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Community and Q&A

Foil-faced polyiso in 2″x6″ stud bays – with shower wet wall

FullerHouse | Posted in Building Code Questions on

Hello all,

So I’m not entirely sure which Category this should be posted, but here’s my question:

For the first time in all my years remodeling, the Design for the Master Bath “requires” that the shower valves/head be located on an exterior wall.  I won’t go into the details of why/how this came to be designed (not by me), but given the sunken costs and logistical challenges of alternative layout options, I’m left to make this work.

First, I’m in Zone 5 (Front Range – Colorado).   Second, the walls are 2″x6″.  Third, we will vertically stack the shower valve, head, and diverter for a supplemental hand-held.

And so now, what I’m trying to determine is whether I can get away with filling the 16″ o.c./96″ tall stud bay(s) behind the plumbing with 2 layers of foiled faced poly-iso (and foaming all seams), getting me +/- R-20 of rigid, plus additional radiant (R3?) for the void between the rigid board and the back of the tile backer board.  And so if we set the valve and water lines inside the void between the poly-iso and the tile backerboard, could/should/would I feel “comfortable” the water lines wouldn’t freeze?

Alternatively, I could forego the void, and add some batts to fill out the full 5.5″ depth.

Or, as initially planned, I could first add some 2″x furring strips to the face of the studs and attach the valves to the furring, and run the pipes through that outside 1.5″ – and then still adding layers of foil-faced polyiso to the bay(s) behind the plumbing.   Clearly, that would add additional R-value to the bay behind the plumbing.  (But it would also add some detail challenges to the wall that continues on to the adjacent double vanity).

So I guess what I’m wondering is whether I really need to add the furring?  And in either case, if I am going with rigid board insulation in the bays behind the plumbing (with the intent to just use batts in all the other adjacent exterior wall bays), is foil faced polyiso the way to go, or would I better off with non foil faced XPS?

And the reason I listed this question in this category, is that, aside from the insulation logistics to consider, I’m not entirely sure what code requirments/violations might be involved in this plan.

A long questions with many variables, I know.  But I surely appreciate your insights.


Chris Fuller
FullerHouse Construction, LLC

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  1. Expert Member
    Dana Dorsett | | #1

    What's the full material stackup of the wall, from the exterior paint to the shower surround/tile?

    A high center-cavity R isn't buying you much even if you CAN get away with stacking polyiso in your stackup. A half-inch of continuous foil faced polyiso over a 2x6/R23 rock wool wall buys you more thermal performance than than filling the cavity with high R/inch and no thermal break over the framing fraction. The simpler parallel path math model of why that's true is outlined here:

    If you shorten the path through the studs the framing fraction has an even lower R. So R20 of cut'n' cobbled polyiso is LOWER performance than an R20 batt if that's the only insulation in the cavity.

    For freeze control install at least R12 between the plumbing and the exterior sheathing (which could be 1.5" or more of polyiso) and on the interior side no more than R8 that between the plumbing and backer board, and fill the rest with fiber insulation. When it's -10F outside and 65F inside (a 75F difference, overnight setback to 65F on the heating) the temperature at the plumbing will be about 60% of the difference warmer than the outside temperature, or 35F. Even if the temp at that layer would drop lower at some outdoor temp, it would have to be sustained for a very long time (not hours, maybe a day) for the heat of fusion to be pulled out converting any moisture in the plumbing to ice.

    Assuming the O.D. of the plumbing is about in an inch, if there's enough room for 3" of polyiso exterior to the plumbing (~R14-R15 at temperatures that matter for freeze control) and 1.5" of blown fiber or well fitted high density batt (~R6.3 if dense packed fiberglass) you have a lot of margin. But 2" of polyiso (R12) and 2.5" of high density fluff ( R10.5) isn't quite enough.

    The high R/inch goods don't have to extend fully stud-to-stud, just 3" or so either side of the plumbing.

    XPS is the least-green commonly used insulation product out there, due to the extremely high global warming potential of it's HFC blowing agents. Also note that as it loses some of those HFCs over the decades it drops in performance toward that of EPS of similar density (~R4.2/inch) . It might be labeled R5/inch, but the the warantee is only for R4.5/inch (90% of labeled performance), and it's not clear that even that will be true in 50 years time.

    When using it for freeze control and it's on the cold side of the assembly assume ~R4.35/inch even for Type-I EPS or 4.75/inch for Type II EPS or foil faced polyiso. Polyiso gets derated from it's labeled R when the mean temp through the foam layer is 25F or colder, but EPS get's up-rated from labeled R.

    A foil facer on only one side of an air gap is buying you at best about R1, not R3. If the facers are in contact with both the sheathing and the plumbing it's R0.

    1. FullerHouse | | #2

      Thanks for your thorough reply, Dana.

      To answer your first question, I can only tell you what I know about the wall assembly.

      There is latex paint on the exterior hardboard lap siding. It's unclear to me whether there is any WRB (many of the homes around this area do not have any WRB - unless of course they've been remodeled). So I'm going to assume there is only 1/2" OSB between the 2"x6" studs and the lap siding.
      On the interior, aside from the options I'm considering for the bay insulation, the plan is to use 1/2" Durock as the substrate for the 5/16" porcelain tiles we'll be installing with mortar. I haven't yet confirmed with my tile sub whether we are going to Redguard the entire shower - but he's intending to use the Wedi system for the shower pan.

      And to further clarify, this is only interior work we're doing on this particular project, so any exterior rigid insulation is out of the question.

      Further thoughts/input based upon this information...?

  2. Jon_R | | #3

    Drill some sideways holes in the studs to be sure that you aren't creating a cavity that can't dry much in any direction.

    I'd err on the safe side - perhaps (and ignoring code) as much polyiso as will fit and no fiber fill. Something like wind pressure and some small air leaks can make normal ratio calculations quite wrong.

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