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Temporary secondary glazing

Polycarbonate secondary glazing can be a good heat loss solution for windows that don’t need to open in winter. It’s cheap and almost invisible.

We’ve been asked to show what polycarbonate secondary glazing is like. Here’s what it looks like on a window in Edinburgh. It’s just a sheet of polycarbonate cut to the size of the window reveal, with sticky-back magnetic tape applied to the edges of the polycarbonate and to the window. In the Session 2 Card Game we refer to a Historic Environment Scotland study which used a fancier mounting system with timber surrounds plus some other options. We haven’t done a formal measurement of the heat loss before and after; they have, and theirs cut the heat loss through the window in half. If the windows don’t fit very well, an added benefit is fewer draughts through the gaps.

And below is a window 4 meters away under similar room conditions, at least until they spent time wiping down the windows. Not nice! It was about -4C overnight and is hovering around zero now.

similar window without the temporary secondary glazing

Polycarbonate is hard to cut, but the vendor (Easter Road Plastics in Edinburgh) cut it to size. It’s a bit flexible so if the window is high you might need a ladder to get the top on straight, but it just snaps in place and peels off. That means if you do need ventilation unexpectedly, that’s not too difficult. It scratches easily and of course it takes room to store it if you need it down for an extended period.

This was a DIY job (by an engineer, as it happens) and after around 7 years the window tape is peeling off in a few places. The only thing wrong with the installation was a minor problem – they didn’t allow for the polycarbonate to expand more than the window in the cold, so in very cold weather the plastic bows out until it is readjusted on the magnetic tape. Luckily, the 1 cm tape is just wide enough to accommodate the difference. 7 years ago, the cost was £80 for this mid-size of window. We don’t know what it is now.

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