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
Interesting point, but Kingspan do make it very clear that the U values stated are only intended as a starting point to assist in reducing the number of iterations that need to be carried out when calculatiing the SAP.ReplyDelete
And Rockwool have yet another set of figues:
Roof .16 to .13
Wall .27 to .24
Floor .20 to .18
As Ecorenovation said, I see the U-Values and constructions given in both sets of documents as guidance as to what sort of wall thickness are required for certain U-Values.ReplyDelete
Although it is very interesting to see the difference in U-Values between the examples in the Kingspan document for mineral wool solutions, verses the U-Values quoted for mineral wool solutions in the Knauf document.
But this says nothing about thermal bridges. In bad designs these easily add 0.15 to the U-values if their effect is spread uniformly over all the building's surface area.ReplyDelete
Result - a building with no thermal bridges could have a wall U-value of 0.25 and outperform a bad design full of thermal bridsges and having a wall U-value of 0.18.
Buildings still appear to be full of thermal bridges, one example being hefty steel lintels extending across the cavity. I don't see the point in arguments between insulation producers until such silly features have been designed out of buildings. BTW, "top hat" steel lintels were virtually unknown before the 1960s.
The thing to watch for is whether they are reporting the same lamda values. Different tests will allow differenr performance to be reported ie best of a sample batch. CE testing samples random batches and quotes worst case performance I believe. We need a level playing field. As to maximum values, PHPP tells us that we need to go beyond even those values sometimes, for new-build anyway.ReplyDelete
50mm kingspan looks fabulous fitted and taped,face on,,,but looking down cavity next lift,even the cleanest bricky has 2 rattle his ties,damaging boards,knocking off clips,ripping tape,,,,,,,,were as full fill earthwool,,,,,, brick or block can be started first,no clips tape,or rattling ties?? im eartwool £££££ toReplyDelete