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My home was constructed in 2016 with exterior insulation around the foundation (see image). They also put in this hanging batt fiberglass insulation on the inside of the walls, which seems superfluous. (On another note, part of me is worried that they did this to hide something...)

I started framing my finished basement and was planning on leaving a 1.5 inch gap between the concrete wall and the framing.

I live in PA, where I think you only need R-10 insulation on the walls.

Is it worth my while to put any insulation on the inside of my exterior walls at all? I was considering cutting up the batt insulation and shoving between the studs at the very least. But I was also considering just trashing it and putting a 1-inch foam board on the wall.

Is this all overkill?

Edit: To clarify: I guess I'm asking, with the exterior insulation > R-10, do I need any insulation at all inside? Or can I throw away this fiberglass insulation that's billowing off the walls?

foundation detail

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    Hello, and welcome to Stack Exchange. Unfortunately, this is likely to be too opinion-based for a good answer here. Oct 23 '18 at 11:17
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    It's not entirely clear what you're asking. If there's already insulation on the interior, why would you need to add more? Could you just frame inside it? Even if you compressed it slightly to get closer to the 1-1/2" gap you mention, it'll still do some good for no cost.
    – isherwood
    Oct 23 '18 at 13:21
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Yes actually, Insulation doesn't 'stop' heat or cold, it only (initially) slows it down. Therefore, your basement walls still are colder in the winter and warmer in the summer than the basement's air.

So, condensation will still always happen unless you foam tight to your walls (the best option) or behind your studs and never allow air to have any direct path to the temperature shock.

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  • Thank you, this makes sense. I'm also battling condensation here and that comes from the inside. Based on the comments here and this article from Building Science Corporation, I'm going to rip off the "diaper" insulation my builders put in, since fiberglass shouldn't contact concrete walls. Then I'm going to put rigid foam board on the walls and put unfaced insulation between the studs (maybe just rip off the plastic part from the diaper insulation).
    – Jack B
    Oct 24 '18 at 10:27
  • Sounds great! The foam by itself is enough to address the condensation and future comfort. Insulated stud walls will only slightly assist, but insulating the ceiling will be your biggest comfort and sound controller. The 'diaper' could even be re-used for the ceiling in a drop-ceiling setup.
    – Iggy
    Oct 24 '18 at 14:25
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The gap would be a good solution if you want air to circulate behind the frame which means you would have to have some holes every so often to do the circulating. If you don't want to have the holes, you would have to make sure that you have a vapor barrier. A foam board would do both...insulate and be vapor barrier.

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