Insulation sounds like it should be a straighforward thing. Use sisalation foil and caulking to seal gaps and make the structure 'tight', to stop heat loss by uncontrollled air movement and radiant heat, and then use bulk insulation installed to all the external surfaces of the house to combat unwanted conductive heat flow through the structure. This will keep the heat out in summer and warmth in during winter. Simple right?
Unfortunately houses have complex geometries with many junctions, pipes and wires, and other penetrations, and multiple trades who have their own focus on what is important. This introduces many opportunities for less than adequate installation of insulatoin. Natirally, heat flows through weakest points, similar to how water would flow out of a plastic cup with holes in it. In fact, if you have 5% of holes in your insulation, the overall insulative value of it drops by 50%. If insulation is poorly installed an 8 star design house can easily turn into a 2 star house, .
To achieve a well designed and well executed house, the design team who understandswhat they are trying to achieve, would emphasise and include relavant details and documentation into the plans to cover weak points and difficult junctions, to minimize penetrations in plaster, keep service pipes off external walls where possible, and provide caulking notes. Likewise it is also then vital to have a construction team, including associated tradesmen, who understand the documentation and insulation system; who values the importance of a high performance outcome of the house. It is through experience that our team learn what a successful job looks like.
Basically it takes knowhow, vigilence, commitment, and lots of communication.