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The Principles of Passive Solar Design

Passive solar design nz

Written by Matthew Cutler-Welsh

One of the first homes I visited in Christchurch, as a Homestar Assessor, was designed by Russel Devlin of Solarchitect. His practice name seems to suggest that he may know a thing or two about designing homes that work well with the sun. Stepping into an example of his work proved Russel knew a thing or two about passive solar design.

Nestled on the Port Hills, the two-storey home includes all the classic features of good passive solar design. Large north facing windows with good shading coupled with exposed concrete on the floor to help absorb heat from the winter sun. Another highlight was the excellent above code insulation. The only disappointment I sensed from the owners was that they rarely make use of their small pellet burner.

The design of the home required some small changes in their daily routines, opening windows and so on were enough to keep them comfortable all year round. Such is the promise of passive solar design.

We were always doing passive solar

Passive solar design is not as prevalent in present day builds as it once was. Vernacular architecture, which dates back to early 1800, are structures made by builders without the input from professional architects.  Vernacular homes used local design using local materials, suiting the local conditions.  For example Mediterranean vernacular homes, often included a fountain or pond in the courtyard; which helped the air to cool.  The cool air was drawn through the building by the natural ventilation system designed in the home.

Romans to Aztecs, civilisations around the world have improved their standard of living with the use of thermal mass coupled with good orientation and ventilation.

Popularised in 1940, the term “solar house” was penned by the Chicago Tribune, commenting on a new style of home based on the architect George F Keck’s “House of Tomorrow”. The theme was famously adopted by Frank Lloyd Wright in 1944 with his “Solar Hemicycle” Jacobs House.

Passive Solar

Passive solar implies that a house will use the sun (solar energy) to stay warm independently (passively). Slightly misleading, a number of designers have suggested that ‘active solar’ would be a more apt term. For the best performance there will be some activity required, such as opening/closing curtains, opening windows and possibly moving external shading, allowing for seasonal changes. Essentially a good passive solar design will require very little in the way of mechanical heating or cooling.

The Principles

The core principles of passive solar design are orientation and thermal mass. Insulation and ventilation are useful to help maintain comfort. Its as simple as this; orientate, aggregate, insulate and ventilate.

Orientate

Most of the windows should face the sun. In New Zealand that means north facing windows should be big, with windows to the south being minimised. Keep in mind that small windows are still useful for effective ventilation and for some natural light.

Aggregate

Thermal mass is a key component to passive solar design. The theory being that heavy materials such as concrete, brick or clay will absorb energy during the day and release it during the night to regulate the building at a comfortable temperature. These heavy materials are sometimes referred to as a ‘thermal flywheel’ for the building.

Thermal mass only works if it’s exposed to the inside of the house. A concrete floor will not be effective for thermal mass if it’s covered with carpet. A concrete block or brick wall will not be effective if it’s lined with insulation and plasterboard.

There is a huge difference between thermal mass with direct sunlight and that which is merely exposed to the inside temperature. If you’re not careful, a well intentioned concrete block wall located within the home can end up acting as a big thermal sink rather than a thermal store.

The best place for exposed thermal mass is in the first two metres of floor directly adjacent to north facing windows.

Insulate

Insulation is important. It’s great to absorb energy from the sun, but if you can’t keep it in then it’s not going to be of much use for very long. Insulation helps to make thermal mass effective by controlling the direction of the heat flow. Insulate under and around the edges of the concrete slab so heat doesn’t escape downwards and out of the sides.

Ventilate

Windows are the best form of ventilation. Ideally they will be well positioned to allow cross flow through the house and fitted with secure stays; this allows for passive ventilation throughout the day while still keeping the house secure. A combination of low and high level windows can be effective in two storey homes, thus creating a stack effect. This helps to keep the house cool during summer.

Passive Solar is Relevant

There has been some debate recently about the relevance of passive solar design principles. This has been in response to Passive House, which relies on airtightness rather than thermal mass. Overuse of windows, particularly those facing north and west, have also increased the problem of overheating in many contemporary homes.

The basic principles of passive solar design are as true now as they were for ancient civilsations. Contemporary passive home designs require important decisions; finding a good designer/architect that understands how to balance the principles of passive solar buildings will ensure a good outcome.

 

 

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