as a ber assessor who regularly plays with u values... i can mak eteh following comments...
u vales suffer the law of diminishing marginal returns..
what i mean is, if you drop from 0.25 to 0.23 you save significantly more energy than going from 0.15 to 0.13.... therefor ethe lower the u value the less impact any more reduction will make, and also, the ability to reduce you u value becomes a lot harder when you are down to low levels... and obviously costs increase exponentially...
generally passive house u value levels are in the region of 0.1 - 0.12....
in my opinion you should be looking for an upgrade of about 20% above minimum levels for optimal ecomomy versus efficiency..
roofs try to get around 0.12..
walls 0.2
floors 0.15
what i have noticed in doing ber assessments on new builds is that its the lower cost upgrades that are worth more in teh long run, than trying simply to increase u values...
create a draught lobby at your main access
increase all your heating controls... have many zones with manytime and temp controls
use delayed start stats
use high efficiency boiler
use LELs throughout
use weather and load compensators, especially with UFH
and most importantly, get your construction detail sabsolutely perfect to minimise cold bridges and maximise air tightness... many builders / self builders say "sure thats grand, we always do..."... which is rubbish.... the standards in open systems of irish construction is terrible.
in my opinion you need a BER assessment done with different specifications which should (depending on teh ability of teh assessor) tell you exactly how much energy you save using different specs / materials / construction methods...