Keeping warm in winter and cool in summer takes a lot of energy, but your windows can help out.
Windows have always been the weak spot in your home’s shell. The thin pane of glass found in most houses built a couple decades ago has an R-value of maybe 1, compared to R12 or R15 for 4 inches of fiberglass insulation. In winter, heat heads outside and cold glass chills you. In summer, outdoor heat fills the room.
To improve insulation and comfort, manufacturers have added layers of glass and sealed-in inert gases. But newer low-emissivity (low-e) glass takes it a step further.
It’s glass with a microscopically thin coating of tin oxide, silver or zinc, applied in single or multiple layers to get different performance characteristics. High performance glazing includes extra layers of low-e film between the panes of glass.
Low-e coatings let in visible light rays (like sunlight), but block or reflect the stuff that produces heat, as well as some ultraviolet rays.
Getting the right windows means looking at both thermal performance and the co-efficient of shading, which affects solar heat gain. You can choose windows that block out the sun’s heat and retain internal heat or capture the sun’s heat and retain internal heat.
Take into account the alignment of your house, the path of the sun, and the location of your windows, and consider adding evergreens or window awnings to decrease the sun that reaches the windows.
When comparing windows, R-values are only one indicator. The R-value of glass varies from the center to the outside edges, and the R-value of an installed window is less than the R-value of the glass itself. Newer standards, which take into account different energy saving properties, help you compare apples to apples.