Trying to get a watertight seal between an outside tap (1/2") and brass fitting

I have been using one of those Polystyrene outside tap covers for years now and have never had a freezing incident. I too have an outside tap without a valve, I will be re-piping it as part of the bathroom refurbishment and will put a valve in the pipe work but it will end up some distance from the tap and will only be useful to prevent major flooding.

I might try a tap cover. The ironic thing is that the tap is on a south-east facing wall so it will benefit (slightly) from any sun heating that there may be in the middle of winter (well, it will benefit more than if the wall was north-facing, anyway!). I'm not sure when it failed, but I ordered the replacement tap from Amazon on 14 Dec 2022 which would have been the day it happened. Looking back at the minimum temperatures around that time (thanks to my weather station) it was between -5 and -9 deg C in the preceding three days, and the *maximum* on the 12th was -0.8 deg C.

I *think* I wrapped a thick hessian sack around the tap over the winter as I had in previous years (if I didn't, I *meant* to!) but evidently that was not enough.

I presume if there is an isolation valve inside the house, the outside tap should be left open so as to allow expansion room for any ice that may form in the residual water that has not drained out.

If you have one with an inbuilt back flow valve leaving them open does not actually drain the pipes. When I re-pipe mine I might put an in-line back flow valve and replace the tap with a bog standard type thus allowing it to drain.

yes. But often there isn't. I didn't fit one to mine :-(

Luckily it is in a sheltered location facing SE and gets the morning sun.

I have another on the NW side but that is at some level connected inside to some plastic pipe - quite a long length - and I suspect that takes any ice pressure.

Another option is to fit a loop of something with some give - plastic pipe - or some sort of vertical stub of pipe that can trap an air bubble.

I am fairly sure that while the pressure is high with ice, it can still move and 'backup' to a non-frozen bit. The problems happen when there is no non frozen bit for a long distance or there is rigid water filled pipe. So fitting a ball valve and shutting it off probably is what split the connector in the first place, assuming the tap was not left open...

During the course of our bathroom refurbishment I have had to have the radiator off and on several times. Over time the connectors between the valve and the radiator have loosened and drips have appeared. Having installed it now for the final time I dismantled the connectors removed any residual PTFE tape and used liquid pipe thread seal. This stuff is really wonderful stuff no leaks rock solid fitting which allowed the compression nut to be tightened without any drips at either end. Cannot praise this stuff enough, this is now my go to sealant for pipe threads on radiators etc.