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Community and Q&A

Transitioning Control Layers for Finished Basement Room

user-413071 | Posted in Interior Design on

I have a project to finish a single room in the corner of my basement. By “finish,” I mean putting insulation and a stud wall on the two outside walls of this room, and putting flooring on top of the concrete floor. The question is about the joining of the floor materials and the wall materials. But before I get more specific with my question, I must first give you some background.

The basement is generally dry and the foundation walls are poured concrete (about 8” thick). The house was built in 1990 by the previous owner. There were a couple of interior 2×3 stud walls built by the owner which, together with the outside walls, define a single room in one corner of the basement. A 2×3 stud wall was also put up in front of the two outside walls (with no insulation in the stud cavities) within this room. I have removed the stud wall in front of the two outside walls to expose the bare poured concrete wall. The two interior stud walls that define the other side of the room remain. The floor in the entire basement as well as in this corner room is and always has been just the bare concrete basement floor.

What I intend to do to the outside walls is attach 3” of continuous rock wool (R12) directly to the poured concrete. I will then build a new 2×4 stud wall up against the continuous rock wool and fill the stud cavities with rock wool (adding another R12 to the total wall insulation). Some sort of wallboard will go on the new studs to finish the outside walls.

Since I have owned the house, there has been some seepage of water from outside into one corner of this room at the level of the floor and foundation footing. I won’t get into the reason for the seepage but I think I have remediated the source of the seepage by adding three layers of waterproofing on the outside of the foundation and over the footing in that area. The poured concrete foundation wall along the rest of this side of the room is also only a half wall because the outside terrain slopes down from the front of the house to the back of the house along this side. The top half of the wall on this side of the room is a stud wall sitting on top of the poured concrete that constitutes the bottom half of that wall.

Before I do anything to the walls, however, I want to install flooring over the concrete so that what I put on the walls is on top of this flooring. The first layer to go on the floor will be a dimpled membrane (e.g. DMX1-Step). This will give me protection against moisture coming up from the cement floor or possibly from that one corner again. I plan to then put 5/8” tongue and groove OSB on top of the dimple membrane and anchor them both to the floor with tapcon screws.  The finished (floating) flooring will go on top of the OSB.  The finished material is likely to be either laminate, engineered wood, or vinyl planking.

The instructions for installing DMX1-Step membrane advise one to bring the membrane right up to the outside wall (which at that time will not have any studding in front of the wall nor any rock wool yet attached to the cement wall). They also advise the installer to use spray foam to seal the edge of the membrane to the wall. Videos I have seen regarding the OSB sub flooring on top of the dimpled membrane suggest leaving a gap between the edge of the OSB and the concrete wall. I understand that this gap is to make sure that no moisture can wick directly from the concrete wall into the OSB. Likewise, it makes sense to me to extend the dimpled membrane a little beyond the edge of the OSB so that no water or water vapor at the edge of the membrane has direct and immediate access to the OSB above it. My first question though is: how important is it to seal the edge of the dimple membrane to the wall with foam? I have some hesitancy to do so. I will explain why as I talk about the walls.

Whether I bring the membrane tight up against the concrete wall and seal it there or not, the rock wool that I subsequently attach directly to the wall will will extend out from the wall by 3” and therefore overlap the dimple membrane and the OSB subfloor that come to or almost to the wall. Likewise, the stud wall that gets built in front of the continuous rock wool wall covering would be resting on top of the dimpled membrane and OSB subfloor already installed. One of the reasons I like the rock wool insulation is that it is inorganic, impervious to water damage, and it can breath. Thus, any moisture that is at the edge of the dimple membrane on the floor or that is coming through the wall theoretically has a chance to dry out through the rock wool – which lessens the opportunity for mold or mildew. Given that theory, my question is: will leaving the edge of the dimpled membrane unsealed (with spray foam) against the wall further enhance the drying process for any moisture that may reside under the dimpled membrane near the edge? This presumes that the gap between the wall or the edge of the membrane and the edge of the OSB protects the OSB from absorbing such moisture.

A related question applies to the two inside walls of this room. Since those two walls are existing finished walls, the directions for the dimpled membrane are to leave a gap between the edge of the membrane and the bottom of the finished wall. How does one protect the finished wall from moisture that may migrate to the edge of the dimpled membrane? Is the theory that the gap between the edge of the membrane and the finished wall provides sufficient opportunity for drying?  Should this gap be hidden by a baseboard made of a material that covers and protects the existing drywall?

