Oddsocks
Ryder Cup Winner
Actually, the fact that hot/warm air rises probably has as much/more to do with it than the open plan. The insulation simply contains the warm air that has risen. If single downstairs rooms, such as a lounge are to be kept warm, then draught prevention is necessary.
If there are no draughts, keeping warm can be much simpler/cheaper - simply put another layer of clothing on.
I get that, when we renovated we insulated to the best levels possible as I was working closely with a builders merchant so the budgets were eased,
Despite keeping the down stairs door shut, double layer foil backed laminate insulation and going 20% over on BTU’s ratings on the new rads, down stairs is very hard to get heat retention in. As you rightly said by the time downstairs is warm upstairs is roasting.