Conventional Attic (Vented) - Most standard homes built in the USA today make use of a ventilated attic space. Basically, the thermal envelope (insulation) is located on the floor of the attic space. The attic space above is then ventilated to help control moisture and relieve excess heat buildup. |
Figure 1. The yellow highlight shows the thermal envelope in a conventional attic. |
Conditioned Attic (Unvented) - The basic idea in a conditioned attic is to move the air barrier and thermal envelope up to the roof line. The most common way this is achieved is by spraying the underside of the roof decking with expanding foam insulation. There is no need to insulate the attic floor in this case. The entire attic space is now included as part of the conditioned area of the building. This is especially beneficial when heating / cooling equipment and air ducting is installed within the attic space. They are no longer located in an extremely hot or cold environment. In addition any air leakage from the ductwork and air handler is leaking within the envelope of the house and not to the outdoors as in the case with a conventional vented attic. |
 Figure 2. The yellow highlight shows the thermal envelope in a conditioned attic. |