Studio Blog

Passive Solar Design

September 12, 2022

If you’re planning a new home and wanting to reduce your impact on the environment while saving on electricity, passive solar design is a wholesome way to make your home more energy efficient. Passive solar home design can also make your home healthier and more comfortable.

What Is Passive Solar Design?

Passive solar design is a method of designing and building a home that maximizes the benefits of sunlight. The set of strategies employed are extensive and begin when the site for the house is chosen and continue all the way to the finishing details of the home. Passive solar houses benefit from the energy of the sun without needing to install or maintain solar panels on the home, and can reduce your home energy bills by as much as 14%.

Passive solar design works by constructing a building with specific materials and angles so that sunlight will help with the heating and cooling of the building. The sunlight comes through angled glass and hits material that is able to collect and trap heat—this can be concrete, tile, stone, brick, etc. The heat is then distributed throughout the home via conduction and convection or a specialized HVAC system. During the hot summer, the glass is shaded, and the home is ventilated naturally. During warmer months, passive solar homes are designed to vent out indoor heat once the outdoor temperature has dropped below the indoor temperature. In winter, the home will absorb heat, keeping it inside where it is needed. A passive solar home may include a separate heating and cooling system, but this may only need to be used for two weeks or less per year.

Because the passive solar system requires such extensive planning, you’ll want to make sure that the design features you choose work best for you, and to enlist an experienced architect to develop the design. Without expert attention to detail, your passive solar house plans may not be as efficient as you’d hoped.

Choosing the Site For Your Passive Solar Home

Because this set of strategies takes advantage of the sun’s energy, choosing the site for your home is important, especially in a place like the Pacific Northwest. The south face of the site should have unimpeded access to sunlight. This means building on the northern sides of slopes, avoiding tall buildings or stands of tall trees on the southern side of the site.

Keep in mind the future development of the area. New construction can obstruct your home’s access to the sun, and small trees (or even new trees!) can also block your access to the sun.
You can also protect your access to sunlight by selecting a lot that’s deep from north to south, and building on the north end of the lot. Having most of the lot to the south of your home means that you get to control what is built or grown there.

An architect can assist you with site selection and understanding the zoning rules and regulations that could affect your access to the sun.

Passive Solar Home Design Basics

There are a few concepts that are important to understand when choosing passive solar design features to add to your home.

  • Aperture or Collector: This is the feature that collects energy from the sun. It is typically a large glass area on the south side of the building
  • Absorber: This is typically a hard surface on top of the thermal mass. It’s usually dark in color to make it more effective in collecting solar energy.
  • Thermal mass: A material that is used to store solar energy in the form of heat. These are usually concrete, masonry, or stone because these materials are able to store a lot of heat.
  • Distribution: This is the method by which the solar energy is then distributed through the house as heat.
  • Control: These are features of the home that prevent over-heating or under-heating due to seasonal changes. These controls may be automated or manual in nature. (See below for a more in-depth explanation of passive solar controls.)

Passive Solar Design Strategies

Most passive solar homes employ some deceptively simple methods to improve heating and cooling efficiency. These are divided into two different strategies: direct gain and indirect gain.

Direct gain is a passive solar strategy that uses the living space in the home itself as the absorber and thermal mass of the passive solar system. Homes employing direct gain passive solar systems will use windows as their collector. These windows need to face within 30 degrees of true south in order to be effective. The air inside the home, or a masonry or concrete floor, will act as the absorber, and the floor will often be dark colored, to make the most of the energy coming in the window.

Indirect gain is a passive solar strategy that uses an external thermal mass rather than sending the heat directly into a home. This means that there will be a stone, concrete, or masonry wall facing south. The wall then acts as the thermal mass, taking the heat and radiating it back into the house. Using indirect gain means that heat from the sun collected at noon will radiate into the house in the cooler evening, as it takes heat roughly an hour to penetrate an inch into the thermal mass. Indirect passive solar systems may have a single or double pane of glass over the thermal mass wall to concentrate the solar energy. In these cases, the pane should be about one inch from the passive solar wall, and the wall is 6-18 inches thick. This is also known as a Trombe wall.

