Heat transfer occurs from warmer to cooler substance or material and continues until there is no temperature difference between them. It happens all around us in one of the three ways:
Conduction is the transfer of heat through matter (i.e., solids, liquids, or gases) without bulk motion of the matter.
Convection is the process of heat transfer from one location to the next by the movement of fluids or gases.
Radiation is the transfer of heat by electromagnetic waves, and it does not need any matter, gas, liquid or solid, to occur.
Energy efficiency is concerned with the heat transfer between our houses and the outside and internally within each room and between the rooms. It is also about efficiency of our appliances because in order to keep the temperature within comfortable range we must continue to run our appliances to heat or cool our house to counteract the effects of unwanted heat transfer. So the cost of running the house depends on how fast is the unwanted heat transfer and how much we are paying to run our appliances. More efficient appliances will use less energy to produce the same amount of heating or cooling, and slower heat transfer means they will have to work less to keep the house comfortable.
In winter we lose heat through ceiling, walls, floors, openings, gaps, and ceiling penetrations. In summer the heat transfer occurs the same way only in the opposite direction.
To reduce the heat transfer we can insulate the walls, ceilings and floors, double glaze our windows and seal all gaps. We can install window covering, internal or external shading, fans and screens
You can slow down conductive heat transfer by making improvements to the building shell. Seal the gaps to stop convective heat loss/gain. Use fans to make convection work to your advantage to warm up the room/house faster. Use the correct size of eaves to shade windows from direct sun (heat transfer by radiation) in summer and warm up the house in winter for free.
What works best for your house will depend on several factors like the location (climate), orientation of the house, size and and materials it is built from, number of residents and their habits and number and type of appliances being used. You can have a professional assessment and advice on what issues exist in your house, ways to improve them, what products to use and even where to get them. You can also do the research yourself, the internet is full of articles about energy efficiency, products and suppliers. Just remember, not everyone is doing it just for the joy of sharing knowledge, some information or advice is put to promote an idea or sell products. So for your research use government, scientific or educational institution's websites and for assessments use assessors accredited with Scorecard or NatHERs as they are bound by code of conduct, privacy and disclosure of conflict of interest rules.