Well, I hate to disappoint you, but curtains work for energy savings. If you are open to changing your mind, here’s an explanation: thermal-engineering.org/thermal-curtains-material…

You always want glass coatings on outside window glass, no matter the climate. Depending on where you live, you want different coatings obviously, but coatings are essential in modern glass panes.

And since you seem to be entirely unaware of what already exists in the construction industry, here’s another article with a bit more explanation: en.aaglas.nl/producten/warmtewerend-glas. A low zta will stop a good portion of the summer sun, while a high lta will still allow through a lot of light from low angles (including from the weak winter sun). Select glass that has a high lta/zta factor and you have glass that is good both in winter and in summer in northern Europe. The Netherlands is at lattitude of about 52°, while most Canadians live a few degrees south of that, so these same solutions would work there as well.