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The case for secondary double glazing

Posted by Tim Pullen on 21 August 2009 at 12:08 pm

I recently helped a lady refurbishing a typical 1960s three-bedroom semi with energy efficiency issues. A quick heat loss calculation gave a peak heat load of 8.5kW which should have equated to a use of about 10,000kWh of gas per year.

Her actual gas consumption was 15,000kWh p.a. which was used entirely by gas fires, as she had no central heating installed. The difference between the two figures was because of the age and inefficiency of the fires.

Insulating the loft and cavity walls got the peak load down to 6.7kW and was paid for in part by grants. Insulating under the suspended timber floor was more tricky but still reasonably do-able and got the peak load down to 5.2kW.

The front door was already u-PVC double glazed and a similar patio door was installed to the rear which reduced the peak load to 4.8kW and got rid of a good source of draughts.

Next was the big one, double glazing to the windows. The lady was looking at a bill of just short of £4,000 and a potential saving of 0.8kW peak load or 864kWh per year. In cost terms that equates to a saving of less than £50 per year – so more than 80 years payback.

Installing secondary double glazing instead of replacement windows got the cost down to under £500 and the peak load down by 0.3kW, which gives an annual saving of about £16 – and the payback down to 30 years.

The more important feature of secondary double glazing is that it eliminates draughts. This gives a great perceived warmth and comfort with the same amount of energy being squirted into the house.

If we replace the gas fires with gas central heating, the overall gas consumption would be less than 5,000kWh and the bill down from around £800 p.a. to less than £300. In fact we suggested an air source heat pump instead, but that is another story.

Photo by D Sharon Pruitt

The author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks

About the author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks.

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

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7 comments - read them below or add one


EverglazeComment left on: 28 January 2011 at 12:59 pm

Great article Tim!

I work for a double glazing company in East London, and you would be surprised by the amount of people that are unaware the benefits Secondary Glazing can bring. Not only is it ascetically pleasing, it is also cost effective both with the reduction of heating bills it brings, as well as the lower cost to install as new windows are not necessary! It is such a cheaper option, we at Everglaze opened up a second premises to promote secondary glazing and we are currently swept off our feet. Feel free to have a look around my blog at

I’m hoping to continually update it with new offers etc.

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Tim Pullen

Tim Pullen from Weather WorksComment left on: 25 September 2009 at 2:03 pm

The U-values are calculated using various software tools - typically Build Desk. There is an optimum air gap for thermal insulation and my understanding (although I have never tested it) is that there is not much benefit over 20mm. You do get better accoustic insulation with larger air gaps and you you get better thermal insualtion with gasses other than air - typically argon.

See my previous response regarding secondary glazing forming effectively triple-glazing. My feeling is (and again I have not tested it) that it would be very effective on draughty sliding sash windows but less effective on less draughty ones. My concern would be over-investment. That is spending too much money for too little return. I am keen on secondary double-glazing as it is a relatively cheap way of dealing with the windows. But turnng them into triple-glazed windows may reduce the commerciality to a point where it is no longer useful.

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vimokshadakaComment left on: 14 September 2009 at 1:23 am

I make secondary double glazing, (with another sealed unit), but am still working out the figures. Could you explain a bit how you get the figures for each glazing system? and what if you add a glazing unit to create kind of triple glazing? Since using a unit rather than a pane should bring down the pay back time. And do you know how air gap sizes effect thermal insulation? Do air currents in the gap reduce it after 20mm or 105mm(as some secondary glazing people say)?

Thanks V

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Tim Pullen

Tim Pullen from Weather WorksComment left on: 8 September 2009 at 5:44 pm

Beyond those figures already given I don't really know. To be honest I don't get too excited about paybacks. You sound like a windows manufacturer so you will know that double glazing typically comes out at something over 90 years. But we do it anyway.

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Safestyle WindowsComment left on: 8 September 2009 at 9:34 am

That's an impressive saving on the double glazing, and the heat and energy conservation all round. What was the overall paybac k time on all of the improvements made?

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Tim Pullen

Tim Pullen from Weather WorksComment left on: 25 August 2009 at 9:29 am

Peak heat load is the amount of heat escaping from the house at the coldest time of year. So we assume a winter temperature of -2C and a desired internal temperture of 21C and calculate the amount of heat escaping under those conditions.

The calculation (put simply) is the surface area of an element (wall, window, floor, etc.) x its U-value x the temperature difference (23C in this case) = a result in Watts.

One of those elements will be ventilation losses, i.e. draughts, which can be the most difficult to calculate and the largest loss.

To that figure needs to be applied a correction factor to take account of system inefficiencies to arrive at the amount of heat to be introduced to meet that heat loss.

For the sake of accuracy the calculation should be done for each room but a whole house calculation is much quicker and is accurate enough to get in the right ballpark.

All this calculation does is indicate the size of boiler needed. It does not tell you how much energy the house will consume.



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paulmukComment left on: 24 August 2009 at 11:36 pm

Could you explain a little more about peak heat load, and how it is calculated?

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