Panels of tempered glass (or a tough plastic) mounted on a rustproof aluminum frame fill garages with light. Choose clear, frosted, or opaque single-pane or double-pane panels. Wood-clad frames are also available. Because of their weight, glass doors require heavy-duty springs. The R-value for single-pane glass is a minimal 1.5; double-pane glazing is R-4. Pricing is similar to composite doors.