We go to great lengths to make our walls airtight and well-insulated, and then we turn around and put big holes in them. Windows. They look simple, but they are highly-engineered machines… pulleys and weights, argon gas, solar coatings and more. Architect Don Powers helps us decode R-value, U-factors and the typical window label, architectural historian Elizabeth Milnarik traces the history of the window, and This Old House general contractor Tom Silva reminds us that old windows are sometimes worth saving.