> Modern windows (especially high end) can be both energy efficient Yes, compared to other windows. Compared to a well-insulated wall with external insulation

> Modern windows (especially high end) can be both energy efficient Yes, compared to other windows. Compared to a well-insulated wall with external insulation and 6" of interior insulation, they're not great. Good windows are an R5. A well-insulated wall cavity can easily be an R30.