zone 5, is a vapor barrier necessary/needed for a garage that is unconditioned? 2x6 wall. R19 batts. 1.5in XPS R7.5 on exterior warming the sheathing. It is attached and I have a VB on the common walls with the home (on the house side). But I'm wondering about the rest of the walls. I figure the interior/exterior temps won't ever be at a significant difference to require it.

What matters is the temperature of the sheathing relative to the dew-point of the garage air. If the garage is pretty air-leaky (most are), the dew point of the garage will track the dew point of the outdoor air, and the sheathing will always be above that dew point.

The reason you may need vapor retarders in conditioned space is that you have multiple humidity sources, between cleaning, cooking, bathing, etc. With modest ventilation rates the dew point of the conditioned space air in a zone 5 location will average 38-40F (30-35% RH @ 70F), which is comfortable and healthy for the humans, but would be an issue for wood sheathing that was averaging 25-30F over the cool part of the winter.

Even if you had R21 in the garage and it was fully conditioned 70F/35% RH space, R7.5 would be sufficient to protect the exterior sheathing:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm

You're letting it coast at some much lower temperature, but without moisture sources in the garage you're not going to have an issue as long as there is some very modest ventilation/infiltration exchange with outdoor air. I've yet to see a garage door tight enough to make that an issue, but if yours is that tight, report back!

The dew point temp will precisely be 37.2-41.2F @ 30-35% RH & 70F dry bulb temp…LOL!

Borst Psychrometrics Calculator

I did air seal the garage. I'd give it a 3 out of 5 as compared to a 5 of 5 for the inside of the house (in terms of effective air sealing). There's still the garage doors as you say. Ok, thanks for confirming.

That link refers to conditioned space, correct? The garage is not, so if I did not have the R7.5 over the exterior, would I need a VB in the garage? I assume no as well in this situation because the sheathing will breathe to the interior/exterior and since it is unconditioned, the temp will be probably somewhat closely track the exterior temp. if it is 25 outside, not enough of a temp difference to cause any dew point condensation on the sheathing. Even if it did, it would dry pretty quickly in the spring thaw...yes?