Insulated slab, do I need DPM under internal insulation before screeding?

[Thorfun]

morning all. I'm pretty sure I know the answer to this one but I'm just after confirmation before I do something stupid. our ground floor rooms not above our basement have this makeup below ground.

 

so the entire insulated slab is wrapped in a layer of DPM. Do I still need to put a DPM below the internal insulation that I'm putting down before the liquid screed. (obviously, I know about the layer of DPM above the insulation before the screed). from my search I've seen other posts where a layer of DPM is done under the insulation but that's because it's direct on the ground so not sure that is relevant in my situation.

[nod]

Yes dmp below and a fly sheet on the top

[saveasteading]

Excuse me going way back on this. Building on bedrock. Why do you need the cordek, which is for the severe expansion of clay?

[Thorfun]

3 minutes ago, saveasteading said:

Excuse me going way back on this. Building on bedrock. Why do you need the cordek, which is for the severe expansion of clay?

coz the SE said so!? 😉

[Thorfun]

1 hour ago, nod said:

Yes dmp below and a fly sheet on the top

but even if my structural reinforced insulated slab is already wrapped in DPM below ground? seems superfluous to then DPM on top of that already wrapped in DPM slab. 😕

[saveasteading]

1 hour ago, Thorfun said:

the SE said so!? 😉

Ok. I was wondering if this was a design borrowed from somewhere else or standard design by a kit building company.

SE presumably knows you site.

Still, it all looks rather over cautious. I don't know the circumstances of course.

You have rock at a "variable" depth which the footings sit straight on....great.

Then an insitu reinforced suspended slab. Presumably the ground is really poor quality? But if it is stronger than eps then it should be usable.

 

Up to you of course but it is trebling your floor cost, and making it specialist.  Perhaps for good reason. Ask again why?

[Thorfun]

32 minutes ago, saveasteading said:

Ok. I was wondering if this was a design borrowed from somewhere else or standard design by a kit building company.

SE presumably knows you site.

Still, it all looks rather over cautious. I don't know the circumstances of course.

You have rock at a "variable" depth which the footings sit straight on....great.

Then an insitu reinforced suspended slab. Presumably the ground is really poor quality? But if it is stronger than eps then it should be usable.

 

Up to you of course but it is trebling your floor cost, and making it specialist.  Perhaps for good reason. Ask again why?

well, it's all a bit moot now anyway as the superstructure is up and all the foundations were finished and paid for about 18 months ago! but we have clay here and are surrounded by woodland and we have a basement which these above ground 'arms' of our building are against. so it's quite complicated but i asked the SE for an insulated slab under the basement and 'arms' and that's what they gave me. if it's overly cautious then i'm ok with that. better safe than sorry.

[nod]

1 hour ago, Thorfun said:

but even if my structural reinforced insulated slab is already wrapped in DPM below ground? seems superfluous to then DPM on top of that already wrapped in DPM slab. 😕

I don’t make the rules

 

If you put any studs on top of the screed BC will still want a strip of dpc under them 

[saveasteading]

Sorry for bringing it up too late!   The chances are that the site had issues i don't know about.

[saveasteading]

Dpc is very cheap and the only concern is wasting plastic.

I see it is also a radon barrier.

 

I wasn't aware of radon in Sussex. 

But i see i should have been... eastbourne all the way west. 

Edited April 24, 2023 by saveasteading

[Thorfun]

9 minutes ago, nod said:

I don’t make the rules

 

If you put any studs on top of the screed BC will still want a strip of dpc under them 

I think we're getting our wires crossed here @nod! our external walls have a strip of DPC under the sole plate. I apologise that the original image doesn't show that detail but here's one that does.

 

 

so this is already built as is shown. we have a DPM under the insulation and structural slab with DPC strip below the sole plate and TF attached to sole plate. so all good so far.

 

above the slab I am having a screed to cover the UFH pipes. below this liquid screed I am adding 50mm EPS100 to build up the depth (which reduces the thickness of the screed) and also to fix the UFH pipes to. do I still need a DPM under that insulation? it is internal and sitting on a structural slab that is wrapped in DPM externally.

 

hope that makes sense!

[Thorfun]

9 minutes ago, saveasteading said:

Dpc is very cheap and the only concern is wasting plastic.

I see it is also a radon barrier.

I wasn't aware of radon in Sussex. 

no. we didn't need Radon barrier. GI didn't show any so we just used basic DPM.

 

13 minutes ago, saveasteading said:

Sorry for bringing it up too late!   The chances are that the site had issues i don't know about.

np at all. 🙂 

[Iceverge]

Seems a strange method. 

 

Did you get the levels for the floor wrong first day? 

 

Have you done any calcs as to the benefit of the extra 50mm EPS Vs just a thicker screed and more heat capacity in the slab? 

