Energy Smart Homes - House Design

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Appropriate window placement, sizing and shading are key elements to energy efficient design, since windows can lose up to 40-60% of heat from a house. Windows serve many important functions. They can act as solar collectors trapping heat from the sun - this is useful in winter but not in summer. They also act as ventilators during summer, funnelling cool late afternoon and night time breezes to rid your home of heat accumulated during the day. Windows also let in natural daylight.

A balance needs to be struck between controlling the sun's access and allowing adequate cross ventilation from breezes, as well as allowing natural light to enter. Standard principles which apply to obtaining the best energy efficiency from your house through considering the impact of windows is discussed here.

North facing windows

It is recommended that around a third to a half of the north face of your home be glass, as it is very effective at trapping winter warmth and can be easily shaded from summer sun with correctly designed eaves.

To calculate the overhang needed, multiply the distance from the eaves-line down to the bottom of the window by 0.7. This will ensure the glass is adequately shaded from September until March. For cooler regions, multiplying by 0.4 will provide suitable shade from October until February.

Eaves design for excluding summer sun on north-facing windows

Eaves design for excluding summer sun on north-facing windows

Deciduous trees and shrubs or creepers growing on an open pergola on the north face of a home can also provide window shading in summer, while allowing the sun through to warm your home once they've lost their leaves in winter. Alternatively, a solar pergola is designed to achieve the same result.

The operation of a solar pergola in winter and summer

The operation of a solar pergola in winter and summer

It is important that shading devices, whether in the form of eaves, pergolas or appropriate landscaping, do not block the sun's access to the interior of your home during winter.

East and west facing windows

While east and west facing windows provide warmth in winter from the early morning and afternoon sun, they are difficult to protect from the sun in summer. This makes rooms on the east and west of your home uncomfortably warm, particularly west facing rooms which receive the hot afternoon sun. To keep cool in summer, it is recommended that the total area of east and west facing windows be kept to a minimum.

External shading devices provide some protection from the summer sun, with complete protection achieved only with full vertical screening, such as outside blinds or shutters. This is because the angle of sun will be close to horizontal early in the morning (east) and in the late afternoon (west), and only vertical screening can block the sun at these angles. Deciduous trees or vines growing on a trellis can also provide shading during summer.

South facing windows

South facing windows receive no direct sun in winter but will receive a few hours of morning and afternoon sun in summer months. For this reason, they lose heat in winter and gain some undesirable heat in summer. South facing windows should be large enough to allow good ventilation and light to enter the home without losing too much heat in winter.

Vertical elements such as external screening or landscaping in conjunction with internal blinds will be most effective at shading south facing windows, since the majority of this sun is at low angle. Basic 'eaves overhang' in combination with internal window treatments will also assist solar control to south facing windows. This is because in mid summer the sun can fall on an unshaded southern façade for approximately 4 hours in the morning and 4 hours again in the afternoon. For the more northerly latitudes (eg Geraldton) provision of shading to south facing windows is even more important. This is because at this latitude there can be an additional 45 minutes of mid summer sun falling on the south face of a building, morning and afternoon.

Insulation Properties of Windows

Different types of windows behave differently in terms of their insulation properties. Choosing a window with good insulation properties can significantly affect the energy performance of your house. Windows in Australia are given a ‘WERS’ (Window Energy Rating Scheme) rating. A WERS rating provides a star rating between 0 to 10 stars for both the heating properties and the cooling properties of a specific window type. Knowing both a heating rating and a cooling rating for a window means that you can choose the most appropriate window for your climate – for example, in colder climates, you would need to ensure that the heating rating is high to ensure that the windows keep the heat in. In climates such as the Perth metropolitan area, which has hot summers and cool winters, an appropriate choice would be a window which has a good heating rating and a good cooling rating (a minimum of 5 stars for both is preferable).

For more detailed information on WERS, please check the Windows Energy Rating Scheme website.

Internal window treatments

While external window treatments are the best way to reduce summer heat gain, internal window treatments are most important for reducing winter heat loss. A window can lose heat five to ten times faster than an equivalent area of wall. This heat loss can be minimised by keeping warm air inside the room away from cold windows.

Closed curtains can be effective insulators and should be made from a heavy fabric with insulating backing for maximum effectiveness. They need to be long enough to reach the floor and should include a closed pelmet. The pelmet is an integral part of the curtain as it reduces air circulation and consequent heat loss through the window glass during winter and heat leakage into the home during summer when the curtains are drawn.

Poorly designed curtains allow heat to escape - Well designed curtains minimise winter night heat loss

Skylights

Skylights can reduce your daytime lighting needs. However, a typical Perth home consumes approximately six times as much energy for heating and cooling than for lighting, and heat can be lost from your home through skylights on winter nights and gained during hot days. To reduce this problem, position your skylight so it is shaded in summer or consider buying one with special glazing that minimises heat transfer and can be closed at night. Non-vented ducted skylights lose less heat in winter, as the air trapped in the duct acts as a thermal buffer.

Tinted glass and reflective films

Tinted glass and reflective films absorb and reflect heat, keeping your home cooler. However, be aware that using them reduces the amount of light and heat entering rooms in winter as well as in summer. During summer the glass itself also becomes hot as it absorbs energy, which will cause some heat to be radiated into the room. These products may be useful where large areas of east and west glazing are unavoidable due to design reasons. However, tints and films will generally not reduce heat gain as much as external shading.

Double Glazing Units (or Insulated Glass Units)

Double glazed units have the potential to significantly improve the energy efficiency of a building. Comprising two or more panes of glass separated by an air (or gas) filled cavity and hermetically sealed, an Insulated Glass Unit (IGU) provides thermal insulation as well as improved acoustic performance through blocking noise.

An IGU is described in terms of the thickness of the outer pane in millimetres, followed by the gap width between the panes and finally the thickness of the internal pane (eg: 6/12/6). A greater gap width (up to 20mm) provides better insulation. The insertion of an inert gas such as argon instead of air and the specification of Low E glass with the coating on surface 3 further improves insulation provided by the IGU.

An appropriately selected IGU has the potential in some climates to prevent up to 70% of heat loss in winter and reduce up to 77% of heat gain in summer when compared with standard 3mm single glazed windows.

Australian quality manufacturers of IGUs are represented by the Insulating Glass Manufacturers Association (IGMA).

Other window products

Windows are also available with other features, such as special coatings on the glass, which can offer improvements in thermal performance.