Thank you in advance for whatever explanation and experience you can offer.

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  1. Expert Member


    You need to use an insulation against the concrete walls that is also an air-barrier. This article explains the ch0ices:

  2. Expert Member
    NICK KEENAN | | #2

    I'll start with the walls. Basement walls should be air-tight and built with the assumption they won't dry. So I don't think the rock wool is a good idea. The reason they should be air tight is that if they are well insulated the concrete surface will be below room temperature year round. Indoor air tends to be humid. When that warm humid air contacts the cool basement wall the moisture condenses.

    The reason you have to assume that no drying is possible is also because the interior is warmer than the wall year round. When there is a temperature differential moisture tends to be driven from warmer to cooler. The moisture drive in a basement wall is from interior to exterior, and basement walls tend to have no ability to dry to the exterior.

    Below ground, basement walls should have a vapor barrier. In most places the ground at basement level is always moist, and concrete is capable of wicking moisture. Without a vapor barrier moisture will continuously enter the living space through the walls. Most vapor barriers aren't perfect anyway, which is another reason to assume that no drying will take place in the wall.

    The vapor barrier is distinct from waterproofing, which is about keeping liquid water from flowing in. This is where the dimple wrap comes in, it allows a channel for water to flow while keeping the other side of the wrap dry. While it's best to keep the water out in the first place, minor leakage can be handled that way if there's some place for the water to go. Note that if you're using dimple mat you have to assume that your concrete is wet and have a vapor barrier on the inside.

    What I would recommend is if you're using dimple mat you run it up the wall against the concrete. Over that a layer of impervious closed cell foam, preferably EPS. That foam should be taped at the seams and caulked on the edges and otherwise scrupulously sealed, so that it forms a vapor barrier against the concrete and an air barrier to keep any interior air from contacting a cold surface. It should run all the way to the bottom of the subfloor of the floor above, and particular attention should be paid between the mudsill and the joists to make sure it is continuous and sealed. Inside of that you can have your rock wool, but that wall should be built with the assumption that it won't dry.

  3. Deleted | | #3


  4. user-413071 | | #4

    I understand that here are those who do not recommend putting anything but foam board directly against the concrete basement wall because anything else (i.e. using rock wool as I had proposed), does not provide a strong enough air-barrier to the concrete. There are those who think the rock wool against the concrete is OK. Having seen the latter approach promoted, my logic was that rather than trying to prevent inside air from reaching the basement wall, it was more important to let the wall breathe to the inside. Nonetheless, I will defer to Martin Holliday's perspective on this and use EPS foam board instead directly against the concrete. I will still put rock wool in the stud wall that will be built on top of the EPS foam board, but the combination of EPS and rock wool order will perhaps be the best of both worlds.

    My original questions, however, still remain. Is it best to put the floor membrane down first and bring it all the way to the cement wall such that the foam board put on the wall will sit on top of the membrane? Should the edge of the membrane be sealed up against the cement wall with spray foam, or should it not be sealed on the presumption that any moisture underneath the edge of the membrane needs to be able to escape in order for it to dry out? Or should the EPS foam board be put on the wall and brought all the way down to the floor first such that the edge of the membrane abuts the foam board rather than the cement wall?

    I would think that doing the foam first such that the edge of the membrane abuts the foam board rather than the wall carries a greater risk because any moisture under the edge of the membrane then is closer to the edge of the OSB sub-flooring on top of the membrane and closer to the stud wall that is going next to the foam board. If the membrane goes all the way to the wall first and the foam board comes down on top of it, then the OSB subfloor and the stud wall on top of the membrane will be separated from the edge of the membrane by the thickness of the foam board. Should the front edge of the foam board be sealed to the top of the membrane by spray foam (e.b. GreatStuff) such that any moisture under the membrane is sealed from everything that is in front of the foam board? This very last question gets back again to the question of how moisture under the membrane can breath and dry out.

  5. Expert Member
    NICK KEENAN | | #5

    You don't want basement walls drying to the inside. Period. They are in contact with the soil, which is essentially a bottomless reservoir of moisture, you're not going to dry out the soil under your house. It's hard to get moisture out of basements and you want to work to keep it out in the first place. Concrete doesn't mind being wet, it actually makes it stronger.

    In a perfect world there should be no liquid water inside the house, so any way of dealing with it is kind of a hack anyway, but you want it draining down and not evaporating. Ideally it's going to a proper drain but a small amount can pass through the basement floor into the soil below -- as long as it is a small amount.

