Dont put the pipes in the cold outside wall. All you have to do to avoid disaster, is a little rethinking. I am assuming at least one side of the bath will abut a warm wall and you are using the three standard bath fixtures bathtub, water closet, and basin. On the tub which I assume runs away from the warm wall, install the taps in the warm wall with drain at that end. The water closet which should always sit next to the tub, 15 on center, bring the waterline up through the floor about 4 inches away from the cold wall. Next the basin, ditto with its waterlines, and plumb it so that the trap is below the floor, the vent can run in the cold wall, no problem. Next build an insulated vapour- barriered skirt around base of bath if no basement is planned, and make provision to keep this space at least at 65 degree F. You will not be wasting heat as it has a nasty tendancy of rising, and it will keep your bathroom floor warm, while preventing your plumbing from freezing, sort of a two for one deal so you are really only using half the energy…..Lou
