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Passive solar energy warms the outside of an apartment building

Step outside on a hot and sunny summer day, and you'll feel the heat and light on your skin. After a while, the rest of your body will begin to warm up and feel that heat and light too.

Well what you’re actually feeling is passive solar energy. Your body is capturing that heat from the sun, storing it away, and then redistributing it to other parts of the body that need cit. Similarly, this is how the concept of passive solar design is applied to buildings. It quietly takes advantage of the natural environment, utilising the surrounding climate to maintain a comfortable interior temperature in an eco & wallet friendly way.

Modern architects & green planners strive to employ passive solar design; creating buildings that capture, accumulate, and distribute solar energy; lowering heating and cooling costs while reducing that building’s overall carbon footprint.  Today, more and more new buildings are going green; passively designed from scratch to take better advantage of our natural resources.

However, utilising passive solar is nothing new! The house is naturally warmed already by sunshine coming in through windows and wall.

Passive solar design can take this even further, specifically optimising energy derived directly from the sun, through the careful planning of the building’s layout to collect the sun’s heat. Equally, precise deliberation over building materials can further reduce the need for heating and cooling systems and artificial lighting. This can make a big impact on the homeowner’s electricity bills, as heating and cooling accounts for roughly 40% of energy use in the average Australian home, as well as their long-term environmental impact.

So how does passive solar design work?

From a basic starting point, paying attention to the principles of good passive solar design is crucial. The importance of being aware of the environment surrounding the building, as well as the orientation of the home itself cannot be overstated. For this reason, the most cost effective time to achieve effective passive solar design in a home is when initially designing and building it. However, considerable renovations to an existing home can also offer the opportunity to invest in passive design strategies.  For best results, passive solar homes need active users — those with an understanding of their typical energy usage and the daily and season climate around them.

If you’re a homeowner interested in passive solar design, there are a number of different ways to utilise it, and we’ve broken down the main few methods for you.

Where you live

The climate you live in may be vastly different to someone else’s and thus what design might be helpful for one person may have the opposite effect for you. Researching the climate characteristics of your location will allow you to identify the most appropriate design features for your home.

The Nationwide House Energy Rating Scheme (NatHERS), with its star classifications, is a great resource for this, and contains plenty of helpful information about different climate zones and their related energy efficiency.

Shading

We’ve sung the praises of correct shading for awhile now, but thoughtfully shading your house and outdoor living areas can truly dramatically reduce heat and save you big on energy. Direct sun can generate the same heat as a single bar radiator heater over each square metre of a surface. Shading can block up to 90% of this heat.

Consider placing trees and shrubs along outside walls to reduce the sun’s heat entering your home and lower the temperature naturally. You can also use artfully placed plants to redirect cool winds into your home but be careful not to deflect breezes away from your house!

Eves and louvres

Eaves are also an essential asset for shading windows and walls outside your home and reduce the heat load entering the building. Fixed horizontal louvres set to the noon midwinter sun angle and spaced correctly allow winter heating and summer shading in locations with cooler winters.

Midwinter and midsummer noon sun angles for locations can be calculated using the formulas below, where L is the latitude of your home.

  • Midwinter noon sun angle = 90 – (L+23.5)

  • Midsummer noon sun angle = 90 – (L–23.5)

  • Equinox noon sun angle = 90 – L

Passive solar design operating through eaves and louvres

Skylights

Surprisingly, even something as common as a skylight can positively impact a house’s energy consumption. They can be a brilliant source of natural light, meaning you don’t have to switch as many lights on, even during winter or overcast weather.

However, it can be important to balance it out with another passive solar element, as skylights can sometimes lead to heat loss in the colder months of the year.

Orientation

Orientation refers to the direction on your home faces on its site and attention to this in the building phase will let you take advantage of climatic features such as sun and cooling breezes. For example, in all but tropical climates living areas would ideally face north, or as close to north as possible, allowing maximum exposure to the sun, and easy shading of walls and windows in summer.

Well considered orientation reduces the need for supplementary heating and cooling and can improve solar potential for solar PV panels or a solar hot water system! So, make sure you do your googling on summer and winter variations of the sun in your area!

Direct Gain

The simplest and most basic form of passive solar design can be achieved through the windows on the side of your home facing the equator. As most of the sunlight will come in through these windows, leaving them unshaded will raise the temperature by several degrees.

If the sunlight strikes dense materials such as masonry (that have efficient thermal mass) inside your home, these surfaces will absorb heat and radiate it over time. Good direct gain is measured or predicted by determining how much heat energy the sun delivers to the interior space throughout the day and year. If you're interested in reading more about direct gain, check out Greenspec's article on 'Direct Gain: Passive Solar Design.'

