There should be no need to apply a membrane. If the insulation is well fitted and no discernible gaps, then the drafts are coming from somewhere else.
Have you paid attention to the joint between floor, wall and skirting boards, this is a notorious draft detail, due mostly to the differing expansion rates of the two dissimilar builds and the need to have clearance for it.
Expanding foam between the edge of the floor and the wall is a typical way to go, however the amount of movement between summer/winter is quite large therefore use of a membrane on the wall here will allow slip and would be good, unfortunately you have to remove the skirting's to achieve this. Another way (could also be inclusive) is to fit dry glazing tape along the bottom edge of the skirting making sure it has good compress-ability - a flexible seal.
If the drafts are coming from internal walls say between kitchen/lobby and lounge, where the temperature settings differ, this same technique will apply, and although it's more work at the time saves money in the long run.
Fit escutcheon plates, if poss, over external door locks, also draft proof the letter box where necessary. The external doors are also a source of drafts and these can be seen in the flame of a candle. Hold a lit candle a few inches away from a closed door/frame joint and move the candle in a parallel path to it slowly whilst watching the flame. In a no draft situation the flame will burn straight, where there's a draft it will burn away from it, small drafts = small amount of bend so watch very carefully. Where a draft is found mark on the door post/facing with chalk, which can be rubbed off easily, if there are a number of drafts replace whatever draft seals may be present, or fit draft seals - as applicable. Do the same with any windows, etc. to be certain all drafts are stopped, a lot of work at the time but cost effective in the long run...pinenot
