How can I keep my outside dogs warm?

I have two pit bull terrier dogs in my backyard. They have a fenced-in area with gr and a dog house. Their hair is kind of short so I'm worried about them getting too cold this winter. I built their doghouse out of thick plywood and it's big enough for both of them to sleep in there, but is that enough to keep them warm? I also nailed a thick rug to the floor of the dog house. I live in Alabama and I believe the lows are about 30-45 degrees or somewhere near there in the winter. I can't bring them inside because my grandparents are allergic and they don't like big dogs.