What performance is more applicable for your climate? Double glazing will perform significantly better than single glazed whatever in regards to heat loss. You can't beat the air gap factor if you're trying to keep avoid heat loss. Yes there are "Low E" single glazed products such as viridian Comfort Plus which depending upon which side of the glass the absorption coating is placed it will perform in different ways, but bear in mind that since it also will reduce heat gain in winter it can actually work against passive solar building performance if not properly applied. In other words, it's not a "cheap" replacement for double glazing. In Victoria, we generally spec standard double glazing and use external controls such as blinds to mitigate heat gain issues in summer. Thermal Performance Specialists such as an Energy Raters understand the technical nitty gritty around this issue the best, and always good to seek their expert advice when applicable.

Thanks, Paul. We're in Adelaide, so we're looking for something that helps keep the heat in in winter and the heat out in summer. I think the winter cold is more of an issue for us though, as we have evaporative cooling for summer and need to have the sliding door slightly open anyway.