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T: +44 (0)845 769 7452 < Springvale (a polystyrene manfuacturer) technical Tel: 01544 388 601 < Kingspan T: 0901 996 0100 < Celotex Call any of the above and ask them what the U Value of their products is. Then post back here and let me know what they say.

No, thermal transmittance / lambda value / thermal conductivity is not a U Value.

Which is exactly the same as putting another layer of 150mm / 170mm / 200mm loft insulation on top of the existing. Which every loft insulation contractor does anyway. The only way round this is to insulate between and under the rafters - which can be a costly exercise.

Yes, it has a thermal transmittance. That is the Thermal conductivity (0.038W/mk) as mentioned previously. Thermal conductivity / lambda value is not a U Value.

That gap is, in essence, a party wall. Is there a specific reason you want to insulate it? If it's for thermal reasons, then it really won't make much of a difference. You could get blown cavity insulation in there, but a better solution would be to insulate internally with a insulated...

If you constructed a wall out of polystyrene, the U Value would be the last of your issues. In that bizzarre circumstance, that the only material in the wall was the polystyrene, then a U Value could be calculated. But the polystyrene itself does not have a U Value. Look, a U Value is...

100mm of Polystyrene still doesn't have a U Value of 0.4. Let me educate you just for a minute. Indulge me. A singular item, be it 100mm Polystrene, or Celotex, or a airated block does not have a U Value. It is impossible. A U Value is the result of a calculation including the...

You did. XPS does have a better thermal conductivity (0.036 - 0.033 (depending on manufacture and thickness) but it is generally harder to obtain. XPS typically has a kPa of 200, but for domestic applications that is overkill. The price per m2 of XPS is also pretty close to PIR...

1) Polystyrene doesn't have a U Value of 0.4. It has a thermal conductivity or lambda value of 0.038 W/mK 2) Celotex GA4000 or Kingspan TF70 has a thermal conductivity of 0.022 W/mK. Roughly 75% improved. 3) Polystyrene EPS70 SDN grade has a compressive strength, or kPa of 70 4)...

Which is fine if you don't want to use the loft ever again. A slightly better solution would be to fit 100mm Loft insulation at ceiling level, between the joists. Overboard this with 100mm Celotex GA4000 or Kingspan TP10. Then you can put some ply, or hardboard, or chipboard flooring down...

Easiest as long as you can dot and dab. If you can't then Celotex / Kingspan against the wall, 25mm counter batten fixed through and plasterboard over the top. Foil tape the joints on the insulation before boarding. And yes, you will need to see to the damp issue first. Edit: There is...