Fiberglass window frames also cost more than the other types of window frames. A divided light window is considerably less energy efficient than other types because of the large number of glazing edges. However, modern multi-pane windows are often whole sheets of glass with optional plastic or wood grilles that attach to the glazing to give the appearance of a divided light window. Spacers separate the panes at the edges. Until recently, most edge spacers were made of metal, which means that the edge of the window has little or no insulating value.
Many manufacturers now use improved edge seals with much better thermal performance.
Metal and vinyl frame corners may be welded or screwed together. Either one is acceptable. However, welding a corner can often distort the weatherstripping groove. Always check how well the weatherstripping meets at the corners. There must be no gaps.