On triple glazing

The highlight of last week’s AECB conference in Oxford was Wolfgang Feist, Mr PassivHaus, who held forth over a 60 minute lecture and four 90 minute seminars on all aspects of the PassivHaus standard. He is a remarkable performer: for one thing, he really knows his stuff and he is able to explain in great detail just why the standard, which he is largely responsible for, is designed the way it is.

The idea is not to construct zero carbon housing, or any stupid notion like that. What PassivHaus aims to do is to provide maximum thermal comfort in a house at minimal energy cost. This thermal comfort thing is important — I hadn’t really appreciated just how important before.

For instance, comfort underlies the PassivHaus take on triple glazing. I have been a voice arguing that triple glazing is “overkill” in the UK climate and that the energy used in making these units would probably never be repaid by the energy saved over their lifetime. However, the main reason for using triple glazing is not to save energy but to provide more comfort, as the internal temperatures remain more even.

Feist produced a table showing what the temperature differences were close to different forms of glazing when the internal temperature is designed to maintain at around 21°C and the external temperature drops to —5°C.
• next to a single glazed window, the adjacent temperature is around 1°C
• next to a double glazed window (2000 vintage), the adjacent temperature is around 11°C
• next to an all bells-n-whistles low-e double glazed window, the adjacent temperature is 16°C
• next to a triple glazed window, with a centre pane U value of just 0.65, the temperature is 18°C.

Feist maintained that in England, the milder temperatures meant that you wouldn’t have to use triple glazing to reach PassivHaus standard, but that it would be silly not to. What he did say was that he thought insulated window frames were more important than triple glazing. Interestingly, when the first PassivHaus’s were being built, there was no such thing as an insulated window frame and they had to adapt existing ones by sticking insulation on the outside. But by 1996, small joinery firms were responding to the idea and insulated frames became commercially available. Now they are available in plastic and aluminium frames, as well as timber.

Feist also discussed where to put the low-e coatings in triple glazing. Apparently, triple glazing will benefit from two coatings, but if the coatings are placed either side of the centre pane, and the pane isn’t toughened, there is a high risk that the panel will crack, due to thermal shock. Now they tend to provide low-e coatings to the inner surfaces of the two outer panes. In passing, it’s worth noting that the inside pane in a triple glazed unit doesn’t need to be glass – however, every transparent material used to date is more expensive than glass, so glass it remains, even though this makes the units very heavy.