The dog house has to be sized properly so that he can maintain his body heat. If it is too large, he will not be able to manage it. Also, you should put some straw in the dog house. Straw is the best bedding as it will insulate even if it gets wet. If you cannot get straw, a wool blanket, like an old wool horse blanket will work nicely.
If it is too warm, the dog will drag it out of his house. Straw will be dragged out if the dog is on a chain. If the dog is not chained, he will make himself a nest in the straw, and it will help him use his body heat to stay warm.
What you need to do is go out there when he is in his house, and reach in and feel his belly. It should feel warm. He will draw his feet up on his belly. If that is nice and warm, and the air feels considerably warmer in the house than it does outside of it, you have it right.
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