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I've got access to some discounted (factory seconds) of rigid foam board and looking to cut it up and place it against the exterior walls on the inside of an old (1894) converted house that is now four 1br apartments. Thinking of going with 1" of rigid foam board and then adding a fiberglass batt on the inside of that. With the old house, the 2x4 studs are actually at least 4", not 3.5. It's in southern Illinois and I'm trying to get some good advice on whether I should be concerned about vapor barrier and if the fiberglass batt makes this better or worse?

  • The XPS (or EPS) acts as a vapor barrier. You dont want to add another one. You can do XPS, then batt insulation, just make sure the batt insulation is unfaced. See this Q&A for more details; diy.stackexchange.com/questions/8636/… – n00b Jan 25 '17 at 14:28
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I am doing exactly this, but the reason for the rigid foam is to create an air channel along the underside of the roof to allow airflow up to the ridge vent. The layering is, from the outside inwards, roof, 1 in. airspace, rigid foam, batts, vapor barrier (plastic), drywall.

Understand that the function of the vapor barrier is to prevent moisture from migrating from the warm side into the total thickness of insulation towards the cold side, where at some point the temperature gradient will drop enough to cause condensation. You do NOT want this to occur inside the insulation. The fact that the rigid foam is also a vapor barrier is not a problem as long as you have a correct barrier immediately behind the drywall.

If by "adding batt inside" you meant "inside the wall - towards the roof", then I would not rely on the rigid foam acting as a sufficient vapor barrier unless you made a point of carefully sealing all the edges with Acousiseal. Preventing air migration past the insulation is critical to its effectiveness, and you must also provided for ventilation of the roof material itself.

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Well, it depends:

  • What is the thickness of your fiberglass insulation?

If you can use the foam board and fiberglass - without compressing the fiberglass batts, then I say go for it.

However, if you end up compressing the fiberglass then you will greatly lose the rated R-Value with an improper installation and almost certainly negate any possible benefits from the foam board. Poor insulation installation invites irritating inconsistencies in hot and cold spots within a room.

With regards to the vapor barrier, you should place a vapor barrier on the drywall side of the fiberglass insulation. (You can use insulation with kraft paper - which counts as a vapor barrier. Otherwise, you can use a thin sheet of plastic.) The vapor barrier keeps moisture out of the fiberglass so it doesn't saturate and effectively insulates.

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I would not put in rigid foam along with fiberglass batting just because you have access to some product cheap. You do not want to trap fiberglass between two vapor impermeable layers. The designs of insulation products have been arrived at by scientific principles and by trial and error. If you try to cobble something together, thinking you are going to get better results on the cheap, you are likely to make a mistake. It would be far better to have an incompletely filled cavity than trap moisture in the walls.

In a cold climate the kraft paper side of fiberglass bats is normally next to the interior drywall. This keeps the moisture generated inside the house from permeating the fiberglass where it would condense.

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