Hand-sprayer and volume control on outside wall

Hi,

I am new to posting here but have lurked for a long time and recognize the wealth of information here on the forums. I would appreciate your help.

I am doing a shower remodel and the main shower plumbing supply lines will feed from the crawl space to an interior wall where there will be the shower heads and valves. Fine, no risk of freezing there. However, we are also adding a hand shower and would like to have the fixture and associate volume control (note: not a diverter!) mounted on the exterior wall (one side of the shower) about 18" away from the corner where the interior shower wall intersects. The exterior wall is 4" thick 2x4 construction, east facing.

I realize it is not great to have the pressurized plumbing (one side of the volume control) on the exterior wall, but the design really works best this way. Of course I would add plenty of insulation behind the pipes (and not infront), but that's not a ton of insulation given only a 4" wall. I live in Oregon and it rarely gets below freezing, but we do have 1 or 2 weeks below freezing and has been down to 18F for a day or two at times.

Here are my questions:

Since the amount of pipe inside the exterior wall is relatively short, would heat conduct from the interior wall through the pipe/water to keep the pipe from freezing?

Is it better to use copper or PEX in this application? The volume control is 1/2" copper.

Is it legit to use automatic heat tape on pipes concealed on an exterior wall? What if I put heat tape on the pipe as it leaves the interior wall, (and made it serviceable from the other side since it is a closet), would the heat conduct the 18" through the exterior wall enough to keep it from freezing?

How bad of an idea is this?

Thanks for your help!

Possibly "horrible". Insulation does not prevent freezing, it just slows it down. ANY water will freeze, regardless of whether it is "pressurized" or not. Depending on convection to carry heat to an enclosed pipe is NOT a sensible idea. Heat tape, in an inaccessible location, would be "non servicable" if something happened to it, such a a burned out wire. Since a handheld shower's "connection" can be anywhere, regardless of where the nozzle itself is located, why do you HAVE to put the piping there?

If you must put water pipes into an exterior wall, then use a high density insulation only on the cold side of the pipe, leaving no insulation between the pipe and the warm side.

Also, care must be taken not to allow air infiltration which can really speed up the cooling.

PEX would be better material in an outside wall. Copper splits pretty well when the water becomes ice.

Thanks guys. The design of the shower becomes too cluttered on the interior wall. There are two rain-shower heads, two volume controls and one thermostatic valve on that wall and for symmetry reasons adding the 3rd volume control for the hand shower on the same wall would look odd. But really, doesn't heat from the interior of the home on the interior wall conduct through the pipes for some distance to the exterior wall? Sure, it probably has no impact if you run several feet, but 18" isn't very far, wouldn't the heat carry through and keep the water in the pipes on the outside wall a bit warmer for freeze protection?

What about the idea of using heat tape in the interior wall (with access panel on the other side for service), and relying on the same conduction effect described above, but with more heat from the tape it would have more significant impact than just the ambient temp indoors. If the heat tape is 18" away on the same pipe wouldn't it still carry some heat through the water in the pipe (even if it is PEX). Eh?

I would not put in heat tape. Run the pipe as close to the inside surface as possible and use heavy galvanized hurricane strapping along the length of it as nail-guard. Tape the PEX to the backside of the metal strap using metal duct tape. The metal will act as a heat sink/collector to carry heat from the room to the pipe.