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Don’t Pass on Passive Solar Heating

A home using passive solar radiation for heating

In short: Passive solar heating is a holistic, “natural” way to heat and cool your home using solar energy without investing in panels or machines. Using good old windows, flooring, and some clever thermodynamic trickery, you can capture sunlight to heat your home, and even “store” it using your home’s surfaces so you can use it later when the temperature decreases. This method can decrease your energy usage dramatically. Combining these techniques with photovoltaic solar panels can eliminate your power bill for less!

What is passive solar heating?

We’re talking a little feng shui today. Much like placing items and furniture in a highly organized arrangement to regulate the flow of (figurative) energy through your home, passive solar heating uses similar principles to regulate the flow of solar radiation (i.e. heat).

Passive solar heating is the act of leveraging a home’s orientation to the sun, the local climate, and the home’s materials to dramatically lower your energy usage while capturing sunlight during the day, and either storing or releasing that energy at night.

In this case, sunlight flows through your home naturally through strategically-placed windows and smart home layouts — without using solar panels. With some thought, a home or condo developer can place open spaces and windows to ‘follow’ the sun, lighting and heating the home during the day.

At night, that heat can be retained with special absorbers (more on that later), or it can be gradually dispersed as the temperature cools. Again — stored heat without machines!

During the winter months, when heating energy costs are the highest, you can reduce your power bill sharply by employing a system that captures heat during the day, and retains it at night.

If you use natural light while you’re at home and leave your lights off, you’re already performing some form of passive solar heating. You’re reducing your energy usage from coal and fossil fuels while capturing the free energy of solar radiation!

The Department of Energy has a great rundown on its support for passive solar heating.

What are the pros and cons?

Pros:

  • Passive solar heating requires very little upfront cost on newer homes. We’ll dive into each component in more detail.
  • Designing apartments with it in mind can dramatically reduce energy usage for multiple people at once.
  • If your roof or location isn’t ideal for solar panels, you can still reduce your energy usage and harness renewable energy with passive solar heating.

Cons:

  • The intellectual and engineering lift of creating a passive system takes professional work and installation. This isn’t necessarily a con, because any major update to your home should involve professionals anyway.
  • Older homes might not have the proper insulation needed to store energy captured during the day. We strongly recommend getting an energy audit done before upgrading to passive solar or installing solar panels.
  • If your home is surrounded by large trees and/or steep hills, it might not receive the amount of sunlight to make a huge difference. Anything that helps you keep the lights off certainly helps, but to offset your energy usage almost completely, south-facing access to sunlight is most ideal.
Passive solar heating system with solar panels and south-facing windows

This passive solar heating system features south-facing windows and control shading.

What mechanisms make passive solar heating work?

Even though passive solar heating doesn’t require machines, there are some mechanisms that make it work. Some you may already have in your home, and some require installation:

  • Apertures are essentially windows or open spaces that have complete, or close to complete, access to the sun, ideally south-facing. Windows specially designed for solar heating typically have a special coating that reflects ultraviolet (UV) rays away from your home.
  • Absorbers and thermal mass are materials — such as brick, stone, and tile — that absorb energy captured during the day and store it. Thermal mass can also absorb heat from the warm air during the colder seasons.
  • Distribution of heat happens in three primary ways, sometimes aided with fans and blowers. Conduction happens when heat is transferred between two objects in direct contact, like your bare feet on a hot floor. Convection happens when heat is transferred via air or water. Warm air will naturally flow to cooler areas. This is why your food is more thoroughly cooked in a convection oven than in a standard microwave. Radiation occurs when you feel the heat from sources around you, like your skin on a hot day.
  • Control mechanisms like awnings, overhangs, and blinds provide custom control over the amount of heat entering your home. This is important in the summer months, when things are generally hotter anyway!

How can I install it in my home?

There are four major methods to passive solar heating; while older homes and large, busy spaces might not be suitable for this type of solar power, you can consult with an environment-oriented construction or renovation contractor in your area to analyze your options.

