Strange pipe freezing issue in bathroom

Hi folks! So glad I found this place. I have an odd issue with "freezing" pipes only in the shower. I used quotes because I'm not sure if it actually is a freezing issue so let me explain.

Issue: Tub/Shower has no running water when it gets down to 5°f or lower.

All other plumbing works.

The knob is a single twist.

The pipes are metal, not plastic.

The very slow drip in the tub is hot.

All hot pipes are warm to the touch going to the tub.

Water goes from the basement through an outside wall then angles to the inside under the tub.

When the water comes back, it is BAM sudden, not gradual.

The pipes are accessible via a panel and the cavity they are in feels warm.

I can see no automatic cut-off device anywhere on the pipes that are visible.

So my thoughts on this are if hot water is dripping, would that not remove any ice over time at least on the hot side so there would be a gradual coming back on? Why would it snap back on so suddenly? Why is everything else perfectly functioning?

I am so boggled by this and any advice/help I could get would be greeted with tacos and cookies. Thanks so much.

Dan

the outside wall is the culprit. or/and the hole under the tub where the drain is hooked up.
try this, the outside wall, insulate the pipes. and the hole under the tub. pack it with insulation

your studs are 16" on center, typical. most likely, both pipes are in the same caviety.
cut the sheet rock out by putting the blade against the stud, make a straight cut all the way down, both sides
install some 2x4s nailed to the studs to screw the rock back to.

use pipe insulation 1'' thick. and 19 r batt insulation in the caviety before you put back the rock

Yep one if not both are freezing. The reason you get a "BAM" back on is because your faucet is pressure balanced. If one side or the other has frozen, you wont get water from the other. When everything is working try this. Turn off the valve to your water heater thereby shutting off all hot water in the house. Now, go try to get cold out of the faucet. If everything is working as it should, you wont get anything. Turn the hot water back on and "BAM" you got your shower back.

The pipes should have never been run in an outside wall. It happens, but it shouldnt. Yeah, I'd definitely take frodo's advice and insulate the hell out of them. I'd go one step beyond though and put in an access panel. On really, really cold days/nights, I'd open that panel up to get a little warmth in there.

Good luck!

I would not insulate the pipes. Insulate the space between outside and the pipes. Leave the uninsulated pipes on the warm side of the insulation so that the warmth from the house will keep the pipes warm. If you insulate the pipes, the house warmth is also prevented from getting to the pipes.

AcornWhole....put the beer down. and insulate them pipes.

Why not insulate the pipes.....without putting the beer down?