I know this one has been done to death in other threads but what are the key points we should consider before going with multi-foil insulation?

We've specified 220 mm i-joist rafters fully filled with glass mineral wool (with 0.032 thermal conductivity) and an Intello Plus VCL to underside of rafters (with a further 25 mm batten/service zone + plasterboard)

But there are currently very long lead times for the glass mineral wool (estimated delivery March 2022), so the contractor wants to look at alternatives. One of which is a multi-foil honey comb product called Hybris between rafters and a thin multi-foil VCL (HControl Hydbrid) to underside of rafters.

HControl Hybrid + 170 mm Hybris is said to provide 0.13 u-value, and at roughly £26m2 considerably cheaper than mineral wool. And with a lead time of only 2-3 weeks. They also come with a suite of LABC certified details.
https://www.insulation-actis.com/pitched-roof.html

So, if those figures are correct, the advantages are obvious. What are the other things to consider? I'm going to list them here as people post them:

I normally use Knauf wool, but I suspect that may be the one with the long lead times. There is also Superglass, based in Scotland, don;t know if you've tried that product for delivery times? It's not as nice to work with, but needs must.

As to multi foils, I had a quick look at the hybris product previously, and it is not the same as the usual thin multifoils - often 15-20 mm thick, so very reliant on outer face conditions. This hybris looks more like a wool type insulation, in that it has considerable thickness, and so likely is more dependent upon air pockets in the 170mm thickness, meaning it shouldn't be squashed. That's much the same for wools.

Could you not go for 210mm of the hybris, so all insulation is between the rafters, as you had planned with the original wool. That keeps your design detailing the same, just a like for like replacement (Hybris shows lambda of 0.033 W/mK). No concerns then with thin multifoil of the Hcontrol additional surface layer?

Your Uvalvue will be better than that calc'd on the Hybris website, as your timber I beams will give significantly better thermal values. On that point, your insulation will need to be a bit wider, as your rafter thickness will be 9mm not the usual 45. Check that the Hybris can still expand to fill the 590mm gap, rather than the usual 555mm gap - it could be slack, and so not prevent air by-pass at the rafter interface?

What about the breathability of (or lack of) the Hybris?

The Intello Plus VCL allows for radiative summer back drying, providing extra protection to that the top layer of OSB, but might the Hybris insulation layer interfere with this process?

I thought that to hit building regs that you couldn't use multifoils on their own, but had to combine them with other forms of insulation such as rockwool or polystyrene?

We used a multifoil that is combined with a waterproof breather membrane (TLX Gold) on the upper (tile-facing) side of out roof, with Celotex PIR between and under the rafters.

It would be interesting if multifoils on their own are now OK for building control.

We found cutting foil was difficult, and in the end purchased some Fiskars Shears.

Recommended (by supplier) Tape to stick the thing together was poor at sticking - bought a different tape in the end.

did you try "superglass 32" alternative for availability? You'll likely be getting a lot of rolls, so several pallets may even be sent from manufacturer directly, without touching the merchant (though he'll still take his cut)? Eliminates any concern over breathability.

There's an excellent example of the proper use of multi-foil in the news at the moment.

The James Webb space telescope uses a 5-layer multi-foil for its sunscreen and achieves a 300 K difference between front and back. The key of course is high vacuum on both surfaces and between all the foils