Insulating behind wall hung toilet on exterior wall

[Birdlove]

We planned to install a Geberit DuoFix in-wall system for 2x6 wall, but are considering using the one for 2x4 wall to allow for more space behind the unit for more insulation. This is going on an exterior wall, which is why we are concerned about insulation. The exterior walls have been gutted and we plan to insulate the exterior wall cavity and build out another wall cavity for the Geberit frame. Another thing that adds concern is the cast iron vent/drain stack in the corner!
So, my first question is, in addition to insulation in the exterior wall cavity, what additional layers of insulation should we add to give us reasonable comfort that the tank will never freeze (we live in central Ohio). And is spray foam insulation sufficient around the vent stack (see the picture)?
Second question, does it make sense to use the 2x4 unit instead of the 2x6 unit to allow more space for insulation without having to move out the finished wall?
Third question, are there any advantages of the 2x4 or 2x6 functionally? is one easier to maintain/repair?
Thanks for any assistance in making this work!

 

[Tuttles Revenge]

We don't get near the cold that you're subject to so I can't provide any advice on insulating the stack. I would think that the tank should be safe with that amount of insulation. The more the better I do know.

The 2x4 tank is 3" deep... the 2x6 tank is 5" deep. I can tell you that changing the flush valve in the 2x4 is not easy.. its doable by a normal sized person.. but if you have XL sized hands.. forget it.

 

[Sponsor]

 

[Jadnashua]

My preference would be the thinner tank and more insulation. Foam has a better R-factor than most other available materials.

Generally, a stack does not need to be insulated. Note, that in cold country, depending on the size, when it goes through the roof, you may need to increase the diameter to help prevent hoar frost from closing it off from freezing water vapor as it rises.

 

[Birdlove]

My preference would be the thinner tank and more insulation. Foam has a better R-factor than most other available materials.

Generally, a stack does not need to be insulated. Note, that in cold country, depending on the size, when it goes through the roof, you may need to increase the diameter to help prevent hoar frost from closing it off from freezing water vapor as it rises.

I am not so much concerned about insulating the stack, but insulating that wall cavity where the stack is located. It will be difficult to get any fiberglass insulation in there, so I'm thinking of using a spray in expanding insulation, like Great Stuff or something like that.
Would GreatStuff be an acceptable insulation around the cast iron?
At best, I will have an inch between the cast pipe and the exterior wall and an inch of rigid foam board between the pipe and the toilet tank. Is that sufficient? Seems like the vent pipe would act somewhat as a "heater" since warm air would be going up the pipe, right?

Thank,
Steve