Replies

1. 
   DCContrarian | Dec 21, 2020 06:54pm | #1

   I can think of four ways that moisture gets into a basement, there are probably more:
   1. Bulk water intrusion from outside.
   2. Leaks and spills on the inside.
   3. Capillary action where moisture seeps through the concrete from the wetter outdoor soil to the drier interior.
   4. Condensation when the basement walls are cooler than the dew point of the air in the basement.

   Each of these requires a different strategy.

2. 
   PLIERS | Dec 21, 2020 07:18pm | #2

   Of the choices you mentioned I know 3 is def true because parts of concrete floor look damp but there’s no bulk water or spills coming from inside. I’m assuming 4 doesn’t happen until you try to add heat to the basement which I’m not currently doing. I guess there is some temperature rise just from my gas furnace running that could be causing condensation from walls but walls are dry to the touch

3. 
   Dadams | Dec 21, 2020 08:06pm | #3

   It seems my house did not have a moisture and mold problem in the basement until previous owner put up plastic vapor barrier and standard stud wall, only in one section of the basement to finish a rec room. Probably we had moisture moving through the wall by capillary action but evaporating away as fast as it came in, as there is no visible moisture issue elsewhere. Once the walls were put up, condensing moisture between the concrete and the vapor barrier drained down and pooled on the floor, soaking the non-PT base plates, wicking up the stud walls, mold and rot everywhere. This thing with moisture migrating through the concrete is no joke. Do you know for a fact that there is a very good vapor barrier below your slab?

4. 
   PLIERS | Dec 21, 2020 08:14pm | #4

   House is over 100 years old, I can guarantee there isn’t anything under the slab, unless they did something to control vapor back then but I doubt it was even a practice.

5. 
   creativedestruction | Dec 21, 2020 10:26pm | #5

   Joe,

   Sounds like for your case the dimple mat roll (DMX or similar) is a good idea to control vapor drive through the slab. Once you've sealed the seams, OSB can sit directly on that. The rigid foam is a 'nice-to-have' but not required. OSB is of course susceptible to plumbing leaks or moisture above, as from a bathroom or otherwise, but the assembly works as a substrate for any finish floor you like.

   IMO the DRICORE panels are both ridiculously expensive and inferior in performance and install time. Too many seams. They seem to target the diy market that can only fit material in the trunk of a prius.

6. 
   PLIERS | Dec 22, 2020 09:47am | #6

   Yes every panel would have a seam on all 4 sides because basically you can’t seal the seams like you can when dmx is your first layer. I was just going to add 1/2 inch eps over plastic dmx before plywood for extra protection. If it’s not doing anything then I will just save the headroom. In terms of building I was thinking dmx over floor and up to concrete wall, then 1inch eps attached directly to walls, then osb/plywood subfloor pushed up against eps on walls, seal edge with spray foam or tape, then build walls on top of that. Would I still need a pt bottom plate, and would I need to drill through bottom plate and subfloor to attach to concrete or if the subfloor is already attached to concrete I can simply use 2 inch screws to attach bottom plate to subfloor

1. 
   creativedestruction | Dec 22, 2020 02:23pm | #7

   I'm no master framer, but that approach seems fine to me. They're non-loadbearing partitions, so risk is fairly low on how much they're attached so long as they feel solid. Sill plates would not need to be pressure treated when not in direct contact with concrete.

7. Expert Member
   Michael Maines | Dec 22, 2020 02:51pm | #8

   An older article but still good: https://www.finehomebuilding.com/2005/03/01/the-no-mold-finished-basement

1. 
   DCContrarian | Dec 22, 2020 06:26pm | #9

   There's some good stuff in that article and some details I would quibble with. The diagram shows the perimeter drain above the footing, it's got to be lower to do any good. The diagram also shows the plywood subfloor touching the concrete walls, I would have the wall foam butt up against the floor foam. With two layers of half inch plywood, unless it just won't lay flat on it's own I wouldn't feel the need to tapcon to the concrete throughout. I would only tapcon under the stud walls.

   In the sidebar, "blocking air infiltration," the piece of plywood at the top the basement walls is presented as air sealing, but I don't think that's its purpose, I think it's there as fire blocking (or it should be with this assembly). This needs to make a continuous piece from the mudsill to the top plate of the 2x4 stud wall. I would run a piece of foam on top of the mudsill, fireblock and top plate.

