I think it needs more insulation to be honest. I too keep my pigs outdoors during the winter, however I am prepared to bring them indoors in severe weather, I keep their carriers at the ready (for fire anyway), I also have cages where i can decamp them into.
My suggestions are you need plenty of snuggle safes (microwaveable pads) or a pet heated pad which needs an electric point.
Additionally you could use a waterproof tarpaulin, when it snows it can really chill the hutches.
If you add a thermometer to the inside of the hutch it will help you understand what the temperature is rather than kinda guessing.
It's always a funny thing because you need air into the hutches, but dry air.
Damp is the real problem to be honest.