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Community and Q&A

Vapor Barrier for Concrete-Block Basement

BasementBuilder | Posted in GBA Pro Help on
Hi Martin!  Your mastery of this medium, and the comment section, is extremely helpful; I’ve read all pertaining to my question but want to clarify in case it helps others.
My questions:
Is it redundant to install a poly barrier against a below-grade concrete block wall, and then install extruded foam over it, or can I just install extruded foam and tape the seams?
What barrier would you suggest, for example is a dimple mat better or is cheaper poly acceptable?
  • Goal: finish my basement to a degree that it is rentable and comfortable year round.
  • I have a house built in the 1950s.
  • According to the previous owner, the basement had water issues in the past.
  • To correct them, he hired a company that installed an interior drainage system.
  • This consists of a small gap at the seam where the concrete block wall meets the concrete floor.  The walls were also treated with some kind of waterproof paint.  This was over 10 years ago.  There is a lifetime warranty on this work.
  • I would like to finish the basement to be a separate living space, which is allowed under local codes.
  • The basement is a walk-out, with 20% fully below grade and 60% partially below grade.
  • On every wall, even the ones that are not very far below grade, there are small patches of paint that bulges out.  When scraped, these are filled with flaky powder, and have bare concrete block behind them.
  • Taping plastic to the walls and floor results in condensation underneath the plastic.  Rubber mats left on the floor have condensation underneath them.  According to my (rather old) dehumidifier, humidity is about 60%.
  • Because there is a lifetime warranty, I think it makes sense to have the same waterproofing company address the bulging paint and interior moisture.
  • They propose covering the walls in a membrane for about $3,000.
  • As I will also be covering the walls in extruded foam panels for insulation, is it redundant?  I’m thinking the membrane can go down into the seam to ensure that all water drips into it, but I am a bit concerned about what I don’t know, for example the risk of mold.  I am in the south where the temperature fluctuates wildly and humidity is naturally high.

To be clear, I am absolutely fine doubling up if that is the correct choice, but want to be sure I am not missing anything.  For example, this article seemed to be against the idea of installing poly against the concrete block:

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  1. Expert Member
    PETER G ENGLE PE | | #1

    The blistering of the paint on the walls is caused by moisture moving through the block and drying to the interior. The dust is called efflorescence and is made up of soluble salts that are naturally in the masonry, carried to the surface by the moisture flow. With some older block foundations, the efflorescence forms just below the surface of the block, causing the surface to spall off. Over time this can cause significant damage to the block. Adding a vapor barrier to the surface of the block can actually reduce this effect by keeping the space behind the barrier damp and therefore, slowing down the rate of evaporation of moisture and deposition of the salts in and on the block.

    If you do add a VB, it must be sealed at the top and all seams. At the floor, the best practice is to insert it into the gap between the slab and the wall (as you suggested above), and then seal the gap between the VB and the floor slab. The goal is to stop all possible airflow behind the VB and also to stop possible moisture and other soil gas movement from below the slab into your wall system and/or basement space. The space behind the VB will be always damp and perfect for mold growth if there is adequate food for it - clean the wall first but know that there will still probably be mold back there. This is not a problem if there is no communication between that space and the basement breathing space.

    I prefer using a dimple mat for the VB because it provides some drainage space, but a thick poly film is OK, too. If you use the dimple mat, use the manufacturer recommended termination bar at the top to seal it to the foundation walls and tape the seams. Any rigid foam on the walls will work, though I prefer Dow Thermax with foil facers for yet another layer of vapor management, again with taped seams. You can build a stud wall inside of that for wiring and plumbing, or just fasten furring strips through the foam to the block wall. Drywall should be mold-resistant variety.

    A cheaper approach is to skip the film completely and use the Thermax with taped seams as the only VB. This approach is "safe" but may not have the durability of the belt-and-suspenders approach. The termination at the floor is a bit tougher but can be done with a high-grade sealant if it's done diligently.

    1. BasementBuilder | | #2

      Phenomenal Peter! Thank you, this is the kind of detail I was seeking before I paid the deposit to the waterproofing company. What kind of tape should I be using at the top and seams? And I agree with the belt-and-suspenders approach for two reasons: First, it's a basement, but I want it to feel like it isn't in terms of humidity. I'll have mini-splits, etc, but this is more of a comfort play vs an ROI decisions. Second, as I will be putting a lot of expensive material down there, like paint, drywall, etc, I don't want to have to redo all of that later. Once mold sets in, tenants will b understandably concerned. If I'm forgetting anything please let me know!

  2. Expert Member
    PETER G ENGLE PE | | #3

    For taping the foil facers of Thermax, there are several foil tapes with butyl adhesives available at the big box stores. For any of the various membranes, go with their manufacturer's recommendations for tapes.

    1. BasementBuilder | | #4

      10 out of 10, thank you!

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