Transitioning Control Layers for Finished Basement Room
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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