It will retain the heat longer - assuming that all the other obvious insulation areas are covered (roof, floor, minimising drafts etc)
It should make a big difference, not only does the double glazing in itself help with heat retention, but the reduction of condensation will greatly take the cold sting out of the air. If you are having a cold night, go and run a dehumidifier overnight (wouldn't do regularly, expensive to run) and see how much more comfortable it makes your house (when the air is drier)
Weigh up the pro's and con's yourself... if you're staying there for a while, its well worth doing. If you're going to sell in the next couple years, well maybe just invest in a heat pump instead.
Yes it will retain heat longer, but above it says "I'd rather get double glazing and floor insulation etc. etc. then have people using heaters" - I just think the expectations are higher than what will happen.
I've insulated my house slowly over time. First I put wool in the ceiling, big win for temperature. Next I put in a heat pump to the lounge, bedroom area, which was awesome. Then I did underfloor insulation and a ground sheet, which was a big win for smell in the house and virtually no change in temperature. Then I did wall insulation, foam pumped in, which again made no real difference. Next I put in more ceiling insulation, which was a big win and made the house feel a lot warmer. Then I did the older retrofit double glazing, which reduced condensation and made the house feel slightly warmer. Next I removed a freestanding fireplace from the kitchen, which was a huge win as the room was then properly insulated and sealed - before that wind whistled in from the ceiling cavity as the roof can't touch the hot chimney. Next I moved the heat pump to the kitchen and put an even bigger unit into the lounge, which was a win for the kitchen / dining area. Next I did the proper double glazing, which has made a small difference over the old retrofit double glazing in the living areas, but I can't tell if it made the kitchen area feel warmer, however it has reduced condensation.
So my experience is:
- Ceiling insulation makes a big difference you can feel
- Removing fireplaces so the ceiling can be sealed is a big win
- Adding a heat pump is a huge win, so long as you have ceiling insulation
- Every other step is small and incremental that I couldn't really feel. It probably reduces the power bill a bit is all. This of course depends on the house.