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If I use foam insulation on the outside (2") and in the 2 x 6 walls (from 4 - 5"), do I need to fill in the leftover space on the inside with fiberglass or Roxul, or just put the vapor barrier over it leaving 1 - 1 1/2" air space?

  • What are you trying to do? There's no reason you couldn't have some air space in your walls. You want to avoid creating a layer that has vapor barriers on both sides (thus no way to dry) but that applies whether the layer contains only air or contains air-permeable insulation such as fiberglass or rock wool. – Shimon Rura Aug 15 '15 at 20:20
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    Where are you from? – DMoore Nov 19 '15 at 23:08
  • Don't add a vapor barrier unless you're in Canada or farther north. – iLikeDirt Dec 20 '15 at 0:59
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The space does not have to be filled but should not be between the two foam layers. The required R value does have to be met. To find your total R value: add the specified ratings for both foams. Why are you putting the vapor barrier on the inside? Is this code compliant where you are?

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