I have a concrete slab on which I am laying 50mm of PIR and then 50mm of screed containing UFH.

The finish on the concrete is not totally smooth, can I just lay PIR and trust it will settle or do I need to lay a blinding with sand or similar first? And if so, What's the best technique? Above or below DPM?

Are you just having 50mm insulation or was that typo?

What's under the concrete slab

There's more insulation under the slab. There are reasons.

54 minutes ago, Ed_ said:

The finish on the concrete is not totally smooth

One man's not smooth is another man's mill pond :-)

Are you talking a few mm difference, the occasional protruding piece of stone in the concrete, or is it an overall tamped only surface (AKA garage floor rough)? The PIR will give a bit and "soak up" the roughness. Have your considered levelling products instead of a sand blinding as this would be a more stable base for your 50mm PIR

A picture will help.

I have thought about levelling but I've got around 90m2. Do you think it would be easy enough to do over that sort of area?

What we see there is roughness. Pir is not strong so any odd stones will puncture the surface and allow the insulation to mate more closely, especially after you have walked over it several times.

Small ups and downs will become voids but they will be insulated pockets in effect.

Levellness does not matter, as the insulation will follow the shape, and the screed can follow it or be laid level, as you instruct.

What is more important is whether the surface is on a consistent plane that the insulation can follow, or has significant undulations. These could result in large areas being out of contact, bouncin in use and the hardened screed cracking.

Test this by straightedge. Officially a 3m one is used, but 2m will do in your case.

Lay it down in multiple positions and directions and see what gaps appear.

And report back.

Thanks for the advice, I will check. I think it is quite flat based on the way the water pools on it.

9 hours ago, Ed_ said:

based on the way the water pools on it.

That's the perfect test. How deep for you think it got?

You could use a low tech abrasive to take lumps off. Crawling around rubbing with a hard brick. That way you also soon get the feel of how wavy it is.

After the pir is laid you will be taping the joints and then laying dpm over it, so it will get plenty of pressure to press into small lumps. The local pressure is never that high again ( unless you drop the grand piano).

5 hours ago, saveasteading said:

After the pir is laid you will be taping the joints and then laying dpm over it

Ah, I had been thinking DPM under the insulation, but I guess that doesn't make sense as could be punctured by the roughness?

So is this a good buildup? -

Finished Floor

Screed 50mm (containing UFH pipes)

DPM

Insulation 50mm

Waterproof concrete slab

5 hours ago, saveasteading said:

That's the perfect test. How deep for you think it got?

Puddles that are 5mm, maybe up to 10mm deep in places probably cover areas 5-8m plus in length.

The buildup looks good.

For the areas that you think are 10mm "out" I would use some leveller to reduce the undulations to reduce the potential of cracking screed as mention by @saveasteading above.

Agreed with @BotusBuild .

I had a brilliant boss once who gave me lots of real world advice. Here's one.

The slab is poured but the building isn't up yet so it obviously has puddles. We know it's OK but the lay client doesn't. Client says the floor slab isn't flat enough. So we say oh dear oh dear and go out to inspect with him.

My boss explains that a few mm is unavoidable and acceptable by all measures. Client agrees.

Boss casually (and visibly) throws a pound coin into the puddle. Remarkably it is not covered by water.

End of concerns....it isn't deep at all.

The meniscus helps this demonstration but it is a realistic demo that it isn't deep.

So back to your floor. Room divisions will reduce the areas and allow the pir to follow the shape. The screed can be poured to a common level (insist on that). And remaining dups will disappear under flooring.

Dpm above the pir prevents any screed getting through and floating it. It is Belt and braces, but you don't want your trousers falling down in public, or even 1% chance of your floor floating.

If you're really worried and the insulation value isn't critical you could downgrade to EPS which is a bit more conformable

22 hours ago, Ed_ said:

Waterproof concrete slab

Why did you use waterproof concrete?

With PIR it is best dry but if the slab is fairly waterproof that should be OK.