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How to maximise the benefit of secondary glazing

Q: Our property is old and the windows are not double glazed but made of oak and have 6 – 8 panes per each half of the window. Any ideas about secondary double glazing appreciated.

A: Thinking back to first principles, the aim is to stop ‘coolth’ getting into the room – draughts of cold air through gaps, and cool air tumbling off cold surfaces. Secondary glazing helps with both these basic issues. But it can be a job fitting the big panes in the autumn then removing/storing them for the summer. 

Once fitted, secondary glazing has two critical but fixable weaknesses:

1) Condensation – because it’s not in a sealed unit, the air between the window and the secondary glass has the same humidity as the surrounding air and will condense onto the cold window glass. Which you can’t easily wipe due to … the secondary glazing. 

2) Overall efficiency – lack of any vacuum between the sheets of glass means it doesn’t insulate as well as double glazing, and the imperfect contact sealing around the mating surface can still allow heat loss. 

There are workaround solutions, of course – if they’re carefully included, secondary glazing is MUCH better than nothing, but will never be as thermally efficient as double or triple glazing. (My Finnish in-laws have triple glazing throughout their 18th century wooden house, but they live through -40C winters as routine. And their wooden walls are a couple of feet thick.)

To address: 

1) Put packets of silica gel in the air gap. They will saturate over time, but quickly lose their absorbed moisture if you heat them eg. in a cooling oven or near a fireplace. But they’ll only work at all if you…

2) Ensure the secondary glazing seals tightly all round its surface with the window. Standard draught-proof strip works well since it hugs all the small surface contours. If the seal’s good, the silica gel should keep condensation down – but you’ll need to swap the packets and heat-dry the originals every so often. 

Alternatively, revert to traditional solutions. Thick, floor-length curtains drawn at dusk can work very well. Lined fabric is a good insulator since it’s not trying to be transparent as well. In extreme cases (like two-inch door gaps I’ve seen in 15th century properties) it’s the only option, but it works. 

Silica gel is typically sold in tiny packets for eg cameras. Better to buy big packets or tubs – ask in a hardware shop, or you can buy tubs or bags on Amazon. 

Photo: Trey Ratcliffe

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