After we'd settled in, we responded to a direct marketing ploy and invited a couple of double-glazing sales-people to visit after work. They insisted that we both be present for the appointment. A man and a woman appeared on the doorstep and we brought them in for a cup of tea. It was quickly clear that the woman's role was to schmooze the woman-of-the-house "what a lovely chiffonier, I love that shade of blue" while the blokes talked about aluminium vs uPVC; BTUs and thermal conductivity.
Wronnnnng!I failed my Physics "O"-level and really didn't know the difference between a BTU and a chip-butty; while The Beloved was working for a Fuel Poverty charity and knew more about loft-insulation, double-glazing and heat-transference than everyone else in the room (combined). But we went along with the script for a while because we had no TV and welcomed the human interaction. After going round and round the virtues of their product and not getting any data about the actual cost - not even an estimate - I pointed dramatically to the large pair of Edwardian sash windows behind me and asked - "how much, roughly, would it cost to double-glaze that". Errrm! But eventually, the bloke admitted that upgrading that one source of energy-loss would cost about £1,200. I told him [you do the math] that I'd be better off buying a new house than double glazing all of this one. Which was a slight exaggeration, but enough to terminate the interview.
The Beloved explained afterwards that the key data is the cost of the installation / annual energy savings = Pay Back Time. Double glazing could have been a good investment IF a) a lot of energy was lost through the windows b) it cost very little to reduce that loss. Houses lose most of their heat through the roof (hot air rises) while only about 5% is lost though the windows. Even with Everglaze triple-glazing some heat is lost through the windows so typically the pay-back time for double glazing is in excess of 20 years.
A very few years later when Pat the Salt and Souad moved into a tiny cottage on the Waterford coast, the first thing we did was install 10cm of fibre-glass loft-insulation. Some people react badly to fibre-glass but as a thoroughly insensitive person, I went up in the attic to un-roll a lot of insulation. It took a couple of hours and a couple of hundred £s. Afterwards, in the November dark, I stripped off and hosed myself down from the garden tap to get rid of any residual fibres. I think we calculated the payback time as between 15 and 18 months. Nowadays, at least 20cm thickness is recommended.