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Climate Zone 5A (Eastern Nebraska)

I’m needing guidance on properly insulating basement walls that already have 6 mil poly fastened to the wall as part of a basement perimeter drain system. I recently noticed condensation on the warm side of the 6 mil poly (not between the poly and poured concrete wall) and am unsure how to proceed. (basement humidity is around 50%, basement air temp around 63 degrees F, outside temp is currently 20 degrees F)

Over the summer I installed a basement perimeter drain like in the attached picture. As part of the system, I covered the walls in 6 mil poly. The 6 mil poly is tucked inside the dimple mat at the bottom and then mechanically fastened at the top with Christmas tree fasteners. I then sealed the top of the 6 mil poly with a sealant and taped the bottom of the 6 mil poly to the dimple mat. This system has been in place for 6 months and appears to have taken care of my damp basement.

However, I recently started to finish my basement. Since I had the 6 mil poly on top of the poured concrete walls, I made the incorrect assumption that I could simply put in fiberglass batt insulation within my 2×4 cavities. Only after getting all of my drywall up did I notice condensation on the warm side of the 6 mil poly (not between the poly and poured concrete wall). The condensation is only on the top 1 foot of my poly with no current signs of condensation below that. The fiberglass batts are obviously wet, so I’ve since pulled off much of my drywall and removed the fiberglass batts in those areas.

While I have a few thoughts on how to remediate, I’m truly not sure what the best option is. I’m also very open to any other ideas you may have.

  1. Instead of filling my 2×4 cavities with fiberglass batt insulation, replace it with rigid foam board within each cavity.

  2. Move all of my exterior 2×4 walls in 1.5″. Then, slide 1.5″ rigid foam board behind the 2×4 walls, creating continuous insulation from the floor to the rim joist. (this is costly and time consuming, but willing to consider if no better alternative)

  3. Remove all of the drywall and spray foam all exterior walls. However, I had someone tell me this was not a good idea as the spray foam will not stick to my 6 mil poly. I also don’t have much space behind some of my walls to allow the spray foam to create a continuous insulation barrier behind the 2×4 wall.

Please also notice that I have open floor trusses. As a result, there is exposed concrete wall above the top of my 2×4 walls. So, while insulating each 2×4 cavity seems appropriate, it doesn’t address the remaining concrete above my 2×4 wall.

At this point I’m fairly desperate and can’t help but feel like I messed up my insulation from the beginning. I would sincerely appreciate any advice you might have! Thanks in advance!

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  • 50% RH is way too high for cold weather. Bring it down 10-15 points and you'll probably be fine. Use lots of air circulation. If you're not running any heat down there, do, at least temporarily. It'll help extract moisture.
    – isherwood
    Dec 30, 2021 at 19:07
  • You said "The condensation is only on the top 1 foot of my poly with no current signs of condensation below that." Is that part of the wall above ground, and is the rest of the wall below ground, or below the frost line? If that 1 ft section is above ground and it's winter, that explains the condensation.
    – SteveSh
    Dec 30, 2021 at 22:18
  • Yes, the first couple feet of the concrete wall are above ground. Certainly makes sense why the interior warm air is condensing on that coldest part of the wall. I just struggle with the idea of closing in fiberglass batt insulation that is wet and hoping it dries out. While a dehumidifier will help reduce the interior RH, I can’t help but think there’s a better insulation option that might reduce my condensation issues.
    – ch2021
    Dec 31, 2021 at 2:09

1 Answer 1

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You are having a condensation issue because of the temperature difference behind and in front of your poly.

https://www.nationalgeographic.org/encyclopedia/condensation/ provides an excellent visual:

Condensation can also produce water droplets on the outside of soda cans or glasses of cold water. When warm air hits the cold surface, it reaches its dew point and condenses. This leaves droplets of water on the glass or can.

Continuing with your available choices:

  1. Fur out the wall with 2x2 strips of wood so that the back of your insulation does not touch the poly
  2. Tear off 1-2 inches of insulation from the back of your, presumably, faced insulation so that it does not touch the poly
  3. Don't insulate. Now that you've discovered a condensation issue you should really allow for as much fresh air back there as possible or else you will face mold
  4. Reduce the humidity to the appropriate level based on outdoor temperature
    • Albeit this would be difficult behind a closed cavity

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  1. Use rockwool insulation because it can actually dry out after getting wet. Although I am not sure how it stands up against continuous moisture

Overall I'd be most worried about the studs touching the poly since they will incur rot. Re-framing a few inches away from the poly might sadly be the best option; I would skip the rigid foam board if you do go this route.

Additionally I think this quote is important:

You have made a huge mistake. The vapor barrier goes against the block wall or poured concrete wall. The insulation comes with a vapor barrier already on the outside (the paper side is the vapor barrier) You want that facing out. Rip it out do it right then you shall have some peace of mind that you know that the interior walls will not be prone to mold and mildew.

https://www.bobvila.com/posts/13337-condensation-on-vapor-barrier

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  • I'm reluctant to upvote because you lead with some fairly extreme (and probably unnecessary) measures. RH is way too high right now, so I'd start with your #7 and not panic.
    – isherwood
    Dec 30, 2021 at 19:10
  • @isherwood If that space becomes a sealed cavity then what good will dehumidification do? Also, I hesitate to believe that "keep humidity at 30-35% in the winter" is information that will be reliably passed from sellers to buyers as the decades pass.
    – MonkeyZeus
    Dec 30, 2021 at 19:15
  • We're not talking about a sealed cavity. The question refers to the warm side of the poly--the interior of the barrier. It's in the room. Once the humidity is brought down it'll dry up, even if it soaks into the insulation and drywall.
    – isherwood
    Dec 30, 2021 at 19:45
  • @isherwood once it's all mudded, primed, and painted then the warm side of the poly will be quite sealed from the dehumidifier in the room.
    – MonkeyZeus
    Dec 30, 2021 at 19:57
  • It should be dried up long before then and any future issues will occur on the sealed drywall, not the poly. But probably not at all. That moisture is from the interior space.
    – isherwood
    Dec 30, 2021 at 19:57

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