aaronm wrote:It was my understanding, from the IECC, that the attic is the exterior of the building envelope and thus the exterior of the house.
The attic is outside the thermal envelope of the house, however, the attic is not the same as "outdoors" with regards to the thermal envelope of the house as the attic requires a much higher insulation R-value than is required for the exterior walls - which do have the "outdoors" on their exterior side.
The air in the attic is considered to be "outdoor" air for many reasons, but the attic air and the outdoor air are not the same temperature and thus are treated differently with regards to the thermal envelope of the house.
Here is just one conflict regarding the attic air: 1) the attic air is considered to be the same as "outdoor" air for combustion make up air; 2) the attic air is not considered to be the same as "outdoor" air for exhausting exhaust fans. There are many such conflicts regarding the attic air and "outdoor" air.