own build and i painted whole house inside in summer months, no heat only natural, anyway I did same thing as empor2010 only the first time i rolled on water down emulsion on ceiling i got quite a shock as paint drips everywhere, so I then bought proper ceiling paint (waste of money as I couldnt see any difference between ord emulsion).
make sure you paint ceilings in bathrooms before tiling as I hadnt managed to get to back door toilet ceiling before tiler and nightmare to paint afterwards.
Good luck, its a tough job but u save a fortune.
Just another point, my brother had a professional painter do his house and advised a creamy colour for ceiling instead of white, really makes the white coving stand out.