A quick rundown on the four methods:

  • Direct gain is the most straightforward method. Sunlight enters south-facing windows, and the energy is stored in thermal mass described above. It is recommended that no more than 12% of your home’s total area should have direct gain glass; past that point, you risk too much heating and fabric fading.
  • Sun-tempering is a more modest approach to passive heating. Usually, no more than 3% of the home’s total area has sunlight-gaining glass.
  • Indirect gain, or a Trombe wall, is a south-facing wall built of heavy masonry (brick) that absorbs sunlight during the day, and slowly releases it at night.
  • Isolated gain is sunlight gained in an isolated space, such as a sunroom, that can be closed off from the rest of the home. Fans can then move the warm air to other rooms as needed.

When considering passive solar heating, it’s best to analyze your home’s energy efficiency, including insulation, air sealing, and window location. While renovating or designing a home for passive solar heating takes some clever strategy, the work pays huge dividends with steep price drops in your power bill!

Home exterior with passive solar heating

The Passivhaus Movement

No, this isn’t an EDM rave party in an abandoned Berlin warehouse. But it does originate in Germany.

Passivhaus is passive solar heating kicked up to the highest notch, but comes with the lowest energy usage. According to the Passivhaus Institute, it is “a building in which thermal comfort can be achieved solely by post-heating or post-cooling the fresh air flow required for good indoor air quality, without the need for additional recirculation of air.”

For a home to adhere to the rigorous standards of Passivhaus, it must pass certification. A home or building with Passivhaus certification has achieved a massive 75% reduction in heating and cooling costs! In the UK, certification meets government’s goals in an 80% reduction in carbon emissions.

To meet the standards, a home (according to the site):

  • Has been designed according to the Passive House Planning Package
  • Has very high levels of insulation
  • Comes with high-performance windows and insulated frames
  • Has ‘airtight’ building fabric
  • Has ‘thermal bridge free’ construction
  • And has a mechanical ventilation system with highly-efficient heat recovery
Map of USA with Passivhaus certified installers

Passivhaus-certified installers are located all around North America.

You can search for a certifier in your area here. You might want to translate the page or download Duolingo first, as the site is in German. Web browsers like Chrome and Safari should auto-translate for you.

How much does solar heating cost? Can I offset all of my energy usage?

Coupled with solar panels, you can easily achieve a $0 power bill with passive solar heating. Not only that, but because you’re reducing your heating and cooling costs, you can invest in a smaller solar panel system, saving you even more money over time.

If you’re designing a new home, it’s absolutely worth assessing your plan and design to leverage the awesomeness of passive heating. You reduce your new home’s impact on the planet, and save money.

You may have heard about active solar heating. Essentially, active solar heating is the process of using solar energy to heat a liquid or air for later use — think your shower later on the day. For water heating, a series of tubes or collectors filled with water or antifreeze absorbs solar energy, then the water is transferred and stored through your home. Active air heating is similar, and is generally seen as more efficient since you don’t have to worry about leaks.

Now back to passive solar heating.

Hypothetically, if you’re in the market for a 6-kW solar panel system, which at $3.50 per watt (there are 1,000 watts in a kilowatt), will cost around $21,000, before any incentives. With the 30% federal solar tax credit, that drops to $14,700.

But let’s say you’ve designed your home for passive solar heating. That means you can reduce your heating and cooling energy usage anywhere from 25% to 75%. Around half of the typical American’s power bill comes from heating and cooling, so you’re essentially cutting your energy needs by somewhere around 25%. That means you can invest in a 4.5-kW solar panel system. At $3.50 per watt, you’d pay get $15,750. With the federal tax credit, your solar panel cost would drop to $11,025!

If you’re planning to renovate instead, you can see whether it would be worth it to choose passive solar heating or a larger solar panel system by simply look to see whether the passive solar improvements you’d make would cost less than the difference between the cost for the larger and smaller systems—in this case, about $3,700.

This can make investing in renewable energy much more affordable for many more people. And building designers have taken note: more and more homes and buildings going up in 2019 have passive solar heating principles applied. Times are a’ changing, and the times are good.

Last modified: August 12, 2019

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Debra Coleman, Architect
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What an informative article on passive solar! I especially like how passive solar savings are shown to reduce the active solar/PV needs. Thanks! And do check out the passive solar house plans by Sun Plans. Internet-based custom design services for those all over the U.S. are also available.

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