It’s easier to shop for new or replacement windows if you know the same things the window manufacturers know. This is the second part of a series on options and features that can make new windows more efficient. Things To Know About Energy Efficient Glass Choices Here’s a list of glass or glazing choices that can make windows more efficient. Quality replacement windows will offer these features. Unless they are handmade in a local shop, virtually any windows you buy today will be at least double pane, which is two layers of glass sealed with an air space in between….