Older windows often leak, letting in hot summer air or cold winter drafts into the house. Double hung windows, since both sashes are designed to move up and down, are even more guilty of these faults. Newer double hung windows are made with more durable materials, like wood and fiberglass, and precision tools allow them to fit their frames much more comfortably. Since newer windows provide much better insulation, older windows should be replaced after twenty years or so. Today's double hung windows use internal springs to support the panes, but older windows typically featured counterweights, taking up room and decreasing the thickness of the wood or aluminum, weakening the window's insulation capability.