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I am helping my brother-in-law finish his basement. I noticed the framing was done first then the insulation was added after.

It looks like rigid foam board was pressed up against the concrete wall and added in between the studs. My primary concern is I am noticing a small pocket between each stud on the wall.

I am adding two images Below. See Both Images for a better example of what I am trying to explain:

My Conerns - Will this pocket be a serious cause for moisture build up? Is rigid foam board up against the foundation wall a bad practice on a finished basement?

Other Notes - We are in Climate Zone 4. His basement has a Radon Mitigation unit (if that remotely helps with humidity below the foundation?). His basement does get moist at times but nothing to crazy. He is using mineral wool insulation.

Real example Additional Explination Revised Image

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What he has going on here is fine for above grade. However for below grade I have some issues.

  1. It seems that the framing should have been spaced better from the wall. We usually leave a good 1" gap on framing in basements to allow air flow. Jamming the framing next to the wall to provide a couple of inches in each direction isn't worth it.

  2. Yes the rigid boards should have gone on first and then framing - it is too late now.

  3. The framing seems to be sitting on the wall and is not PT.

Here is the thing, I do a ton of basements in your same climate zone. There is no impact on insulation after getting past 1-2' below grade in climate zone 4. The heat or cooling loss will be a factor of the venting done around the top of the basement. What ever minor impact this extra insulation has in the winter will have the same or more negative impact in the summer.

I am currently sitting in a house that's basement is mostly below grade. I put crosses between framing 2' below grade all the way around and there is 5-6 feet of empty cavity. I just made sure the top was iron clad. In the winter my kids have to close the vents because it stays so warm. In the summer they have to do the same because it stays cool. That is what you want here.

By over insulating your basement will not cool as well in the summer, except by used air conditioner and you are basically creating a possible mold/rotting risk for no viable reason. I see the picture of the window but not other parts. My initial reaction would be to cut the foam and mineral wool about 3' from floor. You for sure shouldn't be sealing your rigid sheet like that on the bottom. The current recipe you have is for water to hit the bottom plates and wick up into the mineral wool and drywall.

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  • This was VERY helpful. I should be more clear here - the home is a split level home. So the basement is not fully below grade. I would say maybe only 30-35% of it is below grade. I revised this post and included a useful image that would help clear up whats above grade vs below grade. You can also see it here: i.sstatic.net/MKUeS.png My question to you @DMoore would it make sense to keep the Rigid Foam Board and Mineral Wool on the areas which are above grade? (Or just barley below grade). And the areas that are 3-4 ft below grade just cut away the insulation.
    – Andrew
    Commented Apr 23, 2020 at 21:23
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    The walls that are fully exposed I would just leave a foot of air gap (you could keep rigid up and maybe just cut it 1" over bottom plates) at the bottom and the others I would just go below grade 1-2 feet. Being realistic that door area is probably going to let in 50-70% of the cold in the winter. I think you guys are possibly over complicating this. A storm door over the basement door would probably do more than all of this bottom insulation. (you do need a good foot under windows for sure)
    – DMoore
    Commented Apr 23, 2020 at 21:29
  • Appreciate it - the fully exposed walls I am going to leave a foot air gap (remove both rigid panels, and mineral wool). The other areas I am going to do the same but increase it 1-3 feet. Out of curiosity and for my own knowledge how does this help prevent moisture from getting trapped. I have heard some folks say insulate everything and I have heard others say absolutely not.
    – Andrew
    Commented Apr 23, 2020 at 22:17
  • It’s almost as if the concrete walls that are deeper below grade it makes more sense to keep them exposed then everything else insulate.
    – Andrew
    Commented Apr 23, 2020 at 22:31
  • Also I want to note that your biggest heat losses will be windows, doors and right where the floor joists meet the outside. Above the frames walls you should have the mineral wool packed 2-3 feet deep if you can.
    – DMoore
    Commented Apr 24, 2020 at 0:44
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Air layers in insulated walls could help to carry the condensed water out (if the air can circulate) - otherwise it could accumulate moisture.

It depends on the sequence of layers. In this wall here the insulation layer is at the inside of the concrete layer, which could be a problem.

To get precise information, this web site may help:

Ubakus.de

By inputting the layers, possible problems will be shown in a second.

Experimenting/researching is easy by switching on/off of layers.

R-value can be shown by clicking on the spanner/tool icon next to the U-value.

It is free for private use.

Fastest way is to choose an adequate example (menu) and to edit the layers.

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