Had an interesting discussion with a prospective flat roof contractor yesterday, we were discussing the merits of different insulation to lay over our basement roof. I indicated that in one section I wanted to use PIR/Phonolic board due to lack of space and he warned me against it and recommended EPS instead. He recently went back to a large Hotel roof he did in his youth 25 years before where the PIR board had been laid on flat roof. The waterproofing had not failed but the insulation appeared to have broken down, it was spongy underfoot as though the structure had degraded.

Do you think this was a one off or can we expect the same from PIR board today or has technology moved on and today's boards have much longer lifespan? Don't fancy having to dig the garden up in 25 years!

With my sceptical hat on EPS is of course significantly less expensive than PIR possibly just nudging his quote into the cheapest (yuk) category.
Is he offering you savings by using it? He's certainly getting them if he buys it
Check on the spec of the EPS, there are many grades and a price sensitive contractor may be tempted to offer one slightly down the list (packaging material grade)
As for the cause of the sponginess of the PIR its almost certainly saturated due either to a failed vapour barrier or like it or not a leak in the membrane probably around an upstand or where the membrane turns vertically up the parapet. If its a hotel roof then its probably also seen some foot traffic, around plant rooms, TV aerials, window cleaning hanging rails etc. Punctures or rips are very possible.
The reason for the structural collapse of the PIR in both cases would be water pick up by diffusion or immersion. Once saturated the PIR will break down. In a saturated state just below the membrane it will also of course be subject to freeze thaw cycling which will further rupture the cells.
See what he says if you say you'd like an inverted or upside down roof. XPS insulation placed ontop of the membrane, dew point thrown outside the structure so no concerns over a vapour barrier, membrane protected from UV, solar gain and foot traffic from then on. Great solution for flat roofs.
EDIT OK you can't do an inverted roof exactly underground but applying the insulation externally to the structure is still a good way to go

Phil, apologies for maligning your roofer!
The inverted roof system uses XPS (Extruded polystyrene) not EPS (Expanded polystyrene) so there's not an issue with moisture pick up. The inverted roof system has been very popular throughout Europe after starting in the US and in the UK there are hundreds of thousands of sqaure metres in existence especially on large flat roof areas like hospitals, large hotels, apartment blocks and in green roofs. Check out Dow Styrofoam, Knauf Polyfoam, URSA XPS, Jackon Gefinex and BASF Styrodur websites. Here's a link to a US contractor's green roof site http://www.hydrotechusa.com/brochures/GardenRoof.pdf but there are many others http://www.energyefficiency.basf.com/ecp1/EnergyEfficiency/en/function/conversions:/publish/upload/pdf/styrodur_br_eng.pdf
The thermal value of PIR will degrade over time as the blowing agent escapes slowly, the foil face on the PIR boards serves to control this, the same goes for XPS, PUR phenolic, 5milliwatts or so in 20 years depends on application. No reason for PIR to deteriorate structurally unless moisture gets in but that's why XPS is a good safe option for you. Platinum and Pearl are EPS grades not XPS
Yes the electrical testing is pretty common. As you can imagine on a multilayer flat roof build up the leak through to the inside of the building may occur many metres away from the puncture in the roofing membrane as the water tracks along each successive internal layer until it finds a gap. I'm no sparks but electrically you can track the water path