What do over 20,000 neighbors know about glass block windows that you don't? Thick, break resistant glass block windows, properly installed makes a virtually impenetrable glass block window. No more storm windows. Stark Glass Block offers the insulating efficiency of double pane glass with an average R-Factor of 1.96. Start saving money on your heating and cooling costs, now! With glass block windows there's no caulking, puttying, painting or rust. Just hose down, wipe the edges and your glass block windows will look good as new. Glass block windows combine the beauty of light with so many functional characteristics.