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Community and Q&A

Air sealing (with closed-cell spray foam) WITHIN the thermal envelope?

matt2021 | Posted in Energy Efficiency and Durability on

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?

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  1. Expert Member
    BILL WICHERS | | #1

    It doesn't generally hurt anything to insulate or air seal within the building envelope (i.e. between two conditioned spaces instead of between conditioned space and the outdoors), but it's usually a waste of materials. If you do do this, I would advise only spray foaming ONE side of the rim joist so that it's not completely encapsulated in spray foam.


    1. matt2021 | | #2

      Thanks, Bill! For now, I don't plan to insulate the basement's rim joists; but if ever do, and have decided to air seal (by spray foam insulation) the floor's rim joist on the side where it borders the basement, I won't add any insulation, for that segment, to the basement's rim joist.

      All things considered, it sounds like I should skip air sealing the new room from the basement.

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