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I have a 1920s house with concrete foundation that had a semi-finished basement. After a recent 3-day rain storm, my basement flooded (about 1") due to hydrostatic pressure. It wasn't just my house -- all the homes on my row had the same issue.

I removed the drywall and insulation from the finished portion of the basement. Some areas of black mold were found and I am working on getting that taken care.

I am looking at insulating the basement again because it gets cold down there and I am using it as an exercise/home theater area. So, do I need to insulate the entire basement or can I just insulate some of the walls like it was prior?

According to information from buildingscience.com, it is recommended I apply rigid foam insulation to my walls:

The best insulations to use are foam based and should allow the foundation wall assembly to dry inwards. The foam insulation layer should generally be vapor semi impermeable (greater than 0.1 perm), vapor semi permeable (greater than 1.0 perm) or vapor permeable (greater than 10 perm) (Lstiburek, 2004). The greater the permeance the greater the inward drying and therefore the lower the risk of excessive moisture accumulation.

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Just insulating the ceiling will keep the ground-floor floors warmer, if you walk around barefoot, but won't save much energy (air-sealing is a better investment for that) or keep the basement any warmer.

If you want to improve the basement climate, any insulation of the walls will help retain a corresponding percentage of the energy you're losing. Air sealing helps a lot here too, as does an indirect-vented boiler or furnace (so you aren't pulling cold air into the basement to make up what's used for combustion).

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  • This is the correct answer. As airflow probably isn't a factor with a foundation that's probably poured concrete, thermal conduction is the problem that needs solving. Any wall insulation helps in proportion to the total area.
    – isherwood
    Jan 5, 2016 at 14:51
  • How can you answer the question without knowing where the OP lives?
    – DMoore
    Jan 5, 2016 at 18:28
  • It's pretty much basic thermodynamics.
    – keshlam
    Jan 6, 2016 at 1:01
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Try it without the walls & just the full ceiling fully insulated, I usually find no comfort difference with the walls insulated & especially with just some of the walls insulated. I mostly say this because mold takes some months to be readily noticed, you had moisture & ventilation issues for a while. Immediately dispose of any old wall & insulation debris, if you haven't already. And, scrub bleach the walls at least twice. Also, whether you do new walls or not, fully seal the ceiling (new paint & perimeter caulk) so moisture doesn't continue to get up in there at the current rate, assuming the mold was limited to just the bottom of the walls. Open & airy is best, but the rigid foam works as good or bad as anything else...mold really doesn't care what you put there.

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    Are you proposing that the OP insulate the basement ceiling (between floors)? I don't follow your reasoning. Most heat loss in this case is through the foundation walls to the 50-degree soil.
    – isherwood
    Jan 5, 2016 at 14:49
  • Yep. And then, nope...hot air rises. That's why buildings have very little insulation in the walls compared to the attic or roof. Wall insulation might only SLOW the cold transfer, but almost no basement is deep enough for your 50-degree stuff. The cold still gets completely through regardless of how much insulation or air sealing or whatever you've been sold is in place. By definition Basements can't be "drafty".
    – Iggy
    Jan 5, 2016 at 15:23
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    I'm not sure what you're saying, exactly, but here in Minnesota we get frost down to about 40", and below that soil remains at around 50 degrees. What does convection have to do with that? No one insulates between floors unless the basement is unconditioned.
    – isherwood
    Jan 5, 2016 at 15:26
  • Well, don't heat the basement & you'll see nothing stopped the cold. Your houses must be hot as hell up top & chilly at the bottom with that absurdly dumb practice. My last 3 places that I did just the basement ceiling in acted like I had put in zone heating or something. Very different temps on each floor became very close temps, for both cooling & heating.
    – Iggy
    Jan 5, 2016 at 15:44

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