Proper attic ventilation is the keystone in creating effective hot/cool air circulation in your home. This could include vents, soffets, and baffles to create air channels. Without these simple materials your attic will effectively be an oven in the summer and a freezer in the winter. Not only will the hot air increase the overall temperature in your house during summer months, but it will also "bake" your shingles and thus shorten your shingle lifespan exponentially.