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Community and Q&A

Insulation behind Shower surround

GBA Editor | Posted in GBA Pro Help on

I have heard multiple ways to best insulate behind fiberglass/tub surrounds below grade. We always insulate basement remodels with 2” of XPS = R-10 against the foundation wall (sealed with canned foam all 4 sides, seams) and then frame out for the tub/shower surround.

My specific question is the stud cavities behind these surrounds and whether its best to leave open (rely on a single layer of 2” XPS) or go a step further and fill with unfaced FG, or faced FG? Were in MN and my concern is the potential for condensation on the backside of these surrounds. At the same time FG behind a fiberglass surround also makes me a bit uncomfortable with its ability to hold moisture.
Ideally I would double up another sheet of 2” XPS over the original piece for a total of 4” = R-20. However I have read articles by Joe Lstiburek that suggests only using a max of 2” of XPS as using more, reduces the perm rating, thereby reducing the walls ability to dry? What about this concern? If we increased the XPS, by how much? Lastly would you add another layer of XPS directly over the first layer? Or install the 2nd layer so it’s flush with the front/face of the studs (of course you would have a cavity with XPS on both sides). Or simply just rely upon a single layer of 2” XPS against the wall and leave the stud cavities as is.

Any insight would be most appreciated.

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  1. Robert Riversong | | #1

    Does the foundation have exterior insulation, and if so how much? Is it an 8" thick concrete wall buried 7' deep, or what? Is there a vapor barrier and insulation under the slab? Capillary break between footings and walls? Is there evidence of dampness or moisture problems in the basement?

    What else is in or going in the basement - kitchen, laundry, utilities? Is there going to be a bath exhaust fan ducted to the outdoors? Is the finished basement tied into the whole-house ventilation system? What kind of heat?

    Even below-grade, R-10 is minimal for a MN climate. Doubling up to R-20 makes sense, but I would attach both layers to the foundation wall, leave an 1" air gap for breathing space, and then frame for the bathroom. Do not use fiberglass or any additional insulation in the stud cavities (keep the inside surface of the XPS warm), and do not use an additional vapor barrier betwee the framing and the tub surround, as that would create a moisture trap.

  2. Matt | | #2


    These are typically retrofit situations with cinder block construction of houses built in the 40's, Of course no vapor barrier or insulation under the slab or exterior foundation insulation in house of this age. We try to address any moisture issues prior to any project through proper grade, gutters etc. Basements tend to be heated with forced air in which 1-2 supply's are added based on sqft of the space. In bathrooms we always try to add exhaust fan's

    My specific question is the detail behind a tub/shower surround, when you have 2'' XPS attached to the basement wall with a 2x4 wall interior to that. Really I have 3 options before I attach surround to the studs. #1 leave the cavity in the 2x4 wall open. #2 place unfaced FG between the studs #3 cut pieces of XPS or EPS to fit between the studs and attach directly to the existing XPS on the wall.

    As you allude Robert R-10 is minimal for my climate. That is why I was curious about insulation especially behind these surrounds. I would tend of the 3 options to believe cutting some additional pieces of EPS and fit them bewteen the studs laid directly against the XPS on the existing wall would be most appropriate behind a shower surround (an area of high moisture diffusion) . My only question is whether by adding another 1-2'' of XPS/EPS over the existing 2'' XPS = 3-4 total inches would be too thick from a permeability standpoint. Again my hesitation is based on Joe L comments stating that a max of up to 2'' should be used in basement situations. Then again were only talking about a small portion of the basement wall in which would have more than the advised 2'' of XPS attached to it. Of course in the rest of the basement I could add more unfaced FG in my 2x4 wall which is interior to the XPS attched to the basement foundation. But again I’m specifically curious about what to do behind these surrounds.

  3. Robert Riversong | | #3

    With 2" XPS, you're already down in the 0.5 perm range - Class II vapor retarder, or vapor semi-impermeable - so taking it to 3" or 4" isn't going to change the moisture dynamics much. Since there is no exterior insulation and probably precious little damp-proofing, the foundation wall can dry to the exterior.

    Again, I would advise against placing framing tight to the foundation insulation and would leave the stud cavities empty to allow maximum drying potential of framing.

  4. Matt | | #4

    Sounds like we would approach it the same way. Would you advise overalying the existing XPS with additional layer of XPS or EPS? I would think EPS as although your not getting as much R Value you do get more permeability.

    I suspect your advice regarding keeping the framing off set from the rigid board is to minimize and moisture diffusion into the wood. Make perfect sense.

    In regards to your point about omitting FG in the stud cavities “ to allow maximum drying potential of framing” I assume your discussing specifically behind a tub/shower surround? But would be ok having the cavaties in other parts of a basement filled with FG as long as it was interior to the XPS placed against the wall.

    Similar to detail in Figure 1: ” "Unfaced fiberglass batts can be installed in the frame wall for additional insulation”

    Again my focus is the detailing behind these surrounds and at this point is now whether to overlay with XPS or EPS.

  5. Robert Riversong | | #5

    If you're already applying 2" XPS (perm = 0.55), there's no advantage of a more permeable second layer, and the potential disadvantage of allowing moisture to penetrate to the now-cooler surface of the XPS.

    Yes, I was suggesting maximizing drying potential behind the tub/shower by leaving the stud cavities empty and spaced from the exterior foam. F-glass can certainly be added to other framing, but I would also advise an air space for drying because the surface temperature of the XPS will be lowered and could become a condensation plane. Be sure to air-seal the drywall well to prevent air-borne moisture from migrating to the cooler surfaces, and don't forget the band joists for insulation and air sealing.

  6. Matt | | #6

    Thanks Robert
    I will go ahead and and start adding an additional layer of 2'' XPS over the orginal 2'' we normally install to the wall for a total of 4'' of XPS behind these surrounds.

    Thanks for taking the time to respond.

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