50mm polystyrene (or alternative) sheets for insulation....

The Ex wife used to work for Kingspan, their stuff is pretty much the benchmark. Try this spot for panels
https://www.roofingoutlet.co.uk/collections/vendors?q=kingspan

Polystyrene will be your cheapest option but if you want to get the most out of the insultation look at PIR board. Kingsman, Ceolotex being big names but as you say there’s a myriad of other brands.

For a shed I’d be looking at the best priced PIR rather than focussing gonna brand. Delivery costs normally seem kill any saving you make by shopping around so see what you can get locally and collect.

Kingspan or Cellotex are the popular brands. Maybe worth checking eBay or Local FB Sales as often builders look to sell-on excess stuff from jobs or slightly damaged stuff - or see if your local builder’s merchant has some at reduced prices.

Wasn't there a place mentioned on here once that sold seconds?
Can't remember what but I'm sure it was a thing.

Any concerns over fire risk?

I assume this is considered with kingspan and they try to reduce flammability with additives. Less so with standard polystyrene.

Wasn’t there a place mentioned on here once that sold seconds?

https://www.secondsandco.co.uk/

Call the local depot and see what they have in stock - often a few odds and ends which if you don't mind on the shed, you can mix and match.

Seconds and co didn’t seem to offer much discount lst I checked compared to the merchants on account+ have to put up with damage.

For a small project like a shed I’d be checking Facebook market place for surplus from building projects …. Often cutting a they just want rod but low quantity’s of sheets seem to go for pennies in the pound

Phenolic foams (Kingspan, Celotex and others) are approximately twice as insulating per thickness as rockwool and polystyrene (always read the label although they are often confusing and sometimes I'm sure deliberately).
Also tape the joints with the silvery tape. A bit of extra faff but not much extra cost and will make a difference.
Not quite that simple but for a shed a close enough approximation so you can work out if it is worth more to you.

The question is what you trying to achieve.

My garage tin roof has 100mm kingspan in-between the rafters. The adjoining wrap around shed has 100mm rock wool in the walls and 125 rockwool in the roof - sandwiched between osb - with building membrane on the cold side, air gap then a tin roof sheets

It's stays plenty warm and has no condensation drips from the roof - which was the reason I kingspanned the direct to tin roof but not the other roof as it does not suffer condensation due to construction method.

In our concrete sectional with fibreboard roof that suffered bad with condensation in use with the turbo the quickest cheapest way to get rid of the dew point at the roofline was spray foam

And, ideally, foam the gaps* before taping the joints - it's really satisfying!

* Don't bother with the cans of foam with a nozzle already attached - get a proper foam gun and the 'no nonsense' foam from Scewfix. It's much more controllable and lasts for ever despite what it says on the can - I used some the other day that has been attached to the gun for 4 years!!

Rocket dog, are you near North Wales?

I found Wickes was cheap and handy when I needed a few bits of shiny backed foam.

Rocket dog, are you near North Wales?

Midlands

Thanks for all the replies, very useful 👍

@monkeysfeet at the risk of jumping in, I'm also looking for 50mm insulation for the shed (walls) and am not too far away in Wirral, if you have any ideas where to go for some?
Was just walking down the beach looking across at the kingspan factory earlier!

I did my garage ceiling and outer walls, gradually, using bits and pieces I picked up free and cheap from Freecycle/Marketplace/etc. or just keeping an eye out in skips and where people were having work done - builders are often quite wasteful as it's their time but not their money, and will happily let you take offcuts if you ask.

It would have been more efficient to buy them in one go, but I would have had to block out a load of time to do it (moving things around to access was time consuming, and I wasn't about to completely empty the garage). As it was, I got a bit at a time and put it in as and when. Probably kept a lot out of landfill too.

It was easier before lockdown - I think I bought a couple of full 50mm boards and some large offcuts for a tenner, whereas I paid £25 for a single board to finish it off.

While I'm here, the best way I found of cutting PIR if you don't want to invest in specific circular saw blade is to get an old hand saw and take the teeth off with an angle grinder to leave a sharp blade. Goes through an 8' length of 100mm in less than a minute, straight, with virtually no dust.

What about a roll of superfoil, it does work very well, sf19++ will do it to same standard as 50mm pir.

My shed is insulated with 50mm polystyrene. Very warm and when I did it, very cheap. I did a layer of that superfoil taped with aluminium foil tape to make it all airtight and hold it in place.

What about a roll of superfoil, it does work very well, sf19++ will do it to same standard as 50mm pir.

Wouldn't that have the same issues as most foil based insulation products? In that they're not actually anywhere near as effective as their quoted insulation values because they use some sort of fudge to supposedly account for radiation as well as conduction.

My house is full of it (new build) and it's toasty.