Last week it was a little cold for motorcycle riding, but okay for general-purpose living. Suddenly we're down in the teens. The dog bowl is frozen solid in the morning now. Time to put the door back on the dog-house.
People often ask if it's too cold for the dog to be outside. These folks tend to fall into two groups. The first group has a house-dog who goes outside to potty. They fear that he might suffer terrible consequences during the brief time he's outside to eliminate. These are people who have never used an outhouse in the wintertime. It's not great, but it won't kill you. Not that the weather never discourages the dog (or cat) -- in today's "B.C." comic strip he says you know it's cold when you try to put the dog outside and he says, "No thanks, I can hold it." When we have driving freezing rain, I'd be tempted to hold it a while myself.
The second group are people whose dog is going to stay outside. They are just asking to be validated as humane pet-owners. Sure, it's freezing, but that's not too cold for the dog, is it? Well, as we say when choosing undergarments, that depends.
Pounds of body mass generate and retain heat. Square inches of surface area radiate and lose heat. When you're tiny, you have a lot of square inches per pound; you lose heat rapidly and efficiently; you cope well with hot weather. You freeze when it's cold, even if you have a thick coat of fur. You need a warm place to stay, not only insulated but with a heat source of some type. There are armor-shielded heating pads that can stand up to being outdoors with an animal. Whether you use one of those, or a light, or a space heater, make sure that the pet cannot get to the wiring or to an exposed heating element.
Big dogs have the opposite situation: very few square inches per pound of body weight. They don't radiate and lose heat very easily. Thus, in summer they are not happy, even if they are slick-haired with practically no fur. They stay hot. In the winter, on the other hand, conserving heat is an advantage. Once again, the length of fur is secondary. Huge dogs stay pretty warm. It's common for them to abandon their dog-house and seem perfectly fine in the snow.
Of course, thick fur is warmer than thin fur, but the size of the dog is the major factor in this equation. Which brings us to the more difficult questions: If big dogs can stand the cold okay, how big is "big" and how cold is "cold"? Somewhere I am sure that someone has concocted some sort of actuarial table that cross-references the dog's body size with the ambient temperature. If you're above the line, stay outside. If you're below, you gotta stay in the house tonight. Yes, I'm sure that someone has done that, but (like most of the internet, present company excepted) it's probably not reliable. So, we're not getting into that.
Instead, let's discuss shelter. No matter the size of the dog, the winter doghouse should encompass some basic principles. It should not be directly exposed to the wind, so place it in the lee of the house, garage, or fencing -- some type of windbreak. [I cut a hole into the side of our garage storage, and built the doghouse inside.] The house needs to be insulated to reduce the amount of heat conduction away from the dog's body (Do you want to sit on the metal chair with or without a cushion?).
Then there is the size of the house. Think of the doghouse not as a tent, but as a sleeping bag. What keeps you from freezing is a layer of warm air around your body that's separated from the cold air. Your escaping body-heat warms the air around your body. The trick is to trap it there instead of losing it to the winds. The smaller the space, the easier it is to heat. That's why you zip up your coat, or wear extra layers to fill it up. It's why a mummy sleeping bag keeps you warmer than the envelope style. It's why the dog-house should be just big enough for the dog to get inside and turn around. Any bigger, and he's trying to heat too much space, which doesn't work too well. Think of your last winter camping trip. Do you get a lot warmer by going into the tent? Not much; it's when you zip that sleeping bag up tight that things start to get cozy.
Even if you're pretty sure your dog is "big enough" and it's not "too cold", fix a house that gives him the option to curl up in his den. Winter dog-houses are just big enough to turn around in, insulated, behind a wind-break, and have some sort of a door to keep in the warm air. When you have provided that, you don't have to feel a bit guilty when your idiot dog prefers to sleep in the snow.