Blinding beam and block before insulation?

[Dave Anderson]

Hi all, I've read a fair few threads on here, but still have questions! Any advice will be much appreciated.

Our builder has done the shell of our self build bungalow. They have grouted the beam and block nice and solid, and it's now over to me to do the DPM, 120mm PIR and 2nd membrane before the heating company do the UFH.  But brushing-in the  cement grout has left ridges up to 10mm high. I have begun breaking off the worst ones but it is very tough and impossible to get completely level. I'm worried about the insulation rocking or sagging, and any such movement inducing cracking in the screed.  The builder has said it is usual to blind the floor with sand to take out discrepancies in level, but how would you get the sand level? Spreading the sand and dragging a straight edge over it? Once the DPM is down it's too late to adjust the sand if a PIR board is found to be rocking.

Can anyone advise how to get this right?

[Mr Punter]

You can break off any big lumps or ridges with hammer and bolster.  Doing the insulation in 2 60mm layers will help it lay flatter.

[Dave Anderson]

Ok thanks I'll consider that - but probably will cost a bit more.  So would you not bother with sand?

[Russell griffiths]

I hired a floor scabbler to take all the snots off mine. 
did a180m in a day. 

[Dave Anderson]

Thanks for the idea Russell.  Had a quick look and there seems to be a few 110v machines that might do it.  Just wondering what sort of machine worked for you?  Did you use something like this

 

(I think you have to by carborundum or diamond blocks for it)

or this

 

or was it a heavy pneumatic scabbler?

 

Edited October 6 by Dave Anderson
duplicated image insertion

[Redbeard]

Whatever type you use ensure it has good dust extraction/dampening and, because IME it is never perfect, that you have good respiratory protection as well.

[saveasteading]

I think PIR will crush over any lumps, and I would load it to make sure. ie a plywood walkway .

but a skim of sand will help too in filling hollows. it wont need to be dead smooth.

 

I vaguely remember treating a B and B floor by pouring an ultra thin slurry over it, using a watering can.

[Dave Anderson]

Thanks everyone, a variety of ideas all with much merit I think. I guess there is also the option of adding some self levelling, but with an area of 160m2 it's a bit pricey (Toolstation £500 at average 3mm thickness) so prefer to avoid if possible.

I think I'll aim to persevere with chopping out the globs of grout as best I can with the Mutt and a chisel, then try "screeding" some soft sand into the low bits. Then let the PIR bend/crush to the shape under the weight of the wet screed. Unless anyone has any more suggestions?  Still to decide whether to go with two 60mm layers of PIR.

[nod]

Your over thinking it There’s so much weight going on the top of the insulation it will all flatten down once the screed goes down Insulation always rocks till you get some weight on it Spend time making sure the membrane is properly taped Don’t make work for yourself Your builder will tell you the same 

 

[Dave Anderson]

That's reassuring, thanks Nod

[saveasteading]

3 hours ago, nod said:

Insulation always rocks 

Good point. 2 layers with overlaps will always have gaps and movement.

For a thick concrete slab the pir will certainly crush into shape.

For a thin screed I would definitely still do a preload by walking on it in very big flat shoes or moving a board around.

 

70mm of screed will weigh about 15kN/m2. That should do the local bending and crushing.

[saveasteading]

53 minutes ago, saveasteading said:

That should do the local bending and crushing.

Plus  the man with the hose walking over it, presumably very carefully.

[Dave Anderson]

Were actually using traditional hand trowelled screed at 60mm. But anyway I accept that 150 kg/M2 screed should sort it.  So I might revert to a single layer of PIR as cheaper and quicker.

[saveasteading]

23 minutes ago, Dave Anderson said:

traditional hand trowelled screed

so you have a person on their knees moving across the pir on a kneeler board. Also wheelbarrows coming in over planks or boards. well crushed over blips of concrete I'd say.

[Dave Anderson]

Good point, thanks savea