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We're finishing our basement, and after much research have settled on using EPS foam boards attached directly to the concrete wall to insulate and control moisture. However, there is not enough room for the foam between the concrete and the existing stud walls that the builder originally installed. Consequently I am going to have to tear down the walls, install the foam boards and then reassemble the walls.

However, we are not finishing our entire basement as we will be using half of it for storage and workspace. So my question is, can I get away with not tearing down walls and insulating the half that will remain unfinished? Or will this just create moisture or other problems down the road?

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  • I live in Ohio, US. Unless you live north of me, I don't think it is necessary when comparing to the risks of moisture issues. The earth acts as a great insulator. Insulating your rim joist is probably your best solution to temps. Just my two cents. My dad is an architect and we have discussed this topic numerous times. When I finished my basement, I built the walls 1-1/2" out from the foundation walls.
    – Evil Elf
    Mar 28, 2017 at 17:24

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You do not have to insulate the part that will be unfinished, but you will need to insulate between the finished space and unfinished space and include an appropriate moisture barrier. All things being equal, if you don't have moisture problems in the unfinished area now, you are not likely to create them as long as the unfinished space has adequate ventilation. I will follow that statement with the disclaimer that there are a lot of site specific circumstances that can cause moisture problems whether you insulate it or not.

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All outer walls of a basement should be insulated. Period. It just doesn't make sense to not insulate them, especially in your region as the cost is minimal and would probably be regained in the course of the first year's energy bills.

Here are some general notes:

  • you do not need to insulate between the finished and unfinished part of the basement. This is a waste of insulation and should have no effect on anything if other things are done right.
  • you can insulate the ceilings, for sound.
  • you cannot just throw foam boards on the wall in unfinished space unless they are rated to be used in an exposed area.
  • the number one thing you need to do BY FAR is insulate the crap out the joist area in the basement - finished and unfinished spaces. Just doing this is 90% of the energy savings. This can be done by stuff roxul in. I just did mine and 1/4th of the basement is unfinished with about 2 feet (R 100+) of roxul stuffed in all outer joist cavities. The basement is a damn oven in the winter and kids actually have to close the heating vents.
  • as far as other insulation, you can have foam boards on walls or whatever... but if you read the Building Science reports done 7-8 years ago these will have little to no effect on your energy savings if the insulation is below ground level.
  • To further this, not having insulation on the lower parts of your walls will create an air space where moisture can cycle and evaporate without leaching onto insulation.
  • So to sum everything up we insulate the joist cavities only in unfinished spaces (roxul or spray foam) and we do the joist cavities plus anything else above ground level on finished spaces. So during framing we will add crosses about a foot below ground and then stuff roxul in those spaces to header.

So my advice - and I am in a similar climate - is that the EPS is not needed. Keep your existing framing. Add crosses so you can only insulate the area above ground level, stuff the joists, and enjoy the 30 hours of work I saved you on doing something else. (Also this will probably save you a couple thousand dollars)

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  • Thanks for the information. I should probably add some more details however. Firstly, we're in Vermont. Right now, the existing studded walls in the basement already contain fiberglass batts from the time of original construction. The temperature in the basement in the winter is warm, but still about 5-7 degrees colder than the rest of the house. My plan was that in filling the floor joists filled with spray foam and added the EPS on the walls the temperature would remain closer to the rest of the house.
    – user3120588
    Apr 5, 2017 at 14:20
  • There are no existing moisture problems, but I worry that making the basement warmer in the winter will lead to a greater chance of moisture problems down the road. And everything I've read on the Building Science site says inward drying via EPS or XPS is the best way to prevent moisture.
    – user3120588
    Apr 5, 2017 at 14:30
  • Actually the Building Science report that an air gap is the best way to prevent moisture. You are correct that EPS or XPS is the best if your climate requires insulation below grade but that is not the case for you. The air gap plus warmer air will lead to a quick evaporation cycle. One of the biggest impacts the Building Science reports has had on basement building is they basically said putting fiberglass insulation everywhere is the worst thing you can do. Because it holds water and there is no air gap. EVERYONE used to throw fiberglass everywhere before this.
    – DMoore
    Apr 5, 2017 at 14:58

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