The role of high-performance windows in avoiding heating

Zero Energy House

In any building, the use of windows is essential. Windows provide access to:

  • Daylight
  • Solar gain
  • External views
  • Connection with the outside
  • Views of the inside to provide passive security

Windows, however, have the highest thermal conductivity out of all the elements of the building envelope, so they are a weak point for heat loss. Optimising the window-to-wall ratio of a building is key to allowing sufficient daylight, solar gain and views without compromising its thermal performance.

During the design stage of their house project, Shay Brazier and Jo Woods investigated various window-to-wall ratios using thermal modelling and came to the conclusion that an overall building ratio of around 30% was optimal for the thermal performance of the Zero Energy House.

The following three performance characteristics of windows then informed the planning and selection of windows in the house.

Solar gain

Solar gain is the increase in temperature that results from solar radiation. Glazing will provide good solar gain on sunny days in winter, but will also let the heat inside the house escape quickly. In the summer, too much solar gain can lead to overheating if shading and ventilation are not used as control features.

Because the Zero Energy House relies on the sun to keep comfortable indoor temperatures, making the most of solar gain is essential. As a result, the placing of windows on each elevation has been carefully planned to take advantage of solar access:

  • North. Glazing on the north elevation is maximised to allow good solar gain during the winter. Northern (midday) sun is controlled with the use of overhangs.
  • East. The eastern elevation has the-second highest percentage of glazing to encourage heat build-up over the day in the winter. Vegetative shading provides solar gain control on this elevation during the summer months.
  • South. The south and west elevations have the least glazing. The south elevation is the coldest and therefore, higher levels of glazing on this elevation would lead to greater heat loss. The level of glazing on this elevation has been optimised to allow sufficient opening windows for good cross ventilation, natural ventilation of bathrooms and interaction with the street.  
  • West. The west elevation is exposed to strong afternoon summer sun. Minimising the glazing on this elevation helps to reduce solar gain in the warmest months.
South side of the house, showing reduced glazing area to reduce heat loss.
North side of the house, showing larger area of glazing in order to maximise solar gain.

Heat retention

As mentioned earlier, glazing has a significant impact on heat loss. The thermal performance of a building element is usually defined by its thermal resistance (R value). The bigger the R value the harder it is for heat to escape. For example, New Zealand’s Building Code requires that walls of residential homes in Auckland have an R value of R2 (the walls of the Zero Energy House have an R value of R3). Meanwhile, the required R value for windows is R0.26, which is significantly lower.  The thermal resistance of some typical window constructions used in New Zealand is shown in the table below:

The table shows the R value of a window is determined not just by the glazing but also by the window joinery. Timber increases the R value of a window over standard aluminium joinery (thermally broken¹ aluminium will also improve the R value over standard aluminium).

Double glazing adds an air gap between 2 panes of glass and, because air is a poor conductor of heat, this increases the thermal performance of the window. Additionally, low E coatings² reduce the amount of heat that is lost via radiation.

After looking at glazing options for their new home, Shay and Jo concluded that the additional thermal performance of double glazing with a low E coating (to reduce radiant heat loss) was worth the investment. In combination with passive solar design and good insulation levels, high-performing windows allow their family to live comfortably in the Zero Energy House without the need for heating.

Ventilation

The third function windows perform is ventilation, which is essential to keep the house cool in the summer, as well as for good indoor air quality. The window design chosen for the Zero Energy House allows them to be opened inwards at both the sides and the bottom. The bottom hinge means windows can be tilted back to allow ventilation closer to the ceiling level where it is needed most (hot air rises). They're also secure in this position so they can be left open to ventilate the house when the family is not there. Furthermore, their angle of tilt and placement underneath overhangs means they can be left open when it's raining.

North side of the house, showing some of the upstairs windows tilted inwards for ventilation. The full-height bi-fold windows downstairs can also be tilted inwards.

The windows

To achieve their goal of living in a comfortable and healthy home without the need for mechanical heating or cooling systems, Shay and Jo installed high-performing windows throughout the house.

Glazing

The glass used in all the doors and windows is a 24mm double-glazed, argon-filled product. It has a low E coating that helps to trap heat inside the house during winter and reduce heat gain during summer. Unlike other low E glass products that are hazy or highly reflective, the one used in the Zero Energy House has been manufactured in a way that avoids these effects.

Joinery

All of the doors and windows in the home have timber joinery, for the performance reasons outlined above. They include a dual tilt mechanism that allows them to be either opened on a side hinge like a normal window, or tilted back on a bottom hinge, providing ventilation when needed.

The thermal performance of the timber joinery and ventilation control offered by the dual tilt mechanism contribute to the house being comfortable all year round without the need for heating or cooling.

Footnotes

  1. 'Low E' is an abbreviation for low emissivity. By coating the surface of glass with a low E coating the amount of infrared radiation lost out through the window can be reduced.
  2. Aluminium is a very good conductor of heat meaning quite a lot of heat can be lost through an aluminium window frame. Because window frames get colder, condensation often forms on the inside of aluminium joinery when it's cold outside. To reduce this problem a plastic separator (plastic being a poor conductor of heat) is placed between the inside and outside of the aluminium frame, reducing the amount of heat lost through the window frame.
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