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
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