Most of Zone 6 is far heavier on heating than cooling.
The plastic goes near the warmer side of the wall to limit moisture movement in to the insulation.

Don't use regular sheetrock in a bathroom.
At the very least, use 'Blueboard'.

"Don't use regular sheetrock in a bathroom.
At the very least, use 'Blueboard'. "

Regular sheet rock is just fine outside of wet areas (around tubs and showers).

Cement board is still the preferred base for tile, though there are newer membranes that claim enough waterproofing to allow tile on membrane on drywall.

A coat of primer and latex paint is enough protection for drywall in areas not considered 'wet' (tubs and showers).

ALL areas of a bathroom can be considered 'wet areas'
if you got yunguns.
Blueboard everywhere is cheap insurance.

"Blueboard" is very thin veneer plaster on gypsum wallboard that has an absorbent facing but no treatment for moisture resistance other than a harder more durable finish over the entire wall. Blueboard is not available in many areas of the US but where it is available it should be the standard choice in all rooms of the house.

The concept of a house needing to "breathe" is often misinterpreted. It means a that any moisture in an exterior wall should be able to move to the dryer outside air in a cold climate and to the dryer inside air in a hot climate. The air quality of the interior spaces should be addressed with exhaust fans and heat recovery air exchangers and/or by opening a window not by moisture or air moving through the exterior wall.

In a cold climate, moisture inside the house needs to be prevented from passing into a conventionally insulated exterior wall by a vapor barrier in order to prevent it from condensing and freezing on the inside face of the cold exterior sheathing.

I can't imagine why two layers of drywall would be recommended unless it was to reduce the penetration of noise from outside through an exterior wall that contained no windows.

"ALL areas of a bathroom can be considered 'wet areas'"

But they are not, and splashed water from children in the tub is not nearly the same as water from a shower.

The tub enclosure itself is a wet area, but not the walls around the sink or toilet.

Brickeyee, even though you know everything about everything,
you apparently never raised any children. When my kids
were young, they could get water on the house across the
street, when they were in the tub. And others have told
me that their kids were even worse than mine.

If your kids splash enough water to saturate the surrounding wall surfaces the finish on those finishes need to be tile instead of paint or an adhered wallcovering and in that case the nature of the drywall behind it wouldn't matter. Unless your kids use a hose to soak the wall several times a day the wallboard will not be affected. The ceiling in the room below would fail before the wall would.

As I said earlier, "blueboard" should always be used in all rooms if it is available in the local market but it doesn't have any special resistance to repeated contact with water; it's made of the same material as regular drywall.

Perhaps you meant to say "greenboard" (moisture-resistant drywall) but it offers so little protection I would skip it and pay more attention to water resistant finishes.

"apparently never raised any children"
Yes I have.

I sounds like you have a control problem though.

Try some parental supervision.

It seems seriously lacking in may parents today.

I"m sorry Mr. Brickeye, there was never a control problem
in this house, once the 'old man' got home.
I suggest you go back to telling everyone else how wrong
they are about everything, as it's not going to work with
me.

Drywall use limitations from USG catalog

USG brand SHEETROCK (regular gypsum board):
"Exposure to excessive or continuous moisture ... should be avoided."

USG Veneer Plaster Systems ("BLUEBOARD"):
"Not recommended for use in areas exposed to moisture for extended periods ..."