# Insulation Confusion

#### WoOt

Planning to pour a concrete slab for an extension in 2 weeks time, but am a little confused with the insulation requirements.

I had originally planned to go with 100mm PIR insulation in the slab, as everyone else seems to be doing that. But I noticed that certain brands offer far thinner insulation that also complies with what I understand is the minimal 0.022 W/mK requirement from building control.

Am I missing something, or would a 50mm PIR board like the Celotex GA4000 General Purpose PIR Insulation Board be adequate?

Would save a little money as those 50mm boards are cheaper, but more importantly... less digging for me (and less spoils removal).

Any other suggestions on insulation or where to order it welcomed.

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I think you are confusing the numbers thermal conductivity with resistance. The U value is 1 divided by Thermal resistance: 2.25 m²K/W.) You need to hit a U value of 0.22. the above on its own is going to be approx .44. You need another 50mm. Alternatively you could use 100mm of standard polly and opt for thinner insulation via a floating floor?

I didn't take account of any other building materials when doing the above. U values calcs are explained here (skip the first 2:30 seconds) :

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I think you are confusing the numbers thermal conductivity with resistance. The U value is 1 divided by Thermal resistance: 2.25 m²K/W.) You need to hit a U value of 0.22. the above on its own is going to be approx .44. You need another 50mm. Alternatively you could use 100mm of standard polly and opt for thinner insulation via a floating floor?

That's the first time I've understood how R value relates to U value! Thanks!

All these insulation firms try to sell you tools to calculate it. its not hard. Thermal conductivity is a bit confusing as its expressed in watts per meter kelvin otherwise known as the lambda value. So all Celotex GA4000 boards will pretty much have the same thermal conductivity (Lambda) as its for a 1 meter thick slab. So the 20mm stuff has the same Lambda as the 100mm stuff. 5 20mm boards are as thick as 1 100mm board. You need to work out R (resistance for the actual thickness) from L before calculating U: (1 / sum R).

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