Attics

The principal source of summertime attic heat is direct sunlight on the roof of the home.  This is radiated heat, and even on a cloudy day, there is an appreciable amount transmitted to a roof.

This solar heat on the roof is transmitted through the roof material and, in turn, is radiated to the attic floor—or to the top surface of the ceiling insulation material.  This surface becomes heated, and the attic air in contact with the underside of the roof and the top of the insulating material also becomes heated.  Convection allows some circulation of the air so that more and more of the attic air is heated.

Gradually, the temperature increases until the entire attic—the roof, floor, insulation, and air—are extremely hot.  In an unventilated attic, the roof sheathing may reach a temperature in excess of 160 degrees Fahrenheit (F), and the attic floor 150 degrees F or more when the outside temperature is in the 90s.

When the sun goes down, the source of heat, of course, is depleted.  The roof begins to reradiate the heat from the attic to the outside air.  Sometimes the heat absorbed by the structural materials, including the insulation, may not be entirely removed during the cooler night hours.  The heat then builds up over a long period of hot weather.  The heavier the structural material, the thicker the insulation and the amount of stored items present, the greater the amount of heat may be stored.

Intense attic heat is transmitted to the ceiling surface of the living space below.  The ceiling acts as a “hot plate,” not only warming the air in the rooms but radiating some of the heat to the occupants as well.  This, in turn, adds to the air conditioning requirement—both in the size of the unit needed and in operating costs. 

The portion of the solar heat that reaches the living area through the attic is proportional to the difference between the attic floor and room ceiling temperatures.  Adequate ventilation can substantially reduce this temperature difference.  Ceiling (attic floor) insulation retards the rate at which the heat flows to the rooms below.  A cooler attic floor reduces the quantity of heat, which the insulation must keep out. 

Ventilation simply makes the insulation more effective.  Ventilation also reduces the quantity of heat, which is stored within the insulation and other structural materials during the day.  This ensures a quicker and more complete cooling of the attic during the night. 

Seasonal build-up of heat is then minimized or eliminated.