Isolated Gain is a strategy through which you isolate the collection of solar energy to one room in the house. This is typically accomplished through the use of a sunspace, such as a sunroom or solarium. One benefit to this strategy is that you can close off the room to keep the heat out of the rest of the home, or open it to bring the heat into the home, so you have a little more control over heat distribution.

Passive Solar Controls

Control is a strategy for limiting the energy collected by your passive solar system in times when you may not want the extra heat, like hot summer days. This can also be used for solar cooling.

One of the simplest ways to implement a control system on your passive solar home is to place shades over the collector, usually windows, but devices like the Trombe wall can also be shaded in this way. These shades are often overhangs and awnings, carefully placed to block summer sun, but not winter sun. The sun rises higher in the sky in summer, at least for the northern hemisphere, so a properly placed and angled overhang can shade your collectors on summer days. Meanwhile, the lower sun in the winter will still be able to access your collectors. In addition, shaded thermal masses can actually absorb heat from inside the home on hot days.

Another control strategy is natural ventilation. Placing vents on the sides of the home both facing toward and facing away from the prevailing winds promotes a cross breeze through the home, bringing fresh air in and evacuating stale or overheated air. In summer, for example, you can open vents to cool the home at night, and then close them to keep the heat of the summer day out. Wing walls can be placed to encourage ventilation even through windows that are perpendicular to prevailing winds. Wing walls are vertical exterior panels placed between two windows. These panels capture and accelerate natural winds by using pressure differences created by the wing wall.

For areas where there are no prevailing winds, you can still use ventilation to cool the home. Placing a high vent where the heat collects, and placing a low vent, the home can release hot air through the high vent and draw in cool air through the low vent.

Passive Solar Lighting Design

Passive solar lighting is the process of bringing more natural daylight into the home. This can be a byproduct of your direct gain passive solar heating system, but is also often achieved through the use of skylights.

Skylights create less glare than windows in the walls of a home. Skylights for passive lighting are best placed for southern exposure, so that they can allow the light to penetrate the northern side of the home, where the windows let in less light.

Light shelves can be used for passive solar lighting through windows. Light shelves are usually horizontal interior structures placed to divide the viewable portion of the window from the portion that lets in additional sunlight. The purpose of the light shelf is to bounce sunlight to the ceiling of the home, allowing the light to penetrate deeper into the home and maximizing usable daylight.

Exterior light shelves can also bounce lighting into the home, while also providing shade to the windows below it.

Baffles are vertically oriented surfaces placed inside the home. These are usually made from or covered with fabric to help evenly distribute the daylight. These types of passive lighting features also reduce glare, making the passive lighting system more pleasant for the user.

Not only does passive solar lighting decrease the need for electric lighting, but it increases exposure to daylight, improving mood, decreasing daytime drowsiness, increasing productivity, and helping to balance circadian rhythms.

Should You Choose Passive Solar Home Design?

There are a few things to consider when deciding whether to use passive solar heating or lighting for your home. One is whether this is for a new home, or retrofitting an older home. A lot of these methods are difficult to install in an existing home, and may require a complete tear-down and rebuild of the existing home. This is especially true if your home isn’t already designed for energy efficiency. Passive solar heating will be less effective in homes with a lot of air leaks. Additional windows, skylights, and thermal masses are difficult or impossible to add into an existing home.

Since much of passive solar use depends on the design, placement, and orientation of the home, it’s much easier to implement passive solar in a new home.

Climate impacts passive solar systems as well. While your passive solar system can still collect solar energy on cloudy days, it will collect less, making your heating and lighting system less effective. This is true also for homes in very northern areas, where the sun only rises for a few hours in the winter, when you need the solar heating the most.

Including passive solar design in your house plans can also increase the cost of construction. With additional features to manage both light and heat, the process uses more materials and requires precise placement and specific materials.

Passive solar design elements on the exterior or interior of your home may create a look that doesn’t match your personal preference. There are ways to make these features look elegant and beautiful, but there’s not a way to hide them altogether.

So is passive solar design for you? That depends on where you live, whether you’re building a new home, and on your own personal preferences. The benefits are great, though. Passive solar design can create a more efficient and healthier home for you and your family.