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) 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.
Thanks

What's the permeance of the fiberglass surround? (My guess it that it's highly impermeable, but I don't know that for sure.)  If it's very low perm, no matter what you'd be trapping ground moisture behind it, and use of wooden studs probably inadvisable.  (Composite or steel studs would be OK.)  Designing it with ventilation gap behind the surround that communicates with basement air would allow it to dry.

If you want to boost the R-value, going with at most 1" XPS, following it with a layer of 2-4" of unfaced beadboard EPS (or skip the XPS, and use only EPS) to get you the R value with reasonable drying capacity.  EPS is much more permeable than XPS.  At 2" XPS is a class-II vapor retarder, and you don't take it beyond that, but IIRC EPS is good to about 6" before you need to worry about it's vapor retardency. 

Instead of a studwall, furring long-screwed into the foundation puts the wood completely on the warm/dry side of the insulation, and gives you a 3/4" vent cavity behind the surround. EPS will give you ~R4/inch, so a double layer of 2.5" stuff will give you your R20- an inch more than with XPS.  If there are places where you need the more substantial structure of  a stud as opposed to furring you can cut it in (the shallower the better 2 x3, not 2x4s, etc.), but if the whole point of the studwall was to be able to use cheaper insulation, you're better off making the section behind & adjacent to the surround all EPS, then doing the rest of the basement in your preferred XPS + batt insulated studwall configuration. Be sure to avoid the poly or foil-faced stuff.

Alternatively, you can use 3-3.5 of fiber-faced iso (commonly used as roofing insulation) + furring to get your R20+ with less depth (this is how I did my own basement.)  Unfaced & fiber faced ISO is highly permeable, and loses R value a bit with lower temps (but less than fiberglass batting does).  I recently had to remove & replace a section to re-route some drains, and by it's appearance it has drying-toward-interior quite well, without a hint of water staining or mold.

Never use faced FG on below grade foundations, always unfaced batting, and with enough foam insulation on the foundation wall side that the entire stud stays above the interior room's dew point.  Fiberglass & gypsum can pass a lot of water vapor in either direction, so making the foam layer semi-permeable allows the foundation to dry without affecting the humidity in the stud cavity.  If you have vapor retardent facer on foundation/foam side of the cavity the studwall stays dry, but the foundation saturates, driving moisure above grade to cause spalling & efflorescents (or in to the sill, if  not properly gasketed).  If you put the facer on the conditioned space side you've made it even worse, trapping the moisture in the studwall as well, creating mold conditions. All sub-grade insulation & finishes need to be able to dry toward the interior.

Keeping the wood portions as far to the warm side of the structure as possible limits the potential for condensation to form on the wood, creating mold conditions. Warm & dry wood is your goal, so trapping it between insulating layers of XPS isn't ideal, only to be done if there are other compelling reasons.

Dana1

Thx for the detailed reply.   The good news is assuming I  follow you correctly we do indeed attach the XPS directly to the foundation and the 2x4 wall is built interior to that thereby keeping the "wood completely on the warm/side of the insulation".  This of course leaves a cavity for HVAC supplys and the optionality to add additional FG insulation if desired or in this case the face of the 2x4 wall will be what the nailing fins of the shower surround will attached to. 

My specific question is when I have XPS attached directly to the foundation wall and sealed on all 4 corners (this gets me my airbarrier)  and a 2x4 wall interior to that,  in which a tub/shower surround is attached.    You basically have 3 options before you attach the surround to the studs:

1# Fill the cavity with FG insulation as suggested by the article below: "Unfaced fiberglass batts can be installed in the frame wall for additional insulation. These details are shown in Figure 1. Closed cell sill seal should be installed between the bottom plate of the wall and the concrete floor"
This is typically how we approach retrofit situations

http://www.buildingscience.com/documents/reports/rr-0308-renovating-your-basment
(photograph 1 is my exact configuration.  I attach the surround  to the studs)

#2 Put nothing in the 2x4 cavity as the 2'' of XPS  attached to the foundation suffices. And simply attach the surround to the 2x4 framed wall which again is interior  the XPS

#3 Add a layer of EPS over the  2'XPS 's ( which I assume to be your preference due to increased permability which will establish a higher R-Value)

Or does the 2'' of XPS  on the wall suffice?

The main question here is behind these type of surrounds (prone to more vapor diffusion ) if additional insulation beyond the 2'' of XPS attached to wall is necessary.

I assume any tub/shower surround is highly impermeable.

Thanks
Matt

If you're insulating the rest of the basement to ~R22 (clear wall: R10 XPS + R12 batt-stuffed studwall ~ R23) the section of R10 only behind the surround becomes a most-lossy section.

As long as there is good air communication with the stud cavities behind the surround and the rest of the basement you can keep the mold issues at bay though.  But if you want more R value, applying a layer of EPS will work, but still leave a ventilation gap between the surround & EPS to  keep any wood framing dry.