I wouldn't heat the duck house at all! Ducks are very well equipped for cold weather, with the exception of Muscovies. Their caruncles are susceptible for frost-bite when there is some wind chill. Important is to have a good ventilation in the house, but no direct draft at the ducks. Ducks release a lot of moisture into the air, which will condense on cold surfaces and drip down. My duck house, for example, has a metal roof and when snow is lying on the roof, drops of cold condensation drip down. Its awful, if i find the time, i will rip the whole roof off and insulate it before winter. The basics are:
- Keep the ducks out of the wind
- Keep the bedding and the house as dry as possible
- Make sure that water-vapor can leave the house without condensing anywhere