Mar 16, 2015: Our Double-glazed Windows

Last week I wrote in more depth about why we chose to use Galvanised Iron and Plywood on our walls. The bottom line of that discussion was that these were personal choices that had very little to do with the energy performance of the Greeny Flat. There were many other sound and practical reasons why we chose those materials but we could have built an energy positive and affordable house with a very different look and equivalent performance if we had chosen different cladding materials. By contrast I would argue that, at least in the Southern Highlands of NSW, our use of double-glazed windows was not a choice but a necessity.

Double-glazing Essential in Most Parts of Australia

Perhaps, if you live on the coast between Sydney and Brisbane where the climate is VERY mild and is moderated by the relatively constant temperature of the ocean and by cooling sea breezes in the summer time, you might be okay with single-glazed windows. In fact you could live in a tent and be pretty comfortable for most of the year. But almost all of the rest of Australia is either too hot in the summer or too cool in the winter (or both) for single glazing to be appropriate if you want an energy efficient building.

If you want your building to be energy efficient you can start by following the principles of Passive Solar Design, one of which is that you need good insulation and air-sealing around the ‘Thermal Boundary’ of the building. The ‘Thermal Boundary’ is the barrier between ‘Conditioned Space’ (heated or cooled areas of the building) and any ‘Unconditioned Space’ (including the outside, the earth and any parts of the building that are not heated or cooled such as a garage). To put it more simply: in order to minimise the amount of energy required to maintain comfortable conditions inside the building you have to have good insulation and air sealing between the inside and outside.

Windows are always the worst part of Thermal Boundary

In every building that I have designed, built and in most of the buildings that I have tested or seen, the windows and exterior doors are the most poorly insulated and the leakiest part of the Thermal Boundary.

I spent twenty years designing and building in Montana where the summers can get up to 40degC and the winters down to -40degC. In that climate we would often install triple and sometimes even quadruple-glazed windows. Even then the very best windows might have an insulation value of R2.0 (R11 in US numbers) and we would be installing them into walls with an insulation value of R6.0 (US R30). So even the best and most expensive windows we could get only had a third of the insulation value of the walls.

Here in Australia it is common to install single-glazed windows with an insulation value of about R0.13 (US R0.7) into walls that might have R1.5 (US R8). Even with that meagre amount of wall insulation, the windows only have 1/12th as much insulation as the walls.

But even the coldest parts of Australia are nothing like Montana so how much insulation is enough. For the Greeny Flat experiment we chose to try 90mm stud walls with R2.0 insulation PLUS we wrapped the whole exterior of the building with an 8mm reflective foam-and-foil product so our total insulation value is about R2.3 (US R13) and this is proving to be adequate. Of course it would always be nice to have more insulation but then you get into serious cost increases and it’s pointless if the windows aren’t dramatically improved as well.

For the windows we chose to try simple double-glazed windows with standard aluminium frames from a company called Stegbar. The powder-coated aluminium frames are nice because they require no maintenance and will not rot or burn. The trouble with aluminium frames is that they readily conduct heat between the indoors and the outdoors. This can be overcome by using ‘Thermally Broken’ aluminium frames. Basically these have a plastic piece in the middle that stops heat from conducting directly through the frames of the windows. Unfortunately buying ‘thermally-broken’ aluminium frames here in Australia would have doubled the cost of our windows. In order to meet our various goals of being energy positive, low-maintenance and affordable we decided that the standard aluminium frames were the way to go.

These windows have an average insulation value of about R0.25 (US R1.4) which is improved on some of our windows by the addition of a ‘Low-E coating’ on the glass. This is twice as good as a standard single-glazed window but is still only a tenth of the insulation value of our walls.

For future projects I will seriously consider using Stegbar’s Siteline range of wood-framed windows with an aluminium cladding on the exterior of the wood. These have the combined advantages of having better thermal performance (about R0.4 vs our R0.25), low-maintenance exterior, and fire resistance but the disadvantages of requiring maintenance on the interior and about 25% higher cost.

An example of an aluminium clad wood window.

An example of an aluminium clad wood window.

Solar Heat Gain

The insulation value gives a pretty good indication of how good a windows is at KEEPING HEAT IN a building but another vital consideration with windows is how good they are at LETTING HEAT IN OR KEEPING HEAT OUT. This is where the ‘Solar Heat Gain Coefficient’ (SHGC) comes in. SHGC is a measure of how much radiant heat can pass through the glass in a window. The higher the number, the more heat it will let through. Windows with Low-E coatings have significantly lower SHGC numbers than windows with ordinary clear glass so they are much better at stopping heat from radiating in through the windows in summer time. They are also slightly better at keeping heat from radiating out through the windows in winter. The trouble is that they are also much worse at letting heat in from the sun in winter. So for a passive solar design like the Greeny Flat that relies on winter sun entering the building to keep it warm we have to be very careful where we use Low-E coatings. For us the answer was very simple, we used clear glass on the north-facing windows to allow in more of the heat from the winter sun and we used Low-E coatings on all the other windows in order to limit the amount of heat gain in the summer and heat loss in the winter. On all the windows we added insulating blinds to assist with keeping us warm in winter and cool in summer. And for the north facing windows we rely on the correctly sized eave overhangs to keep the summer sun off the windows and out of the house.

Ventilation and Condensation

One of the reasons for using double-glazed windows instead of single-glazed is that you are likely to have much less condensation on the inside of the windows in the winter time. This is due to the fact that double-glazed windows will have a much higher interior surface temperature than single-glazed ones so they are less prone to condensation forming on the cool surface of the glass. Nonetheless, in a fairly tightly air-sealed house like the Greeny Flat it is very important to pay attention to ventilation in the winter time in order to reduce humidity levels and therefor the likelihood of condensation on the windows (or other cold surfaces).

Serious condensation can form on the interior surface of the windows in winter if adequate ventilation is not provided.

Serious condensation can form on the interior surface of the windows in winter if adequate ventilation is not provided.

Conclusion

Overall we feel that the Stegbar aluminium-framed, double-glazed windows we chose to use in the Greeny Flat are a good compromise between cost, quality, energy-efficiency, low-maintenance, and fire-resistance. In future projects we might consider using their Siteline range of wood-framed windows with aluminium cladding on the outside. These would provide the beauty of wood on the interior along with better energy-efficiency while maintaining the low-maintenance and fire-resistance on the exterior. But this would be offset by the higher cost and the need for maintenance on the interior wood surfaces.

P.s. Another hidden benefit of double-glazing is that it adds significantly to the weight of the exterior doors which gives them a real feeling of quality that you would not get with a single-glazed aluminium-framed door.

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