Chapter 5 Windows

5.1 Introduction

It is hard to understate the importance of windows. Confusingly, the thermal performance of windows is usually specified as a U-value. This is the reciprocal of an R-value used in opaque building components. Typical single glazing has a U-value of around 5.5. By comparison, a typical 90 mm wall with R2.5 batts will have an effective R-value of around 2.0, or U0.5. In other words, even a mediocre wall construction provides about 90% lower thermal conductivity than a single glazed window. Moreover, in the summer the sun can easily produce more than 1,000 W/m² on a vertical surface such as a window. So a single window 1 x 2 m could allow about 2 kW of heat into the building. This is the same power as a typical resistance heater, which is the last thing we want on during a heatwave.

5.2 Appropriate glass orientation

There is a trend towards using lots of glass in buildings. Given the poor thermal performance of glass in winter in retaining heat, and in summer of letting in huge amounts of heat, this has profound implications for space heating and cooling loads. So the first suggestion is to think very carefully about how much glass is really required, particularly as glass is also relatively expensive. Second, consider the orientation of windows - avoid south facing windows (as these will only ever lose heat and have minimal heat gains), and ensure north and west facing windows have eaves or external shading that allows the sun to penetrate in the winter but preclude this in summer. The trickiest orientation is the west, as in summer the sun from mid-afternoon can remain very strong but be low in the west such that eaves can rarely built long enough to avoid sun striking the glass.

5.3 Double and triple glazing

It seems to be fairly widely understood that double glazing significantly improves thermal performance. What seems to be less well understood is that there is huge variation in the performance of the best and worst double glazing, and the best and worst triple glazing. Typical U-values are illustrated below; double glazing will tend to have U-values from around 2.5 down to 1.2. In other words, the best performing double glazing can result in less than half the thermal conductivity of poor double glazing. Moreover, the highest performing double glazing can be almost as effective as poor triple glazing (U1.1). The best glass performance will be around U0.6, and requires triple glazing with 16 mm Argon-filled cavities. Even then the glass is not quite as good as a standard insulated wall (around U0.5).

Figure 12: Typical glass U-values

As well as the obvious reduction in heat loss and gain, and therefore space heating and cooling requirements, high performance windows make a building much more comfortable in three ways:

they prevent drafts entering the building on cold, windy days
they prevent the creation of convective air currents inside the building
they maintain a surface temperature close to the indoor air temperature, thereby avoiding feelings of chill when walking or sitting near the window.

To illustrate the latter two benefits consider the surface temperature of the inside of the glass when it is 20°C inside and 5°C outside. This condition is not atypical on a winter morning in Melbourne. As shown in XXX single glazing will drop to 10°C in this situation, while even ordinary double glazing will be much more comfortable at 15.5°C. However, only triple glazing will meet the Passivhaus criteria that requires the internal surfaces to be no more than 2K different to the indoor air temperatures. Moreover, at typical indoor humidity levels when surfaces drop below around 12°C there is a risk of condensation. The single glazing is very likely to have condensation form on the inside in this condition, particularly in locations where there is higher morning humidity such as bedrooms and bathrooms. Conversely, for high performance windows the risk of condensation is on the outside of the glass. This occurs because the heat transfer is so low that the outermost glass pane temperature will drop to close to the outside air temperature. Clearly, it’s far better for the structural integrity and building health (i.e. avoidance of mould) for condensation to form outside than inside.

Figure 13: Indicative internal glass surface temperatures

5.4 Window frames and installation

The whole-of-window performance is influenced not only by the glass U-value but also by the edge spacer, frame and installation detail.

5.4.1 Edge spacers

The edge spacer is the spacer placed around the insulated glazing unit to both seal the unit (i.e. maintain the still air space or keep in the inert gas - usually argon) and keep the glass layers apart. Most Australian glass manufacturers use aluminium edge spacers, as these are cheap and structurally sound. But aluminium is an excellent conductor, so the heat can enter and escape at the edge of the glass. How detrimental this will be will depend on the size of the window; for a small window with high performance double or triple glazing the reduction in performance can be 15% or more (FLIR PIC). The solution is to use a warm edge spacer. These are made from various plastic materials and can very significantly reduce this edge conduction. However, there are few insulated glass manufacturers in Australia that can provide these - however, there are plenty of distributors of imported European windows where this is provided as standard. Two of the best edge spacers on the market are the Swisspacer Ultimate and Rolltech Multitech.

5.4.2 Window frames

Just as aluminium edge spacers are a thermal weakpoint so too are aluminium window frames. These can play no part in a thermally efficient building design. Instead, the options are:

thermally broken aluminium (i.e. the frame is made up of an internal and external component with a insulant between)
uPVC
wood
wood with aluminium outer cover for durability.

All of these options can, in principle, give acceptable thermal performance. The choice will, to a large extent, come down to cost and aesthetic requirements. For our timber building wood was the obvious aesthetic choice.

5.4.3 Installation detail

A high performance window installed with large uninsulated spaces between the window and wall will provide a thermal bridge that will significantly decrease the thermal performance. Ideally the wall framing should be as tight as possible around the window to minimise the gap and whatever gap is present should be filled with low expanding foam or some other approach (e.g. overinsulate the frame if using a render system).

5.5 Airtightness

Windows and doors can be very leaky, especially old style sash windows. High performance windows and doors have at least one, and ideally, several seals to ensure airtightness. This is especially critical for sliding doors; the only way to achieve a reasonable level of airtightness with sliding doors is a lift-and-slide mechanism, as we have on our sliding doors. In this design when the door is closed it pushes down onto the track at the bottom and a rail at the top, and then to open it lifts up such that it slides readily just as a conventional sliding door would do.

5.6 Where can I get high performance windows and doors?

There are few Australian suppliers of truly high performance windows and doors. We suggest Paarhammer as a local (Ballan, Victoria) option. We’ve used their product in our previous home and for the front door of our current home. Another option is to import from Europe. This is not as cost prohibitive or risky as it may appear; there are a number of Australian distributors that can liase with European manufacturers. Moreover, the costs and risks are mitigated as these factories have large economies of scale and long experience producing products far beyond typical Australian products. While we’ve not used these distributors here are several options:

5.7 Cost

Windows and doors aren’t cheap. And on a simple payback measured in terms of energy savings they’d be one the least cost effective treatments. But we would emphasise the benefits of high performance windows go far beyond simply energy savings. They provide a level of comfort thanks to the avoidance of drafts, convective air currents and radiant assymetry which - in our view - make them more than worthwhile.