So you think insulation is only important during summer?
We spend more time Indoors
More time indoors means we generate body heat. With body heat comes moisture. We sweat, bath, cook and sit longer which means the moisture from our bodies penetrate the drywall and try to escape via the insulation. The external air combines with this moist, warmer air and can generate higher humidity levels. Insulation helps act as a barrier between the temperature differentials helping to reduce moisture transfer thus less possibility of mold formation.
Thermal Loss
Thermal loss is the loss of heat due to transfer in the form of heat. Convection, conduction and radiant transfer are the 3 ways heat is lost. As you heat your home, the lack of insulation allows for faster transfer of your heat from inside to outside. Imagine being in the cold with just a T-Shirt. Your thermal loss is much faster than if you have a goose down coat wrapped around you. The cold is not invading your body. The heat is leaving your body. Your home functions the exact same way. So properly insulated and aerated homes keep your heat from leaving prematurely and thus reducing the amount of energy it takes to keep your house warm.