air clean by not introducing strange chemicals.

Keep Backup Systems Small

One great part of having a passive solar designed home is that your heating and cooling systems can be minimized, another immediate cost saver.

This isn’t the Passive Solar of the 1970s…

People might worry that their passive solar house is going to be the odd one on the block. Not so! Any style of architecture can be adapted to passive solar principles.

solar home
A passive solar rebuild/addition in Boulder. Designed and built by architect Brian Fuentes (
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