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How to effectively move moist air in basement when using foam board

[email protected] | Posted in General Questions on

I’m about to kick off a basement renovation. Previous owner finished the basement, but did it poorly. My goal is to reduce humidity, have more consistent temps, and reduce noise from above. I’ll do this by adding insulation (including 2” XPS foam board which will act as a vapor barrier).

I’m in Richmond, VA (zone 4A). Our house was built in 1972.

I’ve done a ton of research and feel confident about the overall strategy. For brevity, I’ll avoid going into all the details, but the approach is 2” XPS foam board on the walls to act as a vapor barrier and insulation, and frame on top of it and add more batt insulation. For the ceiling, just frame and add insulation.

I want to focus on one question in this post. There’s a lot of information about basements and I’ve searched a ton but I’m having trouble understanding how moisture moves through material & air which leads me to this question: will the 2” XPS foam board help with humidity if air can seep into the room from other places?

Ok, some detail: see the first picture (sorry for my bad drawing). That’s my layout; I’m insulating the office and playroom. The walls are all cement block, and the exterior of the house is brick. The bottom walls are below grade, but the top walls face outside because the ground slopes down (which is great because moisture isn’t as much of a problem). Moisture definitely seeps in from the bottom walls; after tearing down the current walls, I can see a lot of efflorescence.

I’m going to cover the walls with 2” XPS foam board, attaching it with small amounts PL300 adhesive which leave a gap between it and the cement block (see 2nd picture). This gap allows the cement to breathe, so moisture can escape.

I’m having trouble understanding where it will escape to though, and generally how moist air will move in this system. For example, if I don’t seal the top of the foam board, it’s useless right? The humid air will simply travel into the ceiling which doesn’t have any vapor barrier, and likely just go into the room.

Even if I seal the top of the foam board (probably with sealing tape), look at the interior wall between the office and the playroom. That’s a cement block wall, which means it’s hollow. These are old walls which have crumbled some – should I assume that air can travel from the bottom wall to the interior wall and out into the playroom? If so, I’m doomed. I can’t put up a vapor barrier from the playroom side because the stairs, and bathroom, and a bunch of stuff is in the way. If I can bet on air not traveling through it, will moisture still seep into that interior wall and come out?

Let’s say I put up 2” XPS foam board on all the walls except the interior one (on both sides, meaning there is also no vapor barrier to the left of the stairs). And I tape the top & bottom of the foam board, which will create an shared air gap with all the walls – will this force moisture to move from the ground to the air in the gap and eventually move to the top wall and to outside? Or is there another way to approach this?

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

    First you've got to understand where humidity in the basement comes from. There are three primary sources: from the ground outside, from inside the house and indirectly from outside above grade air to the inside.
    Moisture from the ground outside is generally in the form of liquid water leaks from high water tables, roof water falling around the foundation and other bulk sources, and water vapor from dampness wicked up and through the foundations to be emitted from the interior face of the foundation. It doesn't sound like you've got any serious liquid water issues because you haven't mentioned any flooding. If you have even occasional flooding or water seepage, this must be your first priority. Moisture wicking is often poorly understood and people generally get this just flat wrong. First, moisture absolutely can wick up through both masonry block and concrete. Whether that moisture adds to the indoor humidity depends on the temperature of the wall and the indoor air. If the surface temperature of the foundation wall is lower than the dewpoint of the interior air, then moisture will condense on the wall from the room and it doesn't matter how wet the wall is: there will be no increase in humidity due to a thoroughly soaked wall so long as the wall surface temperature is below the dewpoint of the interior air. In cold climates with insulated basement walls, this means that there is no contribution to the indoor humidity from moisture in the basement walls because the insulation generally keeps them cooler than the dewpoint of the indoor air. The reason we use a vapor retarder on insulated basement walls is to keep indoor air from condensing on the walls and running out the bottom as liquid water that wrecks the flooring. For this and other reasons, you don't want that air gap behind the insulation to communicate with any other air spaces, indoors or out. The best way to seal the top and bottom of the insulation is with spray foam insulation at the top (extending from the top of the wall insulation, across the sill and up the band joist to the subfloor), and canned foam at the bottom.
    Air from inside the house can contribute to basement humidity, especially when the basement is cooler than upstairs. Upstairs air at 72F and 50% RH is comfortable. If you cool that air to 65F, the RH goes up to 70% and is noticeably clammy. If that same air contact the foundation walls at less than 55F, it condenses into liquid water and soaks into or runs down the face of the wall. So once the basement is insulated, it's best to keep the air temperature about the same as upstairs.
    What I've called "indirect" moisture above is when unconditioned outdoor air finds its way into the basement through building cavities that may or may not include the upstairs indoor air. This one is pretty easy to visualize when hot, damp outdoor air enters the comparatively cool basement, it condenses on any surface cooler than the dewpoint of outside air. With summer dewpoints exceeding 65F and even 70F in much of the country in summer, this is a real risk and one of the big reasons that we need to air seal the entire building, but especially the basement.
    So we've dealt with moisture transactions with the exterior walls. No how about that interior block wall? It can certainly soak up moisture from the damp ground underneath. But, it is at room temperature, not cool ground or outdoor temperature. This means that the wall can be another moisture source and it will if nothing is done to stop moisture from evaporating off the surface of the wall. In this case, we don't need insulation but we do need a vapor barrier. Drylock or one of the other masonry paints is the solution here. These coatings are unreliable as bulk water prevention but they generally provide very good vapor emission prevention.
    And finally, most basements need dehumidifiers, regardless of all of the above. A basement just needs different HVAC setup than upstairs because its loads are so different. An insulated basement might need very little heating or A/C, but still needs fresh air and dehumidification from the moisture sources we can't control.

