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I'm in the process of renovating a bedroom on the second floor and I have a few boards of silverboard (a Polyisocyanurate Foam Board Insulation) lying around and would like to use. The exterior wall is brick followed by a spacing followed by another wall (some sort of concrete). Since the silverboard has radiant shielding on both sides and sandwiches a stiff foam board, would I:

1) install the silverboard in contact with concrete wall, or

2) leave a spacing in between the concrete wall and the silverboard?

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    Are you installing the insulation on the inside or the outside of the concrete wall? – ThreePhaseEel Sep 15 '17 at 4:18
  • The insulation will be on the inside, it's my first time using such a product. If it was regular radiant shielding then I would leave a gap, but because this has a foam board it complicates matters. – nonameswereavailable Sep 15 '17 at 4:21
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    Does the wall already have a vapor/air barrier in the assembly, or is it a pure mass wall with no insulation or barrier layers? Or is it ICF? – ThreePhaseEel Sep 15 '17 at 4:22
  • To @ThreePhaseEel's point, do you already have insulation on ... I mean, IN the wall? I would not use foamboard at all inside of the wall. Instead, I would use a roll foam or heck, the messy "new" spray foam. but not a foam board. it will not serve its purpos inbetween joists, and you do not want it compressing under a drywall either. – noybman Sep 15 '17 at 4:25
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    No vapor barrier, no insulation, its just a brick wall on the outside and a slab on the inside. There is an air gap in between the brick wall and the slab, also no ICF. – nonameswereavailable Sep 15 '17 at 4:28

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