There are several reasons to build a low energy building. It may save you money through cheaper bills over the lifetime of the building. It may reduce your carbon footprint and reduce damage to the environment. It may be eligible for grants or tax breaks that could make building or financing cheaper.
At the risk of coming out with a lesson dryer than the one on humidity, I started off putting my students into a couple of groups, telling them to imagine they were governments and getting them to come up with some standards to reduce energy use in buildings.
First they were to brainstorm a list of ideas. Later they would be able to use judgement and discussion to choose the best. They had various ideas, from banning aluminium windows and setting limits on carbon emissions to setting up bodies to reduce energy use and prohibiting air conditioners.
There are basically three different kinds of standards: prescriptive, performance based and outcomes based.
Prescriptive standards tell builders and designers what they must do, for example that walls must have U values of 0.13 W/m2K. The advantage of prescriptive standards is that they are clear and definitive. The disadvantage is that they may may be too conservative, leading to buildings that are over-engineered, or they may not take into account how prescribed building elements are used, for example the orientation of the windows.
Performance-based standards require calculations to predict how much energy a building will use. For example heating energy of less than 15 kWh/m2a. This gives designers more freedom in how to use building elements in optimal ways, and to make sensible compromises with the over picture in mind. They do mean that designers have to make those calculations. Performance-based standards may also help in the introduction and use of non-traditional features and techniques.
Outcome-based codes mean that the actual performance of buildings is measured and reported, to ensure that standards have been met. If it turns out they don't, it's too late!I gave a brief history of building standards around the world, starting in the 1970's. I briefly mentioned the US's flirtation with energy conservation in that short era of energy democracy, before the energy republic of Reagan took over.
The main story was in Denmark and Sweden, where standards in 1980 were only reached in the UK in 2006. Meanwhile over here in Japan, the standards today in Matsumoto are roughly equivalent to those in 1985 in the UK.
Of course it is very difficult to compare building standards in different countries, and normalising for weather is one issue. Another is how many buildings comply to the standards. Not only are Japan's standards something like thirty years behind the UK and fifty years behind Sweden, they are also applied to well under half of newly built small homes. 
Here are some links if you want to read more:
Global energy efficiency measures could save €410 billion by 2030
Map of global standards