    The dimple mat is a decent product but its marketing is atrocious. They lead you to believe that the dimples are there to allow air in so moisture can dry. No, the dimples are there to avoid the buildup of hydrostatic pressure while the water finds its way out of the bottom of the basement. The edges should be completely sealed away from the interior of the basement.

    I'd run the dimple mat first and then the foam over it. The edges of the foam should be air tight.

  6. PLIERS | | #6

    Interesting I heard on this site to skip the dimple membrane and just go thicker with the foam because the foam itself is a vapor retarder and the extra insulation is more important to reduce condensation.

    Every dimple like membrane on the market claims that their product works so that water is free to evaporate. You are saying it is more effective to prevent hydrostatic pressure and to let any moisture seep back into the basement floor or drain. So are you saying that a dimple mat is a good idea under foam in most situations? Just curious because most messages I have read in the forum are against the use of a dimpled membrane, basically that it does little unless you have consistent liquid water heading into a drain or pump

    1. Expert Member
      NICK KEENAN | | #8

      The only use for dimpled membrane is if there is liquid water.

  7. user-413071 | | #7

    I am seeking input from the GBA site because of the expertise of those on it. However, I have scoured many sources on the internet and it can be very confusing because you can get different opinions and what sometimes seems like contradictory advice. I'm sure the devil is in the details - which sometimes get glossed over. I hope that responses to my inquiry can clear the air a bit.

    I would like to emphasize that in my case, I have no floor drain or sump pump in the basement. That is the way the house was built, and as I said, the basement has generally been dry. The only times that there has been water on the floor, there was a separate event - like a water tank leak, or a busted water pipe inside the basement, or water seeping underneath a sliding door frame that leads onto an outside patio at ground level because of inadequate flashing, or that one corner where an opening in the poured cement wall was filled by cement block and there was inadequate waterproofing on the outside of the cement block. I have addressed each of the issues individually.

    My project only involves one corner of the basement. I am not trying to finish the whole basement, not even most of it. Because events like those I just mentioned can happen even in a dry basement, I find the idea of a dimpled membrane intuitively appealing on top of the concrete floor in the corner that I want to finish. One of the two outside walls to this corner room is fully below grade (except for a 1'x2' casement window at the top), while the other outside wall is mostly below grade at one end but mostly above grade at the other end.

    I have read several articles and discussions that suggest that the "good practice" has evolved over the years with regard to basements. For example, using plastic sheeting anywhere as a water barrier in a basement is seen now as taboo by many. And the idea that basements must be able to breathe has taken on more credence. I am already conceding to the suggestion of putting foam board against the concrete wall rather than rock wool. Since I intend to put spruce OSB on top of the dimple membrane and a simple finished floor on top of that, I don't really want to put foam board down too between the dimple membrane and the OSB because that adds more thickness to the floor and because I am not convinced that I need it.

    I hope to see more input on this thread about the pros and cons of sealing the edge of the dimple membrane to the concrete floor. Remember that the two inside wall of this corner room are finished stud walls sitting on top of the concrete floor. Those walls are not concrete like the other two outside walls. The sheet rock could be taken off those two interior walls; I suppose I could then put a layer of foam board on those exposed existing studs coming down and onto the top of the dimpled membrane if that is highly recommended, then seal the joint between the foam board and the membrane before putting new sheetrock over the top of the foam board on those two walls.

    I can adjust regarding the exact materials I am using and the method I use to install them, but I am pretty set on the idea of using the dimpled membrane on the floor, and am now set on the idea of a layer of foam board against the bare concrete wall. It is on the question of whether and how to seal the edges of that membrane where it meets a wall. I have seen a lot of discussion on "breathing" and am seeking clarity on how it applies to my particular situation. Be as specific as possible. Thanks again in advance.

    1. Expert Member
      NICK KEENAN | | #10

      The stud walls aren't really basement walls, they should be treated like regular exterior walls. In a heating-dominant climate that means a vapor barrier on the interior and the ability to dry to the exterior.

  8. Expert Member
    NICK KEENAN | | #9

    Probably the most eminent building scientist in the US is Joe Lstiburek, PhD. He used to advise that basement walls dry to the interior, he said as much in his book, "Builder's Guide." He has since retracted that advice: “I made a mistake. The insulation just needs to be warm enough to control condensation from the inside. The perm rating doesn’t matter. It’s OK for the concrete to be wet. The concrete doesn’t have to dry to the inside.”


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