Capillary break beneath slab

[Clennls]

Hello everybody,

 

We want to place EPS insulation under our slab and we have very high ground water. We currently think about doing the following (from top = inside - to bottom = underside):

- Concrete slab
- PE foil 0,10 mm
- 20 cm EPS insulation
- 2 x PE foil 0,20 mm each
- Coarse sand 5 cm
- Geotextile
- Gravel 25 cm
- Geotextile

Normally the ground water is 20 cm under the gravel. Is this a correct order to use to make a proper Capillary break? We hear that we don't need to use a geotextile between the gravel and the sand, but then i think the sand will get in the gravel and thus the gravel will no longer work as a capillary break.

[JohnMo]

What does your structural engineer say?

9 minutes ago, Clennls said:

PE foil

Use proper DPM.

 

200mm EPS is pretty poor insulation, would increase to 300mm.

[grantmac]

Your setup sounds about right, but I'd consider JohnMo's point about the DPM (damp-proof membrane) and maybe using thicker insulation if water is an issue. Adding a geotextile between the gravel and sand could help keep them separated and maintain that capillary break.

 

Have you checked with a structural engineer? They might have more specific advice given your ground conditions.

[Alan Ambrose]

I think you’re right to be concerned about the thermal impact of moisture on the eps. I’ll post up my research later today. In short, it seems better to use xps, but there will be a thermal/aging impact on both.

[ETC]

Check your SAP for your floor insulation requirements.

A 1200 gauge DPM is a minimum requirement.

Don't forget about a VCL and perimeter insulation.

Edited February 28 by ETC

[JohnMo]

20 hours ago, Clennls said:

Normally the ground water is 20 cm under the gravel.

If you are concerned about water etc, wasn't block and beam the design solution made for you? Not a fan of block and beam, but it moves all insulation away from possible water and may be the correct solution here.

[Clennls]

Thank you all for the replies.

 

I live in Belgium and we just use the term black PE foil and use the right thickness instead of DPM. But i am sure it is the same thing. For the seams we just use 50 cm overlay. Not sure if i need to add some tape for it. I read about what insulation is better under a plate and if it is above the groundwater, EPS seemed better because it can dry out. XPS eventually gets saturated by moisture from the damp and can't really dry out. However when it is under the groundwater level XPS was better as it does not absorb groundwater. Its also why i think i need a thinner DPM on top of the EPS. That sheet is only so the water from the concrete does not flow in the insulation. And i think if damp gets in the insulation, it will try to dry out on the side with the least ammount of DPM.

 

I will think about using 30 cm of EPS insulation. I was also going to use 30 cm on the roof. 

 

Maybe one last question, how do you do the seams of the geotextile? Is 50 cm overlay enough?

 

Currently al the architect that we ask just seem to really brush the issue away of the groundwater. Here it is standard to place the insulation on the slab. Though there are contractors who want to do this work, i really want to make sure it is done proper.