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Basement inwards vapor drive

user-7600779 | Posted in General Questions on

Hello Green Builder;

I have recently started remodeling my basement. It is a 40´s house located in a mixed humid climate.

I notice that the paint on the walls is destroyed due to inwards vapor drive and salt accumulation. The basement walls are not damp-prof. They are simply constituted by solid pilled rocks with mortar as the most interior finishing layer (painted).

However, there is no record of liquid water penetrating the basement, or any kind of flood event.

I was thinking of directly applying XPS boards to the walls (taped) and a wood frame on top of that with rock wool. However, I am afraid of the possible condensation problems and mold growth between the wall and the XPS boards. Because the vapor is still going to move from the damp soil to the interior.

Am I approaching this right?

Thank you in advance,


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi ER.

    It may just be some inward moving moisture that is causing the paint on your basement walls to fail. Old stone foundation walls tend to hold some moisture and it needs to go somewhere. Make sure you don't have water intrusion problems before you insulate your basement walls and if you do, fix them first. Typically, closed-cell spray foam is the best option for stone foundation walls. I recommend that you read this article: How to Insulate a Basement Wall

  2. user-7600779 | | #2

    Hi Brian;

    Thank you for your comment! I had already read that excellent article.

    I do believe that is solely some innocent moisture coming in. Right now, this moisture can very easily escape and dry towards the interior. There is a lot of moisture in this soil.

    Spray foam is out of the question, due to some allergic problems. My move is going to be with XPS (taped, for air and vapor tightness), held in place with wood strips.

    However, I am afraid that inwards vapor drive (which will continue) will not dry that easily (I am aware that some moisture can move through the XPS, but still ...) creating a sever condensing interface between the wall and the XPS boards and consequent mold growth.

    My second option is to drop the insulation and create a draining plane near the wall with some roofing membrane. I was thinking of leaving a small air gap between the wall and the membrane (to ventilate some inwards moisture). But again, some concern about the mold problem between the wall and the membrane.

    With all these strategies, the wall will not dry as it does now. Instead of having paint defects, I am expecting mold defects.

    1. GBA Editor
      Brian Pontolilo | | #3

      Hi ER.

      You would only need a drainage plane if you had actual water making its way in, and again, if you did, you should fix that first. You can also consider using EPS rigid foam instead of XPS. It is both more vapor open and more environmentally friendly.

  3. Expert Member
    BILL WICHERS | | #4

    My guess is that “salt” you’re seeing is really efflorescence, which is dissolved minerals coming in from moisture. The drylok people make a special cleaner to help remove efflorescence, but you’ll need to correct the moisture issue or it will come back.

    If you don’t have bulk water coming in (wet walls, drips, etc), you’re probably ok with XPS or EPS directly against the wall. If you seal the foamboard, any moisture behind it is trapped so you won’t really have mold problems due to the wall being effectively encapsulated. If you have bulk water problems, and you might — efflorescence is an indicator — dimple mat behind the foamboard and a perimeter drain might be a better way to go.

    You really want to address basement water problems on the exterior though. Grade the ground so that it slopes away from the structure, and make sure your gutters drain water away from the structure.


  4. user-7600779 | | #5

    Than you all for your prompt responses!

    Yes, that right, efflorescence and paint fail all over. However, in more than 80 years, no bulk water or basement flood was detected. So, I think it is just diffusion. My climate zone is nasty, a lot of rain. Furthermore, I can't afford to slope the soil away from the building.

    Yeah, addressing from the outside is top-notch. However, the previous owners replaced all the exterior tiles and redone the landscape. Tackling from the outside is just not feasible.

    Totally agree with you on the concealed mold. However, it is just me who does not like black and yellow mushy things within the wall assembly? Even if they are enclosed behind the foam.

    I was also thinking in allow for some degree of ventilation between the concrete and the XPS to enable that moisture to escape. Conceleing, this way, the future efflorescence (but avoiding mold).

  5. Expert Member
    BILL WICHERS | | #6

    You can use wrinkled house wrap for a little ventilation between the foam and the wall. There is also a mold inhibiting wall sealer (I think zinsser makes it) that is similar to drylok but is specifically designed to prevent mold. You could use that anti-mold
    Sealer on the wall prior to putting up the insulation for a little extra insurance.


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