# Insulation under screed - trying to understand

Discussion in 'Building' started by ey143, 15 Aug 2016.

1. ### ey143

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Im trying to understand how the weight of screed, UFH pipes with water in it, floor tiles and weight of everything else above the floor, does not crush the 100mm kingspan / celotex insulation boards underneath....

3. ### freddiemercurystwin

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Because it's magic.

Did you know motorways or railways are sometimes built on polystyrene.

4. ### ey143

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its a serious question...I dont work in this industry so I'm curious to know.

5. ### freddiemercurystwin

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The simple answer is that it's compressive strength is greater than the loads imposed upon it over a set area. The techy answer is all about it molecular structure, you'll need someone with some brains to explain that though!

6. ### noseall

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Because the weight is spread over a large area.
Place a chair directly onto the insulation and sit on it and the chair legs will dig in. Stick a book under each leg and it will take a lot of electricians sitting on that chair to be able to force the books downwards.

7. ### ^woody^

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Think snow shoes

8. ### Nozzle

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If all the screed that is due to be poured on your floor is instead put to one side and cast into a tall slim cube rather than flat, wide and thin like your floor then the same amount of weight of screed is still there right? It's just the weight then acts on a smaller area. That weight across a smaller area is really pressure. A much higher pressure. The high pressure of your tall, slim cube would easily crush the celotex, but spread that weight out (like someone who is trying to get across thin ice by lying down) and the pressure isn't enough to pierce or crush.

Nozzle

Last edited: 15 Aug 2016
9. ### ey143

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Ok thanks. So it's similar to the old school question of why does a ship not sink despite its weight. All to do with mass and surface area. I guess I thought the answer would be somewhat different because of its foam factor (despite its compressive strength).

11. ### bernardgreen

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How much does she weigh ? 8 Stone ( 50 Kg ) If she sat on just one nail it would stab deep into her flesh. But by spreading her 50 Kg over several hundred nails there is not enough weight per nail to force the nail through the skin and into the flesh.

12. ### Nozzle

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That's an issue of displacement, the volume of the ship below the water line displaces more weight of water than the ship itself weighs.

Nozzle

13. ### ey143

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So are there any exceptions to that in terms of material ie non compressive stuff that would equally take suck weight?

Whilst on that point, running a mains cable channelled under the insulation is ok?

14. ### Nozzle

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There might be limitations to the route the cable channel should take - the electical boys could advise on the "safe zone" layout for it.

Nozzle

15. ### ^woody^

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If you don't already, try wearing your wife's stilettos and walk across the insulation.

Her underwear and lippy too, is optional.

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16. ### Tigercubrider

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i built a floating floor for a recording studio on rockwool slabs, you couldn't have walked on the rockwool but as we went, chipboard was laid and glued up over the wool.

Same principle - snowshoe or displacement.

My car has wide profile tyres and is crap in snow whereas my girfrind'd old Honda has narrow wheels and cuts through snow to hit Tarmac ( reverse principle)

17. ### Nige F

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Think Fakir's bed of nails. Quite a few on here

18. ### DIYnot Local

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