It is difficult to say unless we know your house. Older houses seldom qualify for a significantly better energy rating. This is because energy ratings are based on calculations that take into account a range of factors, including construction year, insulation in the foundations and other conditions that you cannot do anything about unless you tear down all or parts of your house and build again from scratch.
However, we will supply all the documentation you need to recalculate your energy rating. You will receive clear documentation of the energy effects when you refurbish your home with ventilation windows and a LivingBetter solution.
Ventilation windows can help you to achieve a better energy rating. Nevertheless, you should not assume that an older house can achieve energy rating C, no matter how much you renovate and insulate it.