As I will be having my future four-season room (from a covered porch) spray foamed in the ceiling (between the rafters… etc., as some of you already know), an interesting question came to my mind.  I am going to ask the installer to also spray foam the gable areas, and the floor’s rim joists – the latter strictly for air sealing.  (Maybe I will also have the two wall’s vertical posts, where the porch attaches to the rest of the house, sprayed, for the same reason.) The floor is going to be raised above a concrete slab.  On the side of the house, the new floor’s rim joist is going to be at the height of the (unfinished and not conditioned) basement’s rim joist.  Would it make sense to have THAT side of the new floor’s rim joist also sealed?  If I understand things correctly, that would mean air sealing between two spaces WITHIN the thermal envelope.  Would sealing off the new room from the rest of the house that way prevent any heated air, for example, to “leak” into the basement?  Or, in contrast, would such air sealing prevent a potentially beneficial (but for what?) air circulation between the basement and the new room?