My dad is always harping on about keeping doors shut to control draughts. Of course in a well insulated house one should never have draughts, and the idea of zoning is silly. Our house is not well insulated and has plenty of draughts. We keep only a few rooms warm to keep our heating bill modest.
The standard technique for controlling draughts is a sand filled cloth tube which you put against the bottom of the door called a Door snake. But these get in the way and are hard to place firmly with people and cats opening and closing doors.
A simple alternative using only found materials is the door skirt:
To make this, find a strip of wood (in my case a 3mm thick off-cut ripped from a pine 2x4) and some small screws (I used 16mm x 4mm). I used three screw per door. Cut the strip to length (about 81cm/32").
Now find a scrap of material that you don't find offensive, and that your wife didn't buy for some specific quilting project. Cut an 81cm x 2(k + l + 1)cm strip, where k is the width of the wood strip and l is the maximum distance between the door and the floor.
Fold the material in half lengthwise 'inside out' and sew (with a machine is easiest) along the outer edge then in to the fold:
Now use a steel ruler to push the outside out through the end. In a traditional door snake you fill the resulting tube with sand and add a head. In this case we can forgo the sand unless the draught is a particularly strong one. Sew up the open end.
Pre-drill the wood strip and 'practice' in the screws. Remove the screws and screw the wood strip through the tube into the door. Voila, a door skirt, costing a few 10s of cents and easier to manage than the sand varieties.
The efficiency of this is easy to show by the fact that doors become much harder to close simply due to the change in air pressure (you can actually see the material billow out when you close the door, and suck tight when you open).