curiosity: how can celotex insulation hold the weight of the screed + floor + units + people?

[johnhenstock83]

I'm genuinely curious about the above and I was wondering if someone can explain, briefly, how this works. I know it's something that's done often, it's more or less standard practice, but I can't get my head around the numbers.

 

we have a block and beam floor (roughly 20sqm). this is at ground level, with 3 courses to the existing floor level (2 + DPC). builder said he'll insulate it with 150mm celotex and then lay a screed of approx. 80mm on top.

 

how can that sponge hold the weight of the screed and everything else that will sit on it without squashing?

[JohnMo]

It called compressive strength. It holds an incredible weight when spread across a large area.  We have 56T of concrete on top of ours, but it is spread over 192m2.  The screed then acts to spread any point loads out across the floor.

[Adsibob]

It’s called compressive strength. also, keep in mind that the layers above the Celotex spread the load, so there is never very much point loading onto the celotex.

 

 You do however need to calculate if there are likely to be any heavy point loads on your floor. For example, if you had one of those Japanese style deep baths which you sit in (as opposed to lie), and if that bath were made of cast iron, that, plus the water plus a fat bloke like me, might increase the load beyond what the celotex is designed for. You should be fine, but Check which celotex your builder is using and what is compressive Strength is.

[SteamyTea]

Materials have a number of 'strengths'.

The main thing to consider is working at the lower end of the elastic limit (material can recover), and never getting close to the yield point (material cannot recover).

One of the problems is that because strength of materials is a force, and a force has a time element (f = ma, where a is acceleration m.s^-2), what initially seems fine can fail in the future.

Edited April 5, 2023 by SteamyTea

[saveasteading]

The floor loading is mostly concentated point loads, from furniture.

A chair placed on pir, when sat on, would go stright through. But put it on a plywood base, say, or paving slab, and the load is spread over a bigger area that the pir can support.

 

In real life we have a screed over the pir. In very approximate terms, the load from the chair leg is spread at 45°. Depending on the screed thickness, the load is now spread over about 10 x the area, and pir can handle that.

 

Might look at a worked example later.

 

As above, there is a limit, and a bath or bookcase at an outside wall might be worth checking out.

 

In commercial situations pir would fail, so other solutions are necessary.

[johnhenstock83]

So for a domestic rear extension, which will have some typical things such as a kitchen, a dining table and a wooden floor, would celotex be able to hold the load?

[saveasteading]

1 hour ago, johnhenstock83 said:

would celotex be able to hold the load?

What is intended above the pir?

[johnhenstock83]

screed, then some type of flooring (engineered wood as it stands, but potentially laminate).

[saveasteading]

Should be fine then. The timber spreads the load, then so does the screed. The deeper it is, the further it spreads it out.

 

Same principle as walking on any soft surface with flat feet as opposed to heel first...spreads the load.

[Bonner]

Celotex has a compressive strength of 140kPa, about 14 tonnes per square metre evenly distributed! A person sitting on a dining chair might exert 300kPa so it will crush the foam. Human footprints create about 60kPa so you can walk on it without damage (in flat shoes). As others mentioned, the screed and floor coverings serve to spread the load over an area such that the pressure will be much less than the compressive strength.

[Temp]

Insulation can be pretty strong....

 

https://www.dailymail.co.uk/news/article-3829379/Vehicles-CRUSHED-against-roof-burst-water-main-causes-polystyrene-insulation-beneath-car-park-float-raises-floor-four-feet.html

 

Quote

Cars and vans are CRUSHED against roof of underground car park when burst water main causes polystyrene blocks it was built on to float and rise up by 4ft

 

[Bitpipe]

Our 110m2 basement (300mm thick slab and 200m thick, 3m high walls) containing over 16t of rebar PLUS the 300 m2 timber frame house on top with render & slate roof is all sitting on 300mm thick EPS 200 grade - a 2.4mx1.2m block of which was 35kg and you'd struggle to leave an impression on it with a finger. 

 

I guess this (or an equivalent grade) is what you're seeing in the image above. 

 

The SE figured the point loading of the whole construction and we selected the EPS grade that just exceeded the spec.

 

By comparison, the sides of our basement were insulated with EPS70 - much softer (and cheaper) but enough to resist any sideways compression from the ground.

[saveasteading]

4 hours ago, Bitpipe said:

300mm thick slab

Sitting on EPS!

Rather thick, presumably for a good reason.

[Bitpipe]

1 minute ago, saveasteading said:

Sitting on EPS!

Rather thick, presumably for a good reason.

 

Our geology is Thames Valley gravel / chalk which often has solution features (aka holes) so the slab had to be thick for that reason, we also wanted an open box design so the walls cantilever off the slab.