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Context: I am looking to finish my basement, mostly DIY.

  • The original builder installed what I understand to be a "wall wrap" on all exterior cement walls. This looks like ~1.5" of fiberglass insulation covered by a clear plastic sheet.
  • The builder laid three drain pipes where the bathroom would be. I'm looking to use these drains as-is without modification.
  • From what I understand, when framing a basement that has a wall wrap, you want to preserve the insulating value of the wall wrap by not compressing it. Start the 2x4 wall 1.5-2.5" away from the cement.

Problem: It appears like when the original builder laid the drain pipes, they did so without taking into account the width of the wall wrap. This means that all drain locations are off by whatever width I allow for the wall wrap to go uncompressed (or even exist). This is problematic for two of the three drains: the toilet and the shower, both of which need to be in specific locations.

Toilet: This is a problem that I believe I've solved, but would like to confirm. Standard toilet drains are 12" off the finished wall, from the drain's center. In my case, the center of the toilet drain was placed 16" off the cement wall. When measured coming off the cement wall this means: 2" wall wrap + 3-1/2" stud + 1/2" drywall = 6 inches. This leaves 10" of space where I can install a slightly non-standard toilet that has a 10" rough-in size. Any issue you see here?

Shower: This is where I don't yet have a solution. In my case, the center of the shower drain is 8-3/4" off of a preexisting load bearing stud wall, and 17-1/2" off the cement wall. In looking around for basic showers, I have found a few similar to this Home Depot shower. When looking at dimensions guide from the manufacturer, I see this shower expects to be mounted directly to the studs, with the center of the drain being 8-1/2" off one wall and 13-7/8" off the second.

Herein lies the problem of the wall wrap. 13-7/8" + 3-1/2" stud = 17-3/8", which leaves no space for the wall wrap to exist, even compressed. And that is the confusing part; the builder appears to have perfectly laid out the shower drain as to fit some standard shower dimensions, except they forgot about the wall warp? The toilet is a simpler problem to solve, as 10" rough-in toilets exist. But I don't see the same for showers.

Potential Solutions: As I see it, the following are options, but I may be missing something obvious.

  1. Remove the wall wrap from the cement walls along the basement. Frame flush against the cement wall.
  2. Find a pre-made shower with a drain closer to the side wall.
  3. Spend more money and build a custom tile shower. Can't even use a standard pre-made shower pan.
  4. Bust up the cement, and move the drain 2".

What would you do in this situation? Any help would be appreciated. Thanks!

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Is there a reason why you need a 3-1/2" thick wall? If not, the studs could be rotated so the rough wall adds only 1-1/2" in front of the insulation/wrap. It seems that would make all the other problems melt away.

You'd have to figure out what to do about top and bottom plates. You could rip a 2x4 in half and try to nail the studs to that, but it's prone to splitting. Another option is to position the bottom plate like normal, but cut or lift the wall wrap so that the bottom plate can be pressed against the concrete wall. The wall wrap would effectively be recessed inside the wall. The top plate might be done similarly, but it's also possible that this interior wall will be taller than the concrete wall such that there's no real conflict between concrete, wrap, and top plate.

Local codes might require additional insulation be added when the space is finished out. Generally it's allowable to use layers of different types of insulation so long as the total meets the R-value requirement, so if the wrap is left in place, its R-value contributes to the total. You'll probably find that the 1-1/2" cavity is thick enough to allow whatever additional insulation might be required.

Insulation doesn't perform as advertised when compressed, so if you prefer fiberglass batts, you might consider buying 3-1/2" thick batts and separating them to half-thickness for this job.

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  • This is a great thought I hadn't considered. I will need to look into insulation requirements though, as these two cement walls are considered exterior walls, and so must be insulated. I'm guessing I'll need the full 3-1/2" of space to provide adequate additional insulation, as the wall wrap is likely insufficient alone. – Jack Palkens Jul 7 at 20:48
  • Polyiso sheet can do a lot in an inch and a half. You probably also don't need 2x4's turned sideways at all - 2x2s or ripped in half 2x4s (which might be less expensive) are likely adequate (this is a very non-load bearing wall. Furring strips, not "studs") – Ecnerwal Jul 8 at 2:20

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