What is the major cause of heat loss in cold climates?

Infiltration is one of the major causes of heat loss in cold climates. Try to seal the openings and also, have enough ventilation areas (5-10% of habitable spaces floor area) that allow for good air movement during summertime. There are several other factors contributing to the heat loss in cold climates, including low performing windows or thermal bridging. By eliminating these factors, you can obtain a low-cost 6 star energy rating house.