There are many things to consider here – not least the way the house is designed and built, minimising so-called ‘thermal bridging’ which ensures materials that face the outside don’t conduct cold air into the house. Insulation is also critical – ensuring that heat isn’t lost through the roof, walls and floors. The main indicators here are the U-value and the PSI-value.
As you compare different build systems and suppliers, all will list the U-value of the walls, roof, and floors they sell. It’s a measure of the heat loss through the system or window, and the lower the number, the better. It is designed to help you compare the different choices. Although there isn’t a universal building regulation requirement for U-values, the notional target is 0.18 for walls. To offer some context on this, the requirement in 2006 was 0.3; most poorly insulated old homes (i.e. most of them) will have U-values in excess of 0.2. From a thermal bridging perspective, PSI-values measure the heat lost along a metre of junction between two thermal elements. Look at how your system designs it out.
Because it’s relatively cheap and easy to install extra insulation at the construction phase, most experts agree it’s the first thing you should do if you really want to minimise your long-term bills – certainly much easier and more effective than fitting insulation after you’ve finished. Different types of insulation have different levels of performance, of course, but in general terms, the more the better. Modern forms of insulation such as those with phenolic cores perform far better for each inch of thickness than traditional forms such as expanded polystyrene or mineral wool.