What Is Passive Solar Design?

Passive solar design is nothing new — it has probably been around as long as people have been building houses — and it has nothing to do with solar panels. It’s all about creating a comfortable space by optimizing the use of the sun’s energy.

As solar installer Ben Zook explains: “Any system that uses a mechanical pump, an inverter, or other electrical conversion device is not truly ‘passive.’ That means that the only remaining way to have a truly passive solar system is by using the greenhouse effect… the same as an actual greenhouse!”

The transparent walls of a greenhouse allow radiation from the sun to provide indoor climate control, and while that’s a well-known example of passive solar design, it isn’t the only one. I sought help from a panel of home efficiency experts to get a clear idea of what defines passive solar design and how to use it effectively to stay comfortable and save energy.

What Is Passive Solar?

According to environmentalist Aidan Charron: “Passive solar refers to the use of natural sunlight to heat and light your home without using solar panels or arrays or anything mechanical. Instead, you use natural sunlight and exposure to the sun itself to organically capture heat and light inside your home. It’s achieved through clever design features and pre-planning.”

Elements of Passive Solar Design

We all use passive solar already, says Zook. “Imagine living without any sunlight, and you might understand how we ALL use sunlight. Most of it is passive, in one form or another.” When it comes to optimizing the use of solar energy in construction, builders use five strategies:

Orientation

The main wall of the building should be facing in the direction of the sun to maximize heat absorption. In the Northern Hemisphere, this means that it should face in a southerly direction and avoid, as much as possible, trees and landforms that block the sun.

Thermal mass

The materials you use to construct the building should be able to absorb heat and sunlight. Materials such as concrete, brick and stone — and even water stored in barrels — are very effective. They can absorb a lot of heat during the day and release it at night to help keep the building warm. “How well thermal mass works depends on its bulk, thickness, and where it is placed,” advises underfloor heating expert Gordon Chalk. “For it to work best, it needs to be either connected to a warm place or put in full sunlight.”

Windows/glazing

Windows that face south help maximize solar exposure. High-performance windows with low-E treatments and gas fills are best, says Chalk, because they help retain heat. As he says: “Picking windows is important! For windows facing south, choose SHGC (Solar Heat Gain Coefficient) numbers that are high to get the most heat from the sun. To help keep your home cooler in the summer, think about lowering the SHGC of windows that face east and west.”

Insulation

Insulation is a no-brainer. It helps maximize heat retention during the day and minimize heat loss at night. “Focus on continuous insulation to cut down on thermal bridging, which are spots where heat can easily leave,” says Chalk. “Because they often have weak spots, pay extra attention to the areas around doors and windows.”

Shade and ventilation

You need to optimize the use of shade and ventilation if you want your house to stay cool in the summer. Chalk advises using overhangs, awnings, trees, and even well-planned planting to help block the sun. Adjustable shade is best. “When curtains are pulled back in the winter, they let more light in. In the winter, deciduous trees lose their leaves, and in the summer, they provide shade, which lets the sun shine on the building.” It’s also advantageous to deploy fans throughout the house to keep air circulating.

Types of Passive Solar

Builders can approach passive solar design in four ways:

Direct gain

This is the most popular passive solar technique. The most obvious example is orienting windows toward the sun, thereby allowing the sun’s radiation to enter the living space directly and heat up the thermal mass in the walls and floor. The downside is that direct gain techniques, when not used correctly, cause glare and burning, so they need to be used in conjunction with shading solutions.

Indirect gain

The most common indirect gain mechanism is called a trombe wall, which is a dark-glazed wall, usually eight to 16 inches thick, placed behind a glass barrier with a 3/4-inch separation. “The wall that gets hot from the sun moves the heat into the sitting area,” explains Chalk. “The more controlled way that trombone walls let heat out is not possible with direct gain systems. On top of that, they protect against pretty big changes in temperature.”

Isolated gain

Charron explains: “Isolated gain is when the heat hits a sun room or even a greenhouse and then radiates into any adjoining rooms through vents or internal windows.” One of the advantages of this technique is that you can use it on an existing structure that hasn’t already been optimized for solar gain.

Roofing

So far, we’ve focused on walls, but roofer Brian Davis adds this: “Roofing is a key element of passive solar design, playing a crucial role in regulating heat absorption and reflection…Lighter-colored roofs reflect sunlight, reducing heat absorption and keeping homes cooler, while darker roofs retain warmth to help with heating.”

Does Passive Solar Really Save Money on Utilities?

Yes. All the experts I interviewed would agree with this assessment from Zook: “It can and usually does. Especially when done right, passive solar design will save you money on utilities.” According to Davis, even installing a new roof can cut energy costs by 10 to 15 percent.

Passive Solar Pros and Cons

Pros

• Energy savings
• Low maintenance
• Eco-friendly
• Improved comfort

Cons

• Dependent on climate
• Temperature fluctuations that can be difficult to control
• Design complexity leading to high up-front costs
• Usually requires additional active heating

FAQ

What is the difference between passive and active solar energy?

Passive solar design relies on building placement, materials and the surrounding landscape while active solar is concerned with techniques for converting sunlight into energy. Chalk puts it this way: “Active solar is more about technology, while passive solar is more about how it is set up.” Solar panels passively collect sunlight, but because they convert it to electricity, they are part of an active energy system.

Is passive solar heating efficient?

“It can be if you design your home to maximize sunlight capture and have a system to block the sunlight when it gets too warm,” says Charron. “It’s a smart way to naturally make our homes more energy smart.” All panel members agreed.

Is passive solar better for heating or cooling?

Generally, passive solar is better for heating, but if you use shade and ventilation properly, it can also help keep your house cool. One effective cooling technique, according to Chalk, is to open windows at night to reduce thermal mass.

About the Experts

• Ben Zook is the owner and NABCEP Certified Master Electrician at Belmont Solar. He specializes in solar installation, including grid-tied, off-grid, and solar battery backup systems.
• Brian Davis is the AIA, LEED AP, GRP, and Technical Support Director at Brava Roof Tile.
• Gordon Chalk is the managing director at Next Level Underfloor Heating and Screed Solutions LTD, based in the UK.
• Aidan Charron is the Associate Director of Earthday.org and is a global leader of the Planet vs Plastics campaign.

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