What Is Passive Solar Heating? How It Works and Limitations

A passive solar-heated home needs no solar panels to heat or cool it. Rather, the energy used to heat and cool a house comes directly from the sun through skylights and windows. Some of that energy is then stored in the building's walls and floors to be used at night and in cooler months.

With good insulation and ventilation, proper materials, and thoughtful home design and siting, it's possible to reduce heating and cooling costs partially or, in some cases, completely. Paired with an air-source heat pump fueled by solar electricity, passive solar can help homeowners reach net-zero heating and cooling.

How Passive Solar Heating Works

One of the key virtues of a passive solar-heated home is how passive it is. Once the elements of a passive solar heating system are created, the home heats itself, quietly and with little human intervention. Here are some important considerations.

Energy Efficiency

The cheapest form of energy is the energy you never use. The first step in designing a home for passive solar is investing in energy efficiency.

The keys to maintaining a passive solar home are well-sealed doors and windows, double or triple-pane windows, highly efficient appliances and water heaters, and excellent insulation. By itself, a well-insulated home can save up to 20% of a home's heating and cooling costs. A tightly sealed home is also a cleaner home, more resistant to air and noise pollutants, pests, viruses, and bacteria.

Good Siting, Good Windows

In the Northern Hemisphere, south-facing windows gain maximum exposure to the sun when unobstructed by trees, multi-story buildings, or other buildings. (Work with neighbors on a solar easement to increase your solar exposure.) For heating, six hours of direct sunshine in the middle of the day is recommended. For cooling in the warmer months, window shades, awnings, or other coverings help keep the house cool.

In colder climates, where southern exposure to the sun is more limited, tilted rather than vertical glass (such as in skylights on a sloped roof) prove more effective. Even in coastal climates, the diffuse solar radiation created by regular cloud cover can provide significant levels of heat that well-tilted windows can capture.

Triple-glazed windows are increasingly common, especially in new building construction. The space between glazing layers is often filled with inert, harmless gases that minimize heat loss.

Specially treated windows can also increase window temperatures by up to 15 degrees F in cold weather, further reducing heating costs. Of course, it also helps to keep windows and skylights clean.

Good Air Circulation

You don't need to understand the second law of thermodynamics to know that heat flows from hot to cold. Just as hurricanes and cyclones move away from the equator toward the poles, warm air will circulate around the house into colder areas.

Truly passive houses just allow entropy to take its course, without the use of mechanical or electrical devices to circulate air. Other houses designed with passive solar heating and cooling might use fans, ducts, and blowers. “Thermal bridges,” such as walls made of building materials that are highly conductive, also allow heat to pass from one room to another.

Air circulation also brings in filtered fresh air from outside with minimal heat loss (or gain). Proper air circulation is also important to reduce condensation and keep the house free of mold.

Heat Storage

Known as thermal mass, the materials used to build the home (such as concrete slabs, brick walls, tile floors, and drywall, but also home furnishings) absorb heat from the sun and release it into the house at night or during cold months. During warmer months, thermal mass absorbs and stores heat from inside the home when cooling is needed. Darker-colored floors or walls absorb more heat than lighter colors.

Thermal mass is what stabilizes temperatures inside a home. Unlike homes that burn a significant amount of fuel to raise a room temperature on a cold morning from 63 to 69 degrees F, rooms in a passive solar home have smaller fluctuations in temperatures. It is much more energy efficient to raise or lower a room temperature one degree than six degrees, of course, so keeping a home within a stable temperature range uses far less energy.

Temperature Controls

The temperature inside passive solar-heated homes depends as much on the outside temperature as it does on the amount of solar radiation coming in. A sunny winter day can be warmer indoors than a cloudy day in late spring. Likewise, a sunny day in early spring can be hotter indoors than a cloudy day in mid-summer.

Heat controls help moderate these differences: A louvered vent in a roof can dissipate excess heat, while a pergola or awning over windows with southern exposure can provide seasonal shading. Likewise, tall shrubs used as a privacy hedge can block winter winds. Smart planning means most of these controls are self-regulating and need little human intervention.

Limitations of Passive Solar Heating

While passive solar heating and cooling works better in some places than others, its efficiency and simplicity mean it works in more places than expected. Still, there are limits.

Sustained, Not Immediate, Heat

Passive solar homes work by maintaining a comfortable living space, not by providing instantaneous heat on demand. While well-designed houses in prime locations can sometimes solely rely on passive solar heating, most passive solar systems act as base-load heating, while mechanical systems (heat pumps, electric heat, wood stoves, etc.) are used to provide heat on demand.

Location, Location, Location

As with everything in real estate, location matters. While heating a home in winter in northern latitudes may require supplemental heating, cooling a home in summer in southern latitudes may require supplemental cooling. Building design needs to fit the climate.

In northern latitudes, however, homes tend to have sloping roofs, which increases their exposure to solar radiation from the upper portions of the skydome. A roof shaped like an inverted V can catch more hours of direct sunlight than a flat roof can.

Upfront Costs and Payback Times

Building or retrofitting a building can be expensive, but not as much as one might think. Building a new passive solar home only adds up to 3% to 5% of the construction cost. In the United States, 1.4 million new homes are built every year, so there is always a large potential market for passive solar heating and cooling.

Since most of the benefits of a passive solar-heated home come in the design and construction of the building, it is harder (and more expensive) to retrofit a home for passive solar than it is to start from scratch.

The return on investment comes from reducing heating and cooling bills, depending on the amount of sun exposure the building gets and the often-variable price of heating and cooling. In a northern climate with a retrofitted building that reduced its energy consumption by 45%, the return on investment was as little as 7.7 years. In a more temperate climate, the payback period was 13 years.

In either case, the long-term benefits outweighed the costs.

Outlook on Passive Solar Tech

The technology used to support passive solar heating keeps improving, with new glass materials, glazing processes, more efficient insulation, digital tools to measure solar radiation, window performance, and energy use making it easier to plan and design a successful passive solar heating system. Getting to a net-zero home keeps getting easier.