You may have heard a metal roof referred to as a “cool roof”. This is because it works like your loose weave, light-colored shirt. The metal roof “passively” keeps your house cooler in the hot weather, that is, no electricity is used in the process. To understand why this is so, you need to have an understanding of some of the principles of passive solar design.
First principle that effects the efficiency of the metal roof is its mass. Mass can be thought of as the weight of an object or material. Stone has more mass than wood, because a sample of stone weighs more than a similarly-sized sample of wood. A metal roof has a lot less mass than that of a heavy asphalt roof. A higher mass results in a higher amount of heat absorption. Though metal can quickly get very hot to the touch, it does not hold on to the heat when a cloud conceals the sun or a breeze rolls through. A metal roof cools down much more quickly than one made of traditional shingles or tiles.
Next consider the color of the roof. Just like your shirt, a lighter colored roof will absorb less radiant heat. Along with the color, you need also look into the amount of reflection in the color. Modern technology has produced many dark-colored metal roofs with high reflective pigments. Good reflectivity is achieved despite the dark color. No longer are you limited to a silver or white roof in order to claim the benefits of a highly-reflective, energy efficient roof.
Integral Airspace is another Passive Solar principle which effects the efficiency of your metal roof. A gap of air exists between a metal roof and the roof sheathing because the metal roof does not lie tightly against the sheathing. This air insulates the roof sheathing from the conducted heat coming from the metal above. Imagine a pad or trivet put down to protect a counter top from a hot pan. This is the same idea. On the other hand, an asphalt roof lies tightly to the sheathing. The heat is conducted into the rooms below, and, in turn, increases your air-conditioning bill
No matter what the choice of roofing materials, it is a good plan to provide sufficient attic ventilation. Metal roofs do not replace this essential design feature. Although metal roofs tend to cost more initially, money will be saved in air-conditioning/electrical costs. Plus your attic and house will stay cooler in the hot summer months.