So has the double glazing paid for itself in its lifetime taken at 30 years?
Sort of – probably. The graphs below show the total payback, bills for Q1/4 and return on the investment in very simple crude terms. By my reckoning its paid for 98% of itself up to Q1 this year. However if you look at the graph there are two outliers (three if you include this year where the bill so far was unusually low, although part of this explained by a big drop in the rate I paid for gas).
The outliers are 1995 and 1997 where the bills are very low and quite out of keeping with the years around them. 1996 was genuinely high, it was one of the coldest winters I can remember (<-20°C). At one point we left the central heating on for 24/7. The bills for 1995 are incomplete i.e. I don’t have one quarter. 1997 are, it must have been very mild. It has to be said that the price of gas was low (somewhere between 1-2p/KWh) so you got quite a lot of gas for your money. If the 1995 cost was at that of some of the years around it then yes our doubling glazing has just about 100% paid for itself. Its a pity the unit failed this spring since the cost of gas has doubled in April, then tripled (October) compared with the spring – so Q4 this year it would start moving into profit with certainty. The payback has been very slow though and not as good as the PV systems (still thinking about the solar hot water). Finally this takes no account of savings from not switching the heating on since we had double glazing. I can’t prove it but this would definitely put us in profit by now if I could have measured these.
All this raises some interesting questions. First of course why do people put in double glazing? Like us – if your glazing needs replacing – it seems crazy not to. Now (but not then) you have no choice unless you live in a listed building. Lots of people put it in to stop noise at that time. Again less relevant now with quieter traffic. So economic payback with this energy saving product is not the primary driver (I would say uniquely and probably fortunately).
The second question is would double glazing pay for itself now?
Taking inflation into account (https://www.bankofengland.co.uk/monetary-policy/inflation/inflation-calculator) the cost was £1950 in today’s money. Looking for indicative costs on line suggest for a similar sized house its £3000-5000. Two things need to be said here. First double glazing technology has moved on. Its now all low E glass A rated. So you’re getting something that potentially will save you more money. Second energy prices are much higher. Peter Smith in the 2002 book “Eco-refurbishment” suggests 15 years as payback. As you can see from my graph the trend in our bills is ever upwards, something that the new boiler has not arrested. The trend is not uniform as gas prices have gone up and down and the winters have varied. I’ll leave you to make up your own mind. Fortunately for us as the glass fails we can simply replace the units without needing to have new frames which mitigates a large part of the cost. This is true for anyone out there who is looking to replace double glazing. You can be far more confident about that paying for itself sooner.
There is a wider point about energy saving measures concerning heat. All but the most basic (lagging, roof insulation, draft exclusion), take many years to pay for themselves and notionally never will or only in decades. This is true even today with the high natural gas bills. Yet all this needs doing to reach net zero and if you have heat pumps installed. I’ll leave you with that happy thought. Happy new year.