Spaces are placed in areas of the building model to determine the volumes for each area. Revit computes these volumes as an analytical volume and an inner volume. The analytical volume is bounded by the center plane of walls and the top plane of floors and roofs. You use the analytical volume to verify that there are no gaps in the building model. The inner volume is bounded by the interior surfaces of walls, floors and roofs. Inner volumes are computed when the loads calculation is started.
See Calculation for information about how heating and cooling loads are calculated.