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What Are The Key Principles Of Passive Solar Homes?

How to make the most of solar with home design

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Geoff Edwards
Guide

Keeping our homes warm or cool uses a lot of energy. The average family home in the US spends about $1,900 per year on energy, with about half this on heating and cooling. Thus the average spend on heating and cooling is about $950 per year, and much more with larger homes.

Passive solar homes aim to reduce this energy through clever features and design. When combined with comprehensive solar arrays (which can be made quite affordable thanks to tax incentives and net metering), passive houses can even achieve status as Zero Net Energy (ZNE) homes.

5 Principles of Passive Solar Homes

There are five basic principles of passive solar homes. The first is the ‘aperture’ or the place where sun gets in. In the northern hemisphere these are south-facing windows or glass doors. The winter sun comes through the aperture and hits the second element, an absorber. The absorber absorbs the light and changes it to heat. It then transfers the heat to the third element of a passive solar home, the thermal mass. Thermal mass can store large quantities of heat when the heat is available, then release it when the temperature drops. The fourth principle is distribution, i.e. how the heat is moved around the home. The fifth principle is control. An example of control is an overhang on the roof. The overhang can block sun entering the house during hot summer months, as shown in the illustration below

The Aperture

Obviously there should be as much glass as possible on the south facing wall. Windows and doors that collect sunlight should ideally face within 30 degrees of true south and not be shaded during winter by buildings or trees from about 9 am to 3 pm each day. Windows and doors should be shaded during spring, fall and summer to avoid heating.

At night-time, curtains or blinds should be used over glass to prevent too much heat escaping. In very cold climates double-glazed windows and doors can be used to prevent heat loss.

The Absorber and the Thermal Mass

In many instances the absorber and the thermal mass can be the same material. Darker colors absorb more heat than lighter colors and hence are a better choice for the absorber. This is the same reason most solar panels tend to be black in color. Thermal mass must be externally insulated (to prevent heat loss) and internally exposed (to allow transfer of heat into the house).

In a passive solar home the thermal mass is commonly concrete, brick, stone, or tile. These materials can absorb a lot of heat from sunlight during the day and slowly release it during the evening and night. There are other more sophisticated (and expensive) thermal mass materials, such as phase-change materials, however masonry has the advantage of doubling as a structural and/or finish material.

Thermal mass can be incorporated into the design of a house. In well insulated homes, the inherent thermal mass in home furnishings, walls and floors may be sufficient, eliminating the need for additional thermal storage materials. Objects should not block sunlight from hitting the thermal mass.

One very effective type of absorber and thermal mass is a solar wall, or Trombe wall. The wall, made of masonry or even metal, is placed an inch or two behind a glass wall. Sunlight comes through the glass and heats the wall. The heat travels through the wall and then radiates into the house. Heat travels through masonry at a rate of about 1 inch per hour. Thus for an 8 inch thick wall, heat created in the middle of the day will reach the interior of the house at about 8 pm. Gaps can be provided to aid air circulation and heating by convection.

Sufficient amounts of exposed internal thermal mass can ensure that temperatures remain comfortable all night and even through successive cloudy days, if well designed. This is due to a property called ‘thermal lag’. Thermal lag is the amount of time taken for a material to absorb and re-release heat.

Thermal lag times are influenced by:

  • temperature differentials (ΔT) between each face
  • thickness
  • conductivity and density
  • texture, colour and surface coatings
  • exposure to air movement and air speed.

A well designed thickness of thermal mass is the thickness that can absorb and re-release heat during a day−night cycle. For most building materials this is 50−150mm depending on their conductivity. Longer lag times are useful for lengthy cloudy periods.

External wall materials with a minimum time lag of 10 to 12 hours can effectively even out daily temperature variations. Extremely high amounts of thermal mass, eg. earth covered houses, can even out seasonal temperature variations.

Distribution

Various distribution mechanisms can be used to transfer heat around the house using conduction, convection and radiation. Conduction is heat transfer between two objects that are in direct contact with each other. Convection is heat transfer via movement of a fluid, such as air or water. (Solar water heaters, for example, often work by convection.) Radiation is what you feel when near a hot surface. All these mechanisms can be used in a passive solar home. For example walking on the floor can warm your feet (conduction). Fans ducting can move air and heat around the home (convection). And warm thermal masses can radiate heat outwards.

