Q&A: DIY double glazing effectiveness


It was an interesting article about double glazing by Alan Cotterill in ReNew 135. However, I am led to believe that double glazing relies on a vacuum between both sheets to be effective. How did Alan achieve this? Was his objective to cut down sound only? Double glazing is certainly a good idea, particularly where I live in the Blue Mountains, but I don’t have it because of cost.This is why the article interested me, but I doubt its effectiveness for retaining warmth.
—Rod Marshall


Double glazing doesn’t rely on a vacuum, it uses an air gap of a particular width that is wide enough to provide a level of insulation and narrow enough to prevent convective currents in the air between the two panes which would transfer heat from one pane to the other. Many double-glazing units use inert gases such as argon between the panes, as it is a better insulator, but there’s not that much difference between argon and air, so DIY double glazing can work quite well compared to single-pane windows.

There are vacuum window units, although I don’t know of anyone making them here in Australia; possibly Pilkington has their Spacia units available. You can’t produce a vacuum between two unsupported flat panes of glass as external air pressure would press the panes together and possibly shatter them. In vacuum-insulated windows they use many tiny posts between the panes to provide the support for the glass, but obviously this sort of window will not be perfectly clear although it comes close. The space between the panes is much smaller for vacuum glazing than with gas-filled double glazing. For example, Pilkington’s Spacia units have a 0.2 mm gap compared to at least a 6 mm for gas-filled units.
—Lance Turner