I have done a bit of reading, but would like some clarification whether this is the better option.

Attic conversion: originally intend to go with 75mm celotex between rafter and 50mm underneath.

The reading i have done suggests this isnt the way to go as celotex will leak after 10 yrs and becomes impaired.

So i propose to keep the 75mm between the rafters and using multi foil underneath.

What do people think?

I think i remember Tom saying this was the very best solution ?

Oh dear.

The winner is...............

Well 75mm between the rafters will not qualify anyway so i have to put something under, and want to as well.

Ok then so the figures quoted are after the 10 yrs odd time frame. That is more reassuring.

Well the current roof stays so all work will be done from below.

Looks like celotex then, which makes life easier and cheaper.

If you read on through the comments, it's clear that the product complained of isn't genuine Icynene, and there's testimonies that true Icynene doesn't behave as described here, but as the manufs claim. If your slates have to come off, then AFAIC my post above of '5 days ago' says it all. Read up about soffit (= under-surface), OSB (like big-chip chipboard but superior and can be had thinner i.e. 9mm, as recommended) etc.

Posted By: SteamyTeaI think that is the stuff that is sprayed on and then foams up

Yes, but not like the other foams - very significantly different characteristics - Search this forum for Icynene, it's all there. Good stuff.

Posted By: SteamyTeaNot the same as sheet material

That's its main virtue, along with Warmcel. Sheet material is rubbish unless applied as an unbroken 'tea cosy'.

Posted By: SteamyTeathe U-Value

good enough, equiv to standard EPS nominally, but as installed, gapless and airtight, far better performance. It fils every hidden gap, sticks like hell, and being permanently soft doesn't suffer embrittlement or shrinkage.

Posted By: SteamyTeais it breathable

yes

Posted By: SteamyTeais it waterproof

if you're expecting the insulation to do your waterproofing, you're in trouble. If you mean does it degrade if it gets wet, answer is No.

Think again - it's quite good stuff, definitely A-list.

Posted By: SteamyTeaOther techniques should be used to deal with ... water ingression

Too right

Posted By: SteamyTeaOther techniques should be used to deal with condensation

Disagree - much better that the insulation plays am integral part in handling condensation, calculated vapour and air resistance, and airtightness, robustly 'in depth'. Nothing worse than entrusting all that to a fragile sheet of plastic and sticky tape, however sophisticated, for the next 250yrs. No reason why Warmcel, Icynene, gapfilling-glued-and-screwed OSB, breather felt etc shouldn't achieve that, if kept dry, shielded from UV and not physically attacked. Plastic sheet and tape - not a chance.

Icynene isn't generic - it's unique and is really worth looking at. I noticed in the feedback to the above blog, mention of at least one US alternative to the Icynene monopoly.

Posted By: CWatters My understanding is that the performance quoted by companies like Celotex and Kingspan is the long term performance after ageing has been taken into account... or put it another way ..for the first few years it works better than they claim. After that it matches the claimed performance.

My understanding is that companies like Celotex and Kingspan show what they claim to be the performance of PUR after 7 years (0.023W/m.k) and from then on it gets worse.
I've just read a Canadian Construction Institute research paper showing PUR dropping from R50 (0.020W/m.k) to R40 (0.025W/m.k) in 200 days and down to R38 (0.0265W/m.k) after 400 days and still going down! The lower the R value the more heat is lost through the insulation.

Hi Viking, Can I ask if this publication is the same one you posted in a previous thread?

The tests in the Canadian paper relate to samples of 6mm and 12mm thicknesses. Anyone involved in foam insulation knows that the escape of the blowing agent is not surprisingly much faster in very thin foam. These thicknesses are not representative of insulation thicknesses currently required to meet Building Regs nor are they thicknesses used in testing standards for the very same reason and so are largely irrelevant in this case. That fact is recognised in the sample thicknesses required in the thermal conductivity testing standards nominally 20mm to 30mm or thereabouts.
Ageing is clearly important though and that is why foam's thermal conductivities are quoted after a certain number of days and at a certain temperature. The younger the product the better the performance, the cooler the test temperature the better the performance. PUR tends to be measured at 30 days and 10 degC whilst XPS for example is measured at 90 days and 20 degC thus PUR is guaranteed by the test method alone to show a couple of milliwatts better thermal conductivity than XPS. From experience I'd use 0.025W/m2K as a realistic thermal conductivity for a PUR foam regardless of what is claimed

I was involved in foam moulding up until the mid 90's. Only on the tooling side though.
I have been thinking about this problem for a few minutes this morning and was wondering what changes as foam ages. Here are a few thoughts.
Shrinkage. PU certainly shrinks quite a bit initially and I know some elastomers carry on shrinking seemingly for ever.
Offgassing. This happens, but have no idea how the rate changes over time. Probably exponentially though. As it is the air that does the insulation, and making the assumption that initially the PU is mixture of air/other gasses in bubble form within a PU matrix, then it is possible that as it offgasses the bubbles change to predominately air (but may change to something else, I don’t know) which has a different thermal conductance.
Brittleness. As PU ages it becomes more brittle, because of its thermal properties may change.

It is amazing how the term multifoil creates so much interest on this forum. Why do we think that is?

Tom, just because they are all PURs it doesn't mean they will behave the same over time. Yes air will gradually infiltrate the cells as the blowing agent exits but the rate and the impact that infiltration has on final lambda depends on many other factors. cell size, cell wall thickness, open cell content, location etc. etc.
I've had tested an old piece of PUR that had been used as the core of a coldstore door panel. The door had been subjected to abuse from forklifts and the PUR was actually exposed. Theoretically at least the PUR should have been devoid of most of its blowing agent and also saturated as you'd expect in a high pressure differential/high vapour drive situation but disappointingly (it was a competitive product) it stll showed a lambda of 0.024W/mK