    1. [email protected] | | #2

      Thank you for the fantastic response!

      This aligns mostly with what I was planning to do. I'll drylok the interior wall where I can. Unfortunately on the side towards the playroom, there is stairs and space below the stairs that will be difficult to get to. It won't be 100% drylok-ed, but I should be able to get most of it.

      I have on follow-up question:

      > If you have even occasional flooding or water seepage, this must be your first priority. Moisture wicking is often poorly understood and people generally get this just flat wrong.

      We have no flooding. but there is efflorescence on the walls. I believe the wall is getting moisture from the soil behind it. I'm assuming the "water seepage" you're talking about here is NOT that, but actual leaks of water from somewhere? We definitely don't have those problems. (mostly because the ground around our house slopes down so only one wall in this situation is below grade)

      > If the surface temperature of the foundation wall is lower than the dewpoint of the interior air, then moisture will condense on the wall from the room and it doesn't matter how wet the wall is: there will be no increase in humidity due to a thoroughly soaked wall so long as the wall surface temperature is below the dewpoint of the interior air

      I'm having trouble understanding this part. You're saying that because moisture is moving from the air to the concrete that no moisture from the soil moves in the other direction to add to the humidity via evaporation?

      Are you saying it's good for the wall to be below the dewpoint? Let's take some numbers from my current basement: temp of 62 and RH of 70% which is a dewpoint of 52. I don't think our walls are that cold, so would be above the dewpoint. No condensation on them. In this case water is evaporating off the cement block from the soil behind it?

      Regardless, the foamboard will separate the interior air and the concrete+soil and avoid water transfer in both directions. Thanks for confirming that I need to seal the top & bottom of the foamboard.

      1. Expert Member
        DCcontrarian | | #3

        Drylok is waterproof but it's not a vapor barrier. In fact it's quite vapor open. If the wall is dry the best solution I've found is a vapor barrier primer that is rated for masonry with a regular paint wear coat.

        If the wall is wet the paint and primer will bubble off, you need both a moisture barrier and a vapor barrier. The only way I know of is a two part epoxy, which will be expensive.

        1. Expert Member
          PETER G ENGLE PE | | #4

          I guess we need to be more specific. Little did I know that Drylok now has dozens of products. IIRC, their "original" masonry and concrete waterproofer was also a very good vapor retarder. I see they have silicone & silane based sealers that are very vapor-open but resist moisture soaking in. A quick search did not show me actual perm ratings for any of their products on their own data sheets, and if someone else has tested it, I didn't see any results. Like I said, a quick search. But the concept still remains and there are many other masonry sealers, some that provide vapor barrier protection and some that do not. The OP wants one that provides low perm rating (below 1) for the interior walls.

          I definitely agree that a damp wall will usually blow off whatever coating you apply to it. Short of very expensive treatments, the best bet is to brush off the efflorescence, acid etch the wall, rinse, then apply the coating. Sometimes it works, sometimes not.

          1. [email protected] | | #5

            I'm relatively new to all of this, but my gut is the wall isn't nearly wet enough for this to be a problem. The interior wall only has a tiny amount of efflorescence at the very bottom; 95% of it is very clean and in great condition.

          2. Expert Member
            PETER G ENGLE PE | | #6

            You are probably right that the interior wall is not a serious source of moisture.. You should be able to deal with the moisture that does evolve from the wall by using a dehumidifier in the basement.

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