Apologies for not responding earlier to this @michellec

There are three factors which dictate the pipe coverings for solar thermal:

• adequate insulation... especially on the heat output leading back into the house
• preventing ingress of rain
• preventing degradation by the UV component in sunlight

I can see how the installers have used a secondary layer of plastic to keep rain and sunlight off the inner insulation.
So they're clearly aware of the issues.

But I don't understand why they didn't use UV-resistant nitrile-rubber insulation in the first place.

That looks like the standard black Class-O closed-cell insulation, but is usually marked specifically for 'solar' or 'external'.

In any case, it's still important that the upper end of the insulation tube is sealed, usually with mastic.
Otherwise rain will get between the insulation and the pipe. As it runs out, it carries away the heat.

I don't like seeing nylon cable ties used on insulation because it compresses the 'foam', reducing its insulating properties.
Cable-ties can also become brittle in sunlight.

The sides of the split in the insulation should be stuck together.

If the split doesn't already have pre-glued strips, then apply your own glue.
The correct glue for nitrile rubber is a contact adhesive.

Whereabouts are you in the UK?
The slate roof suggests Wales or the West Country.

Is the scaffolding still in place for you to get these photos?

@transparent Thank you so much for replying. Yes the scaffold is still there (the roof they did is still leaking!) Can the insulation you describe be put on now? Could you recommend someone in the southwest (we are near Bude)? Many thanks, Michelle

Hi Michelle,

It's a pity you didn't mention that earlier.

There was a visit by a renowned exponent of good building practice holding a walkabout seminar at Nansledan earlier this week.
That's just 40-miles along the coast from you.

He'd brought along a couple of civil servants from London, to show them how things should be done...

Yes, the UV-resistant pipe insulation can be fitted now.

Are you sure that you don't want to put it in place yourself?
It's not that difficult a job.

Alternatively, you need a teenager with OCD!

I don't know of a builder/plumber who undertakes pedantically-correct work in Bude.
But the usual approach in the West Country is to ask at the post office or the pub.

This an area of UK where you can still get recommendations by simply talking with neighbours. 🙂 

"pedantically-correct work"? You don't mean me, do you? 

@michellec I suspect @transparent means neat and tidy work, which you have reasonably expected. Plus with the detailed implementation to keep water out and heat in.

No Michelle, I'm not making any assessment of your own skills 🤣 

The issue here is that work you need doing is pretty straightforward.
There will plenty of people who would have a go at it.

But the quality of the work is important.

For many years I was working with a German software programmer.
His coding was amazingly slick and accurate...

... as was his grasp of the English language, because he used to correct my grammar.

That's the sort of approach needed to properly insulate those two pipes.

@michellec, this is an area where the classic 80:20 rule is most evident - 80% of the benefit for 20% of the effort.

Any tradesperson will inevitably be applying the rule since their time costs you and them money, but the insulation job you describe is DIY- friendly enough that, with that bit of pedantry applied, brings that other 20% well within reach. It’s not that you have been pedantic thus far but rather that you could probably afford the extra time to apply the attention to detail that marks out a piece of work as first-rate.

Context matters, of course; I am pedantic to the point of anal retention with my woodwork but rather slapdash with any plumbing I (rarely and under duress) undertake.