Insulating in Zone 5. Basement is 90% below grade, maybe 8" to 12" exposed foundation around the top of the basement wall.

Applying R-6 Foil Faced Poly Iso to the poured concrete wall, then standing 2x4 stud walls tight against the foamboard so they help hold it in place while the glue on the concrete side dries. Then going to run wire and such thru the stud cavities and want to insulate that space as well. Since Foil Faced Poly-Iso is considered a vapor barrier with very low perm rating, I am thinking better to use unfaced fiberglass, but on the same note it takes more work to install unfaced fiberglass, and cutting around boxes is now messy and doesn't lay as nice.

I'm thinking that really there should be no moisture between the foamboard foil edge and the kraft faced insulation, but worried that if something happened, say a leak or whatever, that moisture may get trapped in the stud cavity.

Any input, or experience from others who have done this? I believe the foil faced insulation will be great and preventing any moisture from outside migrating or condensating inside the walls.

1 Answer 1


You don't want to apply a double barrier as this may result in the dreaded "moisture sandwich" inside the wall.

Use unfaced batts. In fact I'd recommend rockwool over fiberglass for your use case due to its moisture resilience and easier working.

  • In addition to using unfaced batts, the pros use spray on contact, or packaging glue to assist in keeping the insulation in place.
    – Jack
    Jul 12, 2020 at 22:39
  • I'm not sure how paint and drywall don't cause that to already be a moisture sandwich but I thought unfaced made more since, so unfaced it is. Mineral wool is wayyyyy too expensive. If this were an old home with moisture problems maybe I could justify that cost. It's more than 4 times the cost of the unfaced fiberglass. Trying out the great stuff version of construction adhesive but it's so far sucking. The Polyiso has foil on both sides so probably just doesn't stick to the foil well. Maybe on XPS or EPS foam board it would.
    – CCCBuilder
    Jul 15, 2020 at 17:21

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