Final answer:
Homes in Northern European nations are being built without heating systems and are kept warm by the body heat of the residents. The insulation in these homes helps retain heat, keeping them warm even when residents are not at home.
Step-by-step explanation:
A possible explanation for homes in Northern European nations being warm even when residents are not at home is that these homes are very well insulated. When a house is well insulated, it can retain heat for longer periods of time. This means that even when residents are not at home and not producing body heat, the insulation keeps the house warm.