Zone 5 (chicago)
2x6 wall
2x6 R19 unfaced batts
1.5 inches rigid XPS on exterior
1x4 furring

in the garage (which has the furr strips and XPS) it is unconditioned space, so do I need to put MemBrain on the interior side here (I'd prefer not to as it is a little pricey). I figure the temperature difference will be much less between inside/outside air so condensation shouldn't be an issue and blocking vapor is not necessary?

Also, I have a shower with a common wall exterior wall (4 feet). Should I put membrain where that shower wall will be? Because cement board that is waterproofed with Mapei Mapex will go there anyway so I'm thinking it is not necessary?

I'll guess that taped MemBrain is also your interior side air barrier - so I wouldn't skip it for moisture and air flow reasons.

I would skip if it there is some other taped and impermeable layer.

I'm curious - why XPS over EPS (less expensive and more permeable).

I put it up behind the shower. Do I need it in the garage as a vapor barrier? I was wondering if I could just have the drywall, unfaced batts, sheathing, then XPS then housewrap...?

Now that I'm at a real keyboard. I believe with R7.5 over a 2x6 wall, no inner vapor barrier is required per code - correct? So in the garage, which is unconditioned, with 2x6 wall and R7.5 outside XPS, I should be good to go without the membrain, right? I have R19 batts in the cavity unfaced, but thinking no VB is required. To put up MemBrain would cost about $300 (two rolls) since three walls are outside walls and the walls are taller 12'.

In the house, I'm using it everywhere.

bump for a response on this. unconditioned garage. Do I need a vapor barrier on the walls that are common to outside?

im in climate zone 7 and its always been done that way here.

if the attached garage is insulated and drywalled we put a vapour barrier on the exterior walls 6mil poly

On the exterior wall?

all walls in the attached garage and ceiling get vapour barrier if its insulated. the wall that is attached gets the vapour barrier on the warm side inside the house.
been done like that since 1960s houses here

Posted By Surfsup on 10 Nov 2014 08:00 PM
Now that I'm at a real keyboard. I believe with R7.5 over a 2x6 wall, no inner vapor barrier is required per code - correct? So in the garage, which is unconditioned, with 2x6 wall and R7.5 outside XPS, I should be good to go without the membrain, right? I have R19 batts in the cavity unfaced, but thinking no VB is required. To put up MemBrain would cost about $300 (two rolls) since three walls are outside walls and the walls are taller 12'.

In the house, I'm using it everywhere.

In zone 5 with R7.5 on the exterior of a 2x6 wall you'd be able to skip the interior vapor even for the fully conditioned space, per the exceptions in IRC 2012:

Insulated sheathing with R-value ≥ 7.5 over 2 × 6 wall.

In an unconditioned garage it's even safer (despite the lower average room temperature), since garages all leak air pretty badly, and the moisture sources are few.

Even unconditioned garages can have high moisture rates at certain time due to snow melt from the vehicles.