   A picture shows 2" EPS on the rim joist. Depending on your climate that may not be enough.

   Finally, I don't agree with his description of the vapor drive. With this assembly, I don't think there is going to be any drive at all through the walls. Materials inside the foam "cooler" will come into equilibrium with the moisture content of the ambient air.

1. Expert Member
   Michael Maines | Dec 22, 2020 07:25pm | #10

   I should have been more clear, that I was referring to the floor system Andy described, which as far as I know was a new idea at the time: to not use sleepers and to use two layers of subfloor instead. Steve Demetrick used Andy's article as inspiration for his floating floor system: https://www.jlconline.com/how-to/foundations/a-basement-floor-without-concrete_o.

   But for your other points I would mostly agree. We've all learned a lot in the 15 years since this article was published. I don't see a problem with his description of vapor drive, however. It looks to me like the illustration that notes "vapor barriers" should be pointing to the interior face of the studs, where 15 years ago it was standard practice to place poly sheeting, not the face of the concrete. There will absolutely be vapor drive through foundation walls--unless you're in a desert, the soil will be saturated at least some of the time, and the indoor air is going to be dry in comparison. If the wall insulation was foil-faced polyiso and not vapor-open EPS I'd agree with you.

1. 
   DCContrarian | Dec 22, 2020 07:48pm | #11

   Right, to clarify on vapor drive: 1) you should do everything in your power to prevent vapor drive from outside to inside, nothing good can come from that. 2) You should assume that there will be zero vapor drive from inside to out, the physics are against you. The only moisture transport you can count on is the interior finish materials equalizing moisture content with the ambient air.

   Unless I was absolutely sure the exterior damp-proofing on the concrete was perfect I think I would put a vapor barrier between the foam and concrete.

8. 
   PLIERS | Dec 22, 2020 08:40pm | #12

   So going full circle here to the original question I’m still confused. I was trying to replicate this panel

   https://www.lowes.com/pd/DRIcore-DRICORE-R-Panels-Insulated-2x2-0-sq-ft-Premium-1-in-Flooring-Underlayment/1002437526

   It’s 1 inch thick with a r3 rating. As Jason points out these panels create so many seams it’s not worth the expense. To replicate this and use a better system I would lay down 1/2 inch eps and 1/2 inch osb/plywood on top. Only raising the floor one inch. The article linked about used 1 inch eps with 2 layers of 1/2 inch plywood raising the floor 2 inches in a low ceiling.

   Jason also mentioned that you could use a dmx plastic dimple Matt and lay osb right on top. The eps is nice but not necessary.

   So the question basically boils down to what system can I use successfully with the least amount of head room added. Mold prevention being #1 priority

   1. Dimple Matt with 1/2 inch osb on top
   2. 1/2 inch eps with 1/2 inch osb on top
   3. 1inch eps with 1/2 osb on top.

1. 
   DCContrarian | Dec 22, 2020 11:48pm | #14

   I don't see the dimple matt being necessary unless you have liquid water flowing, and if that's the case you have bigger problems.

   There are two types of moisture you're trying to block. The first is capillary action, moisture wicking through the concrete. That can be blocked with a layer of polyethylene against the concrete and taped, it adds essentially zero thickness.

   The other type is condensation caused by warm, humid interior air making contact with cool concrete. In most places this happens in the dog days of summer. It can be blocked with a layer of insulation, which is sealed so no air gets through the cracks. EPS with the seams taped is a good choice. Then the question is how thick, half inch or inch? You want it to be thick enough so that the inner surface is warm enough that it's above the dew point. You could calculate that if you know your soil temperature, summer dew point, r-value of flooring, etc., but as a simplification I'd say if it gets humid in the summer where you are go with the full inch, otherwise the half inch should be fine.

1. 
   PLIERS | Dec 23, 2020 12:59am | #15

   It’s get humid in NY so I would think 1 inch. I’m going to start with a layer of poly directly on floor, would I have to do the same on walls? Also how level do floors have to be. I could patch a few spots but does it need to be perfect if I’m basically layering over it?

9. 
   krom | Dec 22, 2020 09:39pm | #13

   to me any chance of moisture means avoid osb.
   you could prob get away with something like advantech subfloor, which has weather resistance, unlike osb that turns to mush and mold food when it gets damp