This can depend on factors such as walls, thickness, type of windows, overlap material, and so on. To insulation of walls, it is better to approach last. Try to warm the windows first, or ceilings. For better persuasiveness, consult a specialist. And if it turns out that the cause of the minus temperature is cold external walls, then this issue must be approached with caution. If the wall insulation is incorrect, you can make it even worse. Condensate and increased humidity will be added to your problem. Almost all experts, recommend insulation of walls outside. To warm the walls inside, you must first find out the temperature of the dew point. Very good if it is inside the outer wall. If this is not, then it is better to refuse to warm the wall from the inside. The choice, of course, is for you, but listen to the advice of professionals.
