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The wall is in a 45 year old house that has standard UK cavity walls (red-brick on the outside, cavity, concrete blocks on the inside). Kooltherm K118 Insulated Plasterboard is what I'm thinking about.

Kingspan building regulation's page shows a target W/m²·K figure for the wall type. I'm in Scotland and for an existing heated home, 0.30 W/m²·K applies.

On the the product page it says:

As part of our Kooltherm 100 range, it provides a level of performance that is unrivaled in the world of rigid thermoset insulation materials with a thermal conductivity of just 0.018 W/mK across all thicknesses.

So (help me here) 0.018 is a lot less than 0.30, and it says "any thickness", so I could choose their thinnest sheets: 32.5mm and still be compliant with the regulations, right?

Maybe I'm getting lost in W/mK vs W/m²K..

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You do need to adjust between the different units. To work out the thickness of insulation needed for 0.30 W/m²·K you can do 0.018W/m·K / 0.30W/m²·K = 0.06m. So you will need 60mm sheet of insulation to get that target with the foam (note that this is the thickness of the foam, not the combined foam+plasterboard).

Dimensional analysis is useful for a sanity check, it doesn't make sure we have the right formula but it does help make sure we aren't wrong. W/m·K / (W/m²·K) = m which is good since we want to work out the thickness of the insulation and m is a measure of thickness.

In practice it is much more complicated because your existing walls provide some amount of insulation, how much depends on your existing wall construction, if you cavities are already insulated then this will be quite significant. So you will need less than 60mm of foam to get 0.30 W/m²·K thermal conductivity for your wall. Working this out is beyond the scope of this answer. However in the product brochure pdf on the Kooltherm K118 Insulated plasterboard webpage you linked to there are tables with estimated combined U values including a "Masonry Cavity Walls" section. So this should hopefully give you the information you need.

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  • Thanks Johnathan. So my next task I think is to find out what type of concrete/cinder/breeze block the inner leaf is made from, per this table imgur.com/a/0k7YOZt. Dense/Medium/Lightweight/Aerated. I'll ask my neighbors on NextDoor.com in case one of them know.
    – paul_h
    Sep 10, 2020 at 12:22
  • If you can't figure out what type you have, can always just assume the worst case (dense). From that chart it looks like the thinnest product which offers <0.30 is 57.5mm. I don't know how much of a cost difference that is vs 47.5mm (best case). Anything smaller looks like it doesn't meet the target.
    – Khrrck
    Sep 10, 2020 at 20:25
  • Thanks again Johnathan, so we took down the ceiling too, today. The last ceiling plasterboard / sheet rock, had been put up with nails and panels were beginning to part - leading to cracks in the paintwork. So heres a fully exposed block - imgur.com/a/KMhFqfA (zoomed in). I think it is "aerated" to some degree - comparing to pics from greenspec.co.uk/building-design/blocks/#lightweight. Thoughts?
    – paul_h
    Sep 11, 2020 at 15:59
  • I agree that it does look more like the aerated block than lightweight. I have zero previous experience visually identifying types of blockwork though (more of a physicist with some minor building knowledge) so maybe it is worth asking another question to see if you can find someone more knowledgeable about this.
    – Jonathan
    Sep 14, 2020 at 9:33

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