Convection currents in particular need to be well managed. These are created when warmer air rises to the ceiling, and air cooled by windows and external walls falls back down and along the floor to the heat source. With careful design, convective air movement can be very beneficial. Poor design, on the other hand, can cause a major source of thermal discomfort.

Controlled convection can be used to heat rooms not directly exposed to heat sources. Vents that can be opened or closed can help achieve this. Openable panels over doors control movement of air while retaining privacy.

For multi-story homes, upper levels should be able to be closed off to stop heat rising in winter. Stairs can be used to direct cool air back to heat sources. Open rails on stairwells and balconies should be avoided as that allow cool air to fall like a waterfall into spaces below. Ceiling fans can be used to push warm air back to lower levels.

Control Strategies

Control strategies include properly sized roof overhangs that give shade to south-facing windows and doors during summer months. Other control elements include electronic sensing devices, for example a temperature sensor that can turn fans on, controllable vents that allow or restrict heat flow; low-emissivity blinds, operable insulating shutters; and awnings.

A rule of thumb for overhang widths, reliable for latitudes north of about 27.5 degrees, is shown in the figure below.

A diagram shows the rule of thumb for calculating the width of eaves, with reference to the height from the window sill to the bottom of eaves. To correctly angle the summer and winter sun, the outer extent of the eaves should be 45% of the height from the window sill to the bottom of the eaves. In addition, the height above the top of the window pane stretching to the base of the outer extent of the eaves should be at least 30% of the height of the window sill to the bottom of the eaves. These measurements
Rule of Thumb Calculation for Overhead Roofs, for latitudes north of 27.5 degrees.

The location of the home onsite can be critical. For example, if trees are present that block the sun during winter then passive heating will obviously be reduced. Orienting the house on the block to face as south as possible is also obviously important, just as you would need to orient solar panels to receive as much direct sunlight as possible.

Insulation

High insulation levels are essential in passive solar houses.

Ceilings and roof spaces account for 25-35% of winter heat loss and must be well insulated. Most of insulation should be placed next to the ceiling. Floors account for 10-20% of heat loss. In cooler climates the undersides of suspended timber floors and suspended concrete slabs can be insulated. The edges of ground slabs should be insulated. Walls account for 15–25% of winter heat loss. Insulation in walls is often limited by cavity or frame width. In cold climates, alternative wall systems that allow higher insulation levels can be used.

In high mass walls such as double brick, rammed earth and reverse brick veneer, thermal lag slows heat flow on a day−night basis. Insulation is still required in most instances; strawbale walls are an exception as they have a high insulation value .

How Effective are Passive Solar Homes

Passive solar buildings can create energy savings of up to 90% for heating compared with traditional buildings, and over 75% compared with the average modern, best-practice buildings. Importantly, warm climates that require more energy for cooling than for heating can achieve similar savings.

Solar passive homes may cost about 5% more to build, however they will clearly pay that back in no time, then deliver significant energy savings for years to come.

When combined with residential arrays of alternative, renewable energy generators, such as wind or solar, passive solar homes may cover their every energy need without relying on the grid at all.

What if My House Doesn’t Face South?

Don’t fret. There are some modern-day designers that argue that passive solar design is less relevant today than it was 30 years ago when it began to be popular. This is because of the falling price of solar power. For a house that doesn’t face right, putting up solar panels can allow heating (or cooling) during the day at little cost. However several of the passive solar design principles can still help, in particular thermal mass, distribution and insulation.

Conclusion

It’s relatively straightforward to incorporate passive solar features into a home, and doesn’t cost much extra. These features can save up to 90% of energy used for heating or cooling. Given that the average US household spends about $950 each year on heating and cooling, it would seem that passive solar houses are an incredibly smart investment.

FAQs

Do I need to have a wall that faces south?

The short answer is yes. Windows and doors work best when oriented within 30 degrees of south.

What other elements are important in solar passive design?

After the sun comes in, it needs to be converted to heat by an absorber, then the heat needs to be stored in thermal mass. The heat then needs to be distributed around the house. Control elements like roof overhangs to shade windows in summer are also important.

How effective are solar passive houses?

Solar passive houses can be very effective, saving up to 90% of heating and cooling costs.

Do solar passive houses cost more?

Yes, but not much. Around 5% more depending on where you live.

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Author Bio

Geoff Edwards has worked in the renewable energy sector for more than 15 years, initially at the forefront of lithium ion battery technology, and more recently in solar power combined with energy storage. He has over 15 patent applications in various fields. Geoff has a degree and PhD in engineering from the University of Queensland in Brisbane, Australia.

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