Replies

1. Expert Member
   DCcontrarian | May 15, 2024 10:01am | #1

   There's a lot going on here. First, are the exteriors of the walls above ground or below?

   There's three main ways that water gets into basements. The first is bulk water intrusion, usually rainwater, sometimes from other sources. The solution is to direct it and remove it, usually by waterproofing the exterior of the walls and providing perimeter drains.

   The second is capillary action, where dampness in the soil gets sucked into the concrete and then evaporates into the interior. Also known as "rising damp." The solution is to have a capillary break between the walls and the footers and damp-proofing on the walls and floor. Damp-proofing can be either internal or external. When people talk about a vapor barrier in basements they're usually talking about damp-proofing.

   The third is condensation, where warm interior air comes into contact with cool basement surfaces and dew forms. The solution is to have an impermeable layer of insulation against the walls and floor. It can help to have a vapor barrier on the interior walls, but only if everything behind the barrier is moisture tolerant. It also helps to dehumidify the interior air.

   The description of liquid water running down behind the poly sheeting doesn't sound like condensation, that would be forming on the interior side. Nor does it sound like capillary action, the capillary action is driven by an imbalance between wet and dry and once the inside was saturated it would stop. A poly film should be an adequate barrier against capillary action, it would prevent evaporation into the interior. Once the concrete reaches the same moisture content as the surrounding soil the water stops moving, and being wet doesn't harm concrete.

   So it sounds like bulk water intrusion. Which means you have to give it a way to drain.

   I'm not a fan of carpet in basements. The problem is that basements have very little drying action, so if the carpet ever gets wet it's quite difficult for it to dry out. Even a spill or a plumbing leak and the carpet is ruined.

2. 
   Nat_Elliston | May 15, 2024 11:20am | #2

   Thanks for the reply! I am 100% picking up what you are putting down with water movement through buildings, and I think I have ruled out bulk water - it is the first thing I look for.

   Three walls are above ground - 2 were once below grade and are partially treated with an old school drainage matt and what appears to be an oil-based spray-on sealer. The moisture pattern inside is consistent with the sealer pattern outside! The fourth wall has a conditioned crawlspace behind it and is 50/50 above/below; moisture behind poly on this wall is none to minimal. Exterior of foundation was dry when observed during a heavy rain event, with site grading and footer drains transporting water out and away from the foundation. Considering this, I am 99% sure the moisture is coming from capillary action as the quantity of moisture behind the poly has been in relative stasis for a few weeks now. As I mentioned (or I think I did), there is no capillary break between footer and stem wall.

   All that said, John Tooley once told me that as builders or building investigators it is our responsibility to come up with a hypothesis and then do everything in our power to prove our hypothesis wrong - I like to rinse and repeat this philosophy until I can't prove myself wrong, then ask someone smarter than me if I am barking up the wrong tree or overlooking something (hence this post!).

   I'm not too worried about the walls, more the floor as I don't like carpet in basements either, but it may be non-negotiable. So the questions are:

   1) Without an interior drain to capture and remove the small amount of moisture draining down the wall from behind the foam (poly will be removed and replaced with a semi-permeable insulated assembly), should I be worried about this moisture making its way to the floor and wicking into the carpet? (I am hoping that the semi-permeable and insulated wall assembly I have planned will both prevent seasonal condensation while also allowing some interior drying, combining to prevent any liquid water from making its way to the bottom of the wall.)

   2) How much do I worry about moisture transport through the slab? Do I treat the slab with a semi-(im)permeable coating or do I put down an interior vapor barrier like poly or EPDM, completely limiting vapor transport and interior drying from the slab to conditioned space? In the case of the latter, I would detail the floor vapor barrier into the wall foam with "reverse flashing", and keep any drained wall moisture below the floor vapor barrier.

   If I had my way, I would either leave the slab alone as it dries fine now, or I would install a semipermeable, high density foam and sleeper system with a finished flooring that allowed drying to the conditioned interior (likely conditioned by a floor mounted mini-split or low-static ducted system, possibly with independent dehumidification if needed). But I may not get my way on this either.

   In my mind, moisture is going to get into assemblies no matter what we do. I want to slow it down as much as possible and give it a way to get out.

3. Expert Member
   DCcontrarian | May 15, 2024 11:13pm | #3

   >All that said, John Tooley once told me that as builders or building investigators it is our responsibility to come up with a hypothesis and then do everything in our power to prove our hypothesis wrong - I like to rinse and repeat this philosophy until I can't prove myself wrong, then ask someone smarter than me if I am barking up the wrong tree or overlooking something (hence this post!).

   OK, the hypothesis you're putting forth is that the moisture under the poly sheeting is getting there from capillary action of moisture in the soil pulling it into the concrete, where it evaporates into the space between the concrete and the sheeting and condenses against the sheeting. Disproving that hypothesis would come by investigating the conditions and seeing whether moisture is in fact moving up into the concrete, and whether conditions for condensing exist in the basement.

   An alternative might be that wind-driven rain is entering the concrete and flowing through to the sheeting. Or some other bulk water intrusion.

   The reason it's important to know what's going on is that the response is going to be different. If it's capillary action, all you need is something on the inside of the concrete to prevent evaporation -- a vapor barrier. It's OK for concrete to be wet. Since the gap is causing condensation, you want the vapor barrier to be tight to the concrete, which probably means something painted or sprayed on. Same with the floor.

   However, if the problem is bulk water intrusion, you need to provide a place for the water to go. Some sort of spray coating is going to cause the water to concentrate, which will result in bad things ranging from the coating bubbling off to the wall collapsing.

1. 
   Nat_Elliston | May 16, 2024 07:32am | #4

   Correct and great synopsis. Perhaps my next step is to get more data; I have a couple small tests planned for the floor assemblies I am thinking about implementing and I can add in some extra data collection while I am at it. My skin tells me it is colder inside the basement right now than outside, which would indicate the proper conditions for condensation on the block side of the poly. I will get some temperature and humidity data and go from there. If I cannot rule out bulk water, I will recommend an interior drain and adjust wall detailing accordingly, as this presents the most severe consequence (wall collapse - though without any pressure from exterior fill, this seems very unlikely in a wall entirely above grade).

   A little more background on the existing observed water: It is the worst on the side of the house that is most protected from rain. There is a 12' covered porch running the full length of foundation wall in question. During a heavy rain, everything under the porch was totally dry. Additionally, in my 3 weeks of observation, the moisture between the block and the poly appears to stay the same regardless of the weather. We have had some dry spells, and some moderate to heavy rain events. In past projects, I have observed walls wetting and drying with the weather - these moisture issues were typically solved with correcting gutter and downspout issues, correcting site grading, and installing footer and curtain drains where none existed. This wall does not appear to have that cyclical wetting and drying pattern.

   I am trying to wrap my head around how bulk water would be the culprit with the entirety of the walls in question being above grade. Even if bulk water is getting below the footer, the transport mechanism would still be capillary action, right? And if it is bulk water rising from below, I would think I would be able to observe it as it pushes up and out during or shortly after heavy rain events. Any suggestions? What am I missing?

   Thanks again!

1. Expert Member
   DCcontrarian | May 16, 2024 07:09pm | #5

   For an above-grade wall I'd say the only way there could be bulk water would be rainwater.

   I'd also say that a wall that is completely above ground isn't really a basement wall, it should be treated like any other exterior wall. In a heating-dominant climate you'd want it to dry to the exterior and have a vapor barrier on the interior.

1. 
   Nat_Elliston | May 16, 2024 09:04pm | #6

   I think I feel comfortable moving forward from here. Thanks again for the thoughtful insights!