In reply to veteye:
> but that UPVC doors/windows have transmittance values measured at 1.22(which is better). I suspect that there is a blind acceptance that composite doors are better at insulation without any proof.
Just to clarify, lower U-values are better, higher R -values better.
You can get a minute rebate under the Green Deal, but you need proof of the U-value off the installer, which should be better than blind faith.
The financial savings of energy are tiny, less than a tenner a year, also the U-value includes the frame, so putting in a wood door in an existing frame is largely guess work, sealing the draughts probably better than anything else.
You can get wood U-values down to about 0.5 (Passivhaus), try the Wood Window Alliance, but often, because these have functionally identical frames, it can be difficult to tell whether it's wood or not. Be a shame to spend the money and have people think it was really realistic fake wood.