Ages ago I wrote a spreadsheet for doing what-if comparisons to see whether it was better to invest in more insulation in the walls, roof, floor, fit better windows and doors, or fit a better MVHR system.  Others have found it useful and I've been reminded that I've not re-posted it over here, so here's the latest version. It should be self-explanatory, you fill in the cells with your wall, roof/ceiling and floor areas, add the areas of each door and window, put in the U values for each and