Over the summer one of our doubling glazing units ‘failed’, the dried air has gradually leaked out of the gap between the sealed panes. The symptoms of this are that moisture appears in the gap and the glass looks dirty, but of course cleaning it has no effect. Today this unit was replaced twenty-seven years after it was put in. All this got me thinking – does doubling glazing pay for itself in monetary terms in its life time? My suspicion has always been that it wouldn’t. First a bit of background rules and comment.
Most of the double glazing was replaced twenty-seven years ago, but not all. Twenty-seven years is a long time and in that time and things change. According to the double glazing company that quoted for the job you can expect a lifetime of 30 years. We’ve had two glass doors replaced with double glazing since. Like the single glazed window they open up onto the conservatory – so not directly onto the outdoors unlike the other double glazing. One of these in particular has made the dining room warmer. However this was done more than 10 years after the initial tranche of doubling glazing.
Two smaller windows were also double glazed at a later date. One of these is in the down stairs toilet and is so small a child couldn’t get through it (thieves note despite this it still has bars on the outside). Another is above the main door and is also small.
We’ve also added a lot more insulation and in 2008 a wood-burner.
In 2018 we had a new boiler. This cut our consumption massively. This was not due to a massive gain in efficiency per se – although it is a bit more efficient – but because the old boiler heated the water when the central heating was on whether we wanted it to or not.
In 2000 we had an extension built. This was heated for a while by an electric heater on the wall but after a few years we extended the gas central heating system and put a radiator in. The unit that failed was in this room. The window was at the end of the kitchen which the extension was built onto. We moved the double glazed unit which h has failed into the new room and reused it.
I’ve taken no account of inflation which until this year has been low. In any case in my view it cuts both ways since inflation reduces the size of the original sum invested. We didn’t borrow money to do this, but paid out of our own capital.
I assumed a 10% heat loss saving on single glazed units. As far as I can gather the U value of a single pane of glass is 5.6W/m2K. This means every metre of glass transmits 5.6W of heat. This doesn’t sound much, but adds up. A double glazed unit of that era is probably half that*. It might be more in our case. The old windows were very bad fitting with lots of drafts. We only had one winter in the house pre doubling glazing, but I can remember the house being very cold and the curtains in the lounge flapping around when the wind blew.
The cost of the installation was £950 (1995 price).
I have almost all the Q1 and Q4 bills or price and gas usage data with the exception of 1995. This means the % payback is slightly too low.
All this means the calculation is a bit crude. Part two will cover the results.
* In the UK the part L of the building regulations cover energy use in England. Scotland or Wales. These are not available on-line so far back but the amendments to the 1991 building regulation act barely mention energy, which shows how much things have changed. Nor does any of the literature we have from the doubling glazing that fitted the windows company mention U values. Some people (https://great-home.co.uk/building-regulations-u-values-how-have-they-changed/) put it at 3.1 for double glazing of that era, but I’m sticking to a 10% saving partly because as I say the drafts were horrendous and partly due to the fact that most double glazing companies put the saving at that.