 

In any case I wouldn't put another layer of DPM under the insulation.  You'll end up with 4 X separate layers of impermeable plastic in the floor buildup. I can't see any justification for this. 

 

[Thorfun]

Just now, Iceverge said:

Seems a strange method. 

 

Did you get the levels for the floor wrong first day? 

 

Have you done any calcs as to the benefit of the extra 50mm EPS Vs just a thicker screed and more heat capacity in the slab? 

 

In any case I wouldn't put another layer of DPM under the insulation.  You'll end up with 4 X separate layers of impermeable plastic in the floor buildup. I can't see any justification for this. 

 

floor levels are correct from plans in this specific section. it was always the plan to have a layer of insulation under the screed and on top of the slab. not done any calcs on benefit for heat capacity but 50mm EPS is a lot cheaper than an extra 50mm of liquid screed. 😉

 

above the basement, though, we were originally going to have a hollowcore floor but at the last minute changed it to block and beam. this meant that the floor level was 50mm lower that originally planned so we have 150mm to fill to get to FFL and so i'm adding 100mm EPS above the B&B above the basement to get a 50mm liquid screed level.

 

plus there are extra savings as i can use UFH pipe staples rather than click tracks fixed to the slab.

[saveasteading]

23 minutes ago, Thorfun said:

GI didn't show any so we just used basic DPM.

See map here. If you are on the yellow then i would upgrade to radon quality. 

If it is marginal then there is a more precise map.

 

[saveasteading]

11 minutes ago, Iceverge said:

can't see any justification for this. 

I'm just thinking that there seem to be lots of changes since this drawing. Keeps the bco happy for £30 of unnecessary plastic. 

Slip layers are often helpful too.

[Thorfun]

18 minutes ago, saveasteading said:

See map here. If you are on the yellow then i would upgrade to radon quality. 

If it is marginal then there is a more precise map.

 

no yellow where we are. 🙂

[Thorfun]

15 minutes ago, saveasteading said:

I'm just thinking that there seem to be lots of changes since this drawing. Keeps the bco happy for £30 of unnecessary plastic. 

Slip layers are often helpful too.

no changes, it's just that the SE drawing didn't have the insulation under the screed and is missing the sole plate but the architect's BR drawings that went to BCO did and BCO didn't say anything.

[saveasteading]

36 minutes ago, Thorfun said:

No yellow where we are. 🙂

Good. Shouldn't have been on the drawing though.

 

Are you getting the drawings revised to 'as built'.

 

While on the subject , if you ever needed proof of no radon requirement, it comes up on that map as a postcode search , and a formal  certificate costs £5.

[Thorfun]

18 minutes ago, saveasteading said:

Good. Shouldn't have been on the drawing though.

 

Are you getting the drawings revised to 'as built'.

 

While on the subject , if you ever needed proof of no radon requirement, it comes up on that map as a postcode search , and a formal  certificate costs £5.

thanks. not planning up updating the drawings tbh (costs more money!) but if BCO insist then i will. our ground investigation also shows no radon requirement but good to know i can get a formal certificate if required.

[Temp]

4 hours ago, Thorfun said:

but even if my structural reinforced insulated slab is already wrapped in DPM below ground? seems superfluous to then DPM on top of that already wrapped in DPM slab. 😕

 

I believe the one on top helps stop water loss from the screed making it weaker. Something like that.

[Temp]

Deleted.

 

Edited April 24, 2023 by Temp

[Thorfun]

8 minutes ago, Temp said:

 

I believe the one on top helps stop water loss from the screed making it weaker. Something like that.

oh, right. so water from the screed going through the insulation downwards which would hit the slab underneath? nothing to do with damp rising up from below?

 

but would've thought the concrete slab would be pretty impervious to water penetration, no?

Edited April 24, 2023 by Thorfun

[Temp]

45 minutes ago, Thorfun said:

oh, right. so water from the screed going through the insulation downwards which would hit the slab underneath? nothing to do with damp rising up from below?

 

but would've thought the concrete slab would be pretty impervious to water penetration, no?

 

No I mean water loss from the screed makes the screed weaker not the slab below the insulation. 

 

Water also reduces the insulating properties of the insulation.

 

Our builder even covered the screed with a temporary plastic sheet as soon as it could be walked on to stop water loss into the room by evaporation.

Edited April 24, 2023 by Temp

[Thorfun]

27 minutes ago, Temp said:

 

No I mean water loss from the screed makes the screed weaker not the slab below the insulation. 

 

Water also reduces the insulating properties of the insulation.

 

Our builder even covered the screed with a temporary plastic sheet as soon as it could be walked on to stop water loss into the room by evaporation.

ok. but with a DPM on top of the insulation the screed will be poured on to that and so water loss in to the insulation will not happen? understand about evaporation to the room though.