Thermal mass

Thermal mass is a material's resistance to change in temperature as heat is added or removed which is vital to good passive solar design! The ability of a material to absorb and store heat energy. A lot of heat energy is needed to change the temperature of high density materials such as concrete, bricks and tiles: these materials have high heat storage capacity and are therefore said to have high thermal mass.

In contrast, materials that more lightweight (such as wood) have low thermal mass so are not as effective for passive solar design. While the simplest way to optimise thermal mass is during construction, thermal mass can also be retrofitted. Thermal mass acts as a thermal battery. During summer it absorbs heat during the day and releases it by night to cooling breezes or clear night skies, keeping the house comfortable. In winter the same thermal mass can store the heat from the sun or heaters to release it at night, helping the home stay warm. For more information on thermal mass, Your Home has a detailed breakdown here. 

Insulation & Ventilation

Insulation acts as a barrier to heat flow and is essential for keeping your home warm in winter and cool in summer. Being able to isolate the temperature on one side of the building element, you can ensure that the heat regulated will stay where it should. The most economic way to execute insulation is to factor it in early, at the construction stages.

Adding extra thicknesses or a reflective barrier where necessary can make all the difference to the final performance. Ventilation is also a very efficient way to cool internal thermal mass in your building during the night time. Your house plan should account for the way the air will flow through your home.

This will enhance the effectiveness of the ventilation in taking heat out of your walls and floors. When you close up your building the insulation will trap the cool air in, helping to maintain the right temperatures the following day. Without careful planning, the wrong use of ventilation will quickly disperse heat in the winter.

An infographic demonstrating the different methods of passive solar energy

Windows

Windows are frequently to blame when it comes to gain heat in the summer and loss in the winter. To control how heat is allowed in and out through windows, think about the size, height, shading and glazing of the windows before you enter the construction phase of building your home.

Although it’s fairly common to consider the placement of windows, glazing is just as important when it comes to bringing in light and fresh air. However, glazed windows can sometimes lead to unwanted heat gain in summer and heat loss in winter. This issue can easily be fixed by ensuring you choosing the correct glazing for your particular location, orientation and climate – refer to all that climate research you will have already done!

Sealing your home

This one is simple yet can be highly effective! Air leaks can waste a lot of your energy dollars. Leaks can let hot air inside during the warmer months or drafts during the cooler season. One of the quickest energy- and money-saving tasks you can do is caulk, seal and weather strip all cracks and large openings to the outside. The more extreme your climate, the more of a difference you’ll notice by sealing leaks.

Sunspaces

In addition to direct gain heating, homes designed to take advantage of passive solar often use what are called sunspaces or sunrooms. Much like a greenhouse, these rooms feature lots of windows and are perfectly placed to best take advantage of the sun’s rays.

The sun warms the air in a sunspace quickly, and with the help of open spaces and open plan living throughout the house, the heated air will circulate throughout the building. There’s also no better place to curl up and read a book on a winter’s day!

Trombe Walls

A Trombe wall is a popular passive solar design element where a wall is built on the winter sun side of a building. It has a glass external layer and a high heat capacity internal layer separated by a layer of air. Trombe walls are thermal storage walls, named after the French inventor Felix Trombe. 

A typical Trombe wall consists of a 20 - 40cm (8" - 16") thick masonry wall painted a dark, heat-absorbing color and faced with a single or double layer of glass. The glass is placed between 2 - 15cm (1" - 6") away from the masonry wall to create a small airspace. Heat from sunlight passing through the glass is absorbed by the dark surface, stored in the wall, and conducted slowly inward through the masonry.

A diagram of a Trombe wall generating passive solar

Trombe walls can provide carefully controlled solar heat to a space without the use of windows and direct sunlight, thus avoiding potential problems from glare and overheating!

Hot Water

The sun’s energy can not only be used to heat the air in your home, it can provide hot water as well. Solar hot water systems use thermal collectors mounted to a series of pipes. When water flows through the system, it absorbs heat from the collectors, raising its temperature.

You can also consider a courtyard with evaporative cooling water features to allow night cooling with wind protection. Garden ponds and water features outside windows can also assist greatly in providing evaporative cooling.

Put simply, passive solar design keeps out summer sun and lets in winter sun while ensuring that the building seals heat inside in winter and allows any built up heat to escape in summer. Orientation, thermal mass, sealing and other elements all contribute to the design of a house that benefits from passive solar heating. So if you’re building a new home or renovating, keep these things in mind and you’ll be reducing your energy bills and your carbon footprint in no time!

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