which scrub down the surface to refresh it.

Weather stripping

Weather stripping is the insulation around a window sash that prevents air filtration when the window is closed. In general, better weather stripping means airtight windows, which means better efficiency.

Casement and awning designs tend to use squashable weather stripping – the sort that is compressed by the window frame when you close it – and are generally more airtight than sliding models. However, as with all good-better-best comparisons, a lot depends on the manufacturer and product quality, so check available documentation before making a purchase decision.

Warm edges

A "warm edge" is a fairly new innovation that helps reduce heat transfer through the window sash (where the glazing joins the frame). It's a simple idea: a non-conductive spacer is inserted between the two panes of a double-glazed window (two spacers in triple glazing) to prevent heat being transferred out.

Warm edges are surprisingly efficient. They can add 10% to a window's efficiency and increase the temperature of the frame by as much as 5°F. As a bonus, they also reduce condensation around the edges of windows, reducing maintenance and extending the life of the installation.