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Spray foam only the southern face of a roof?

MALCOLM TAYLOR | Posted in General Questions on

I have a friend in climate zone 3 who is suggesting spray foaming only the south facing roof plane of their existing vented gable attic to try and slow heat gain. Any thoughts on how effective that would be?

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  1. Expert Member
    DCcontrarian | | #1

    This is in addition to insulation on the attic floor?

    My intuition is that the same money would be better spent adding more insultation to the attic floor.

    1. Expert Member
      MALCOLM TAYLOR | | #2


      Yes. It sounds like it's a trussed roof with gable vents at each end. He doesn't want to disrupt the exterior, or condition the attic. I've never heard of doing what he is suggesting.

      Worrying about excess heat it not something I've ever needed to turn my mind to here, so I know very little about it. Would a radiant barrier with a small airspace under the top chords of the trusses do much the same thing as the spray foam?

      1. maine_tyler | | #3

        "Would a radiant barrier with a small airspace under the top chords of the trusses do much the same thing as the spray foam?"

        That was my thinking, since it sounds like the spray foam would be to simply reduce the temp on the underside of the roof. Also, I wonder how much more irradiance is experienced on the southern side to justify a fairly large expense (the spray foam) ONLY there and nothing at all for the north side.

      2. Expert Member
        DCcontrarian | | #4

        I would back up and ask him why he thinks he has "excess" heat. A lot of people think the purpose of venting an attic is to cool the attic, not realizing the true purpose is to vent moisture. So if the attic is at 120F or 130F on a summer day they think something's wrong. The attic needs to be hotter than outside in order for venting to happen.

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