will the OSB not act as a VCL if joins are glued and taped (foil tape/air tight tap ?)

I doubt Jablite would accept it either. They're expecting a a high performance membrane of some sort, turned up to the full thickness at all perimeters.

Jabdec provides significant resistance to the passage of water vapour, but it should not be considered as a vapour-control layer. An appropriate vapour control layer should be incorporated in the construction below the Jabdec. BS 8217 recommends a minimum Type 3B roofing felt (to BS 747), either fully or partially-bonded to the decking; if the decking is of profiled metal, the felt should be a minimum Type 1F. Alternatively, a high performance vapour control layer can be used as recommended by the manufacturer of the weatherproofing membrane. The vapour- control layer should be turned up to the full thickness of the Jabdec at all perimeters and upstands.

I used PIR (non foil faced) glued direct to OSB3 and went with no VCL on the underside - the upper layers of the build-up are non hygroscopic, and a WUFI sim showed that the moisture levels in the timber elements were acceptable. Dunno about EPS tho'.

The WUFI sim showed slightly dryer timber in the long run if you use a VCL below the PIR, but I reckoned that if water ever got in (which is surely will on a flat roof at some stage), then you'd end up scrapping the insulation and with possible timber rot too (in my case it was only feasible to install the VCL at ceiling level). To be on the safe side, I embedded some temp+humidity sensors in the roof build up at various points - next to the OSB3, so that I could keep an eye on the RH - must get around to connecting them up, so that I can see if what's really going on matches the simulation in any way.

Posted By: wavyI have used a self-adhesive bitumen foil product from Sheffield Insulations (SIG) called Alutrix which is easy to install & effective but not cheap.
Any penetrations, mechanical fixings etc. will need special attention.
Dont take risks with this element or the roof will deteriorate over time.

So I got some Alutrix 600. The EPS insulation (Jabdec) will be screw-fixed down over it. What "special attention" do we need to take when we do this?

Posted By: Nick ParsonsI gun the holes full of silicone when I am drilling through insulation/VCL. It's not foolproof, I know, but 'feels' better than relying on a screw to be a VCL, if you see what I mean.

Okay thanks Nick, not easy to gun through 150 mm of 150 kPa EPS though. I assume you drill a pilot hole through the insulation?

Posted By: ShevekSo I got some Alutrix 600.

But didn't buy any primer, arghh. Perfect weather now turning to rain tomorrow. Is primer a must on timber decking?

Edit:

Where the insulation is to be mechanically fixed, ALUTRIX 600 can be applied without primer
Where the insulation is to be bonded onto the ALUTRIX 600, (please consult with us regarding the adhesive) the substrate must be primed with FG35 primer before installing the ALUTRIX 600

Given that we're screwing down the insulation over the Alutrix I think we're in the clear...

Edit 2:
Oh and this too:

*) For mechanically fixed or ballasted roofing systems, priming the roof surface is not usually required

Think 150 mm Jabtherm and a 150 kg/m2 green roof will be sufficient ballast. Back to work

I think I've got some primer left in case you decide you need it - will check - what size area are you covering?
I found that the edges of the Alutrix lifted from my ply deck where primer was missing. The VCL was a medium term temporary roof so if it had lifted wholesale it would have caused some problems.
We had a zinc finish which was mech fixed through the insulation. I didn't worry about the screw penetrations as the bitumen in the alutrix appeared to 'self-heal' around screws.

Make sure they're stainless....

Is the roof ballasted (sedum or other)? or is there another deck or a membrane finish.
If you are relying on the screws to resist wind uplift I fear the heads will pull through the insulation.
There are dedicated insulation fixings with a very large head available and I would get a calculation from the manufacturer based on the fixing and insulation type.