Several decades ago the spray foam insulation industry made possible the concept of bringing the attic into the house thermal envelope, this design is known as a conditioned attic. It simply moved the insulation from the attic floor to the underside of the roof deck; the result was an attic that never got hotter than the outside air.
A conditioned attic does not require direct-vented conditioning; the attic atmosphere is naturally conditioned by allowing the free flow of conditioned air from the living spaces below the ceiling into the attic. In this design all duct leakage is not wasted energy; it simply contributes to cooling/heating of the building. No vapor barrier or air impermeable barriers are permitted on the attic floor as these will hinder natural conditioning of the attic. The temperature of this attic will be within 3-10 degrees of the living space.
Before the introduction of spray-in-place open cell spray form insulation there was no cost effective method to insulate directly to the underside of the roof deck which effectively blocked conduction, convection and radiant heat transfer into the attic space; making it a HVAC friendly atmosphere. A good example of an ineffective attempt to insulate the roof deck is a cathedral ceiling insulated with fiberglass or other air-permeable insulation; it does such a poor job the room below is almost always uncomfortably hot.
In a spray foam conditioned attic, spray foam insulation is applied to the underside of the roof deck and all the exterior walls of the attic, joining with the exterior wall insulation, acting as a thermal break and as an air barrier, sealing all air leaks blocking pathways for convective heat transfer. This construction method is so energy efficient that no radiant barrier is required and can be achieved with only an R-20 value.
The most important energy saving features of any residential building are its airtightness and the ability to eliminate the hot attic; this is exactly what the spray foam insulated house with a conditioned attic assembly has done.
The HVAC equipment can now be placed in a friendly atmosphere, permitting proper HVAC sizing, prolonging equipment life, resulting in energy cost savings of up to 40% every month.