Part L has long since given up on giving us elemental U values and has left it to us to work them out from first principles. The insulation manufacturers have no such qualms and are happy to go into print with their views on what the position is. And, surprise, surprise, they don't agree.
In one corner is Knauf, who run the old Pilkington plants in St Helens, and specialise in glass and mineral wool (now re-branded Earthwool). Plus a little extruded polystyrene (Polyfoam). In the other is Kingspan, makers of phenolic foam and polyisocyanurates (I hope I've spelt that right): high performance plastic foam boards which sell for twice as much as Knauf's products but get the job done in less depth.
You don't have to be a rocket scientist to figure out that low U values tend to favour Kingspan products, because the space consumed by their insulation is much less than Knauf's, and as insulation gets thicker, the space used becomes more of an issue. This is particularly true for cavity walls, where Kingspan products have to be installed with a 50mm air gap, the so-called partial fill solution.
But Part L is scientific and unbiased, so surely the manufacturers can't play fast and loose with the U values. Can they?
Well here's what they reckon the maximum U values should be for various elements. Kingspan here. Knauf here.
External Walls: Knauf 0.25, Kingspan 0.18
Pitched Roof (rafter level): Knauf 0.18, Kingspan 0.13
Ground Floors: Knauf 0.16 to 0.20, Kingspan 0.13