On 20/01/2021 15:02, R D S wrote:
We've a 15mm plastic mains water pipe on an outside wall.

I haven't got around to lagging it yet so as a precaution i've been
turning it's supply off in the evening and had been expecting to open it
up and find it frozen.

Having had no bother and it being zero and under recently around these
parts i'm wondering at what sort of temperature you'd actually get
freezing?

It is generally a combination of temperature and wind speed that does
it. Metal pipes being somewhat more prone to it IME. How fast the heat
gets out of the water determines whether it freezes or not.

I realise that rather than debating this, a better use of my time would
be to go and get some lagging!

It is actually quite mild today 9C but the road is running like a river.

--
Regards,
Martin Brown