NOW IS THE BEST TIME! (obviously)
However some energy efficiency retrofitting can be quite expensive and disruptive. But if you intend to carry out any home improvements whether DIY or contracted, then it can be useful think about what can be done while the works are going on anyway.
New floor covering?
We have met several people who regretted spending a lot of money on upgrading a floor but sadly didn’t think to install underfloor heating and/or insulation while the work was being carried out.
If you are adding a fitted carpet, floor tiles, laminate, engineered board or new floorboards then you will probably need to empty the room/s and have the original floor exposed. So this is a great time insulate or adapt pipework.
If you can’t face or afford the disruption of lifting up floorboards (and it may need only a few of the boards) then at least when having a new carpet fitted it’s worth getting thermal insulating underlay.
Loft conversion or adapting for storage space.
A loft conversion to create actual living space such as bedrooms and bathroom will require the work to be approved by building control, and so building regulations will automatically require insulation to be done along with new joists.
However if you are simply boarding out a loft to use for storage this is definitely the time to think about installing or beefing up any existing insulation. See Loft insulation