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How to maximise the benefit of secondary glazing

Posted by Simon Forsyth on 2 February 2012 at 9:35 am

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

About the author: Simon Forsyth focuses on the links between carbon management, Environmental Management Systems, and cost-saving through resource efficiency, and produces simplifying software.

If you have a question about anything in the above blog, please ask it in the comments section below.

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4 comments - read them below or add one


Anthony525Comment left on: 21 February 2012 at 3:33 pm

Nick, I fitted ecoease last autumn....fairly simple process as each of the four windows was less than 2m3.

The result has been really impressive.

When we walk into the room, the first effect is it feels warm. The heat seems to wrap round us.

To see how the system works we  treble glazed four out of eight windows, leaving the remaining four double  glazed.

 Comparing the temperatures at the  centre of the eight windows the four that are treble glazed read 3 to 4 degrees C above those that are double glazed.....this with an outside temperature of minus 9C and a room temperature between 21.5 and 22C.

The immediate visual difference is that condensation forms on the double glazed windows to a height of five feet from the floor and condensation forms a line about half an inch along the inside surface of the double glazed units inside the plastic sheet.

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utility-exchangeComment left on: 2 February 2012 at 3:06 pm

Interesting post and thanks to Nick for the info about Ecoease. I have no experience of it but it looks like a good idea. But agree the problem is storing it in the summer months - tho in the 80's my parents kept their secondary glazing on all year round!

I think you can still get Silica gel or at least something similar from hardware stores.  

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Rudge Energy

Rudge EnergyComment left on: 2 February 2012 at 2:55 pm

Just had a look at ecoease

This is very similar to the old polycell system in the 80's.

That used to work well, keeping that cold 'glare' away from single glazed windows.

The only downside is finding somewhere to store the panels in the summer, which often get removed to enable you to open the windows.

 Also.. I've been having problems buying silica gel for other projects.. Is it still sold, as I've also notced we no longer have equipment packed with it!

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Nick Hanna

Nick HannaComment left on: 2 February 2012 at 11:47 am

Hello, I wonder if anybody has any experience with magnetic secondary glazing panels from Ecoease?

Also sold through Nigel's Ecostore.

They look relatively simple and much easier to remove and handle than the big old-fashioned secondary glazing (I know, I used to have horrible heavy panels in my previous house which had fiddly rubber bits which fitted into grooves...took hours to do).

And it would be easy to apply Simon's tip of using silica gel if you can just pop them on and off.


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