Below Basement RC Insulation

[Internet Know How]

My Architect has specified 150mm insulation beneath the entire basement slab, as well as 150mm on all external RC walls to the basement. Is it typical to insulate under a basement slab or is this a complete waste of time?

 

I understand having to insulate the walls, but the slab seems a little overkill.

 

There are questions over whether the design will meet the SAP calculations if the under slab insulation is not included.

 

Slab is 240sqm, so quite big, and with a pool in the basement we are going to insulate with 150mm under the slab where the pool is location which will take up some 80 SQM

 

Your advice would be appreciated, thanks

[Thorfun]

we have built a basement and put 200mm EPS 300 underneath it and others, like @Bitpipe put 300mm under the basement slab. just do it and I agree that you might have issues with SAP and building regs if you don't put any insulation underneath.

 

I note that you don't mention what type of insulation the architect has specified but it's my understanding that PIR won't have the compressive strength which is why our structural engineer specified EPS300 for under the slab. if it's EPS then I would up it to 200mm minimum in my opinion. 

[PeterW]

Yes you always insulate under the slab otherwise you’re basically heating the earth. It won’t meet building regs without. 

[Conor]

What's the rest of the floor build up? I'm assuming there will just be screed on top? Where is the tanking membrane in this design? Do you have drainage under the slab? If the insualtion is perpetually wet, it won't work so well! 

 

Your engineer is basically suggesting an insulated raft foundation, which is the best type of foundation in the world*

 

 

*I am not a structural engineer

[Internet Know How]

4 hours ago, PeterW said:

Yes you always insulate under the slab otherwise you’re basically heating the earth. It won’t meet building regs without. 

Hi Peter,

 

Our basement slab is also the pool base, but not the basement floor level. We have dig deeper to accommodate a pool in the basement, so at pool level we have insulation and underfloor heating across the entire basement floor. There is a 1 m void between the basement slab and the basement floor.

 

Thanks

[Internet Know How]

2 hours ago, Conor said:

What's the rest of the floor build up? I'm assuming there will just be screed on top? Where is the tanking membrane in this design? Do you have drainage under the slab? If the insualtion is perpetually wet, it won't work so well! 

 

Your engineer is basically suggesting an insulated raft foundation, which is the best type of foundation in the world*

 

 

*I am not a structural engineer

Hey Conor, Swimming pool in basement, and slab is the bottom of the pool. It means we have a 1m void, then we install a block and beam floor which will have insulation, UFH pipes and screed. We are in the water table and its not possible to drain away water because the basement slab sits below the water table level. My view is that the insulation will be good for the pool area only to keep some heat of the pool water in, but wont offer much else. All other floors in the house have UFH and insulation under the screed.

 

If I install 150mm or 200mm insulation across the entire underside of the slab, I then have to reduce my pool depth by this amount, which was planned at 1200mm (a standard depth). A 1m pool will work but isnt the best

[Thorfun]

doesn't digging down to just have a void increase the costs? why not dig down to the basement floor level and then dig the pool area out a bit further? seems counter intuitive to do it the way you suggest and more costly! not only the increased depth but then also paying for block and beam flooring and supporting walls etc.

 

seems crazy to me.

 

Also, you'll need some form of sump and pump to pump out water from around the basement.

[Internet Know How]

5 hours ago, Thorfun said:

we have built a basement and put 200mm EPS 300 underneath it and others, like @Bitpipe put 300mm under the basement slab. just do it and I agree that you might have issues with SAP and building regs if you don't put any insulation underneath.

 

I note that you don't mention what type of insulation the architect has specified but it's my understanding that PIR won't have the compressive strength which is why our structural engineer specified EPS300 for under the slab. if it's EPS then I would up it to 200mm minimum in my opinion. 

Architect specified 150mm Recticel Eurothane® GP PIR board (thermal conductivity 0.22 W/m.K) or equal approved. im not sure on compressive strength of this

[Thorfun]

2 minutes ago, Internet Know How said:

Architect specified 150mm Recticel Eurothane® GP PIR board (thermal conductivity 0.22 W/m.K) or equal approved. im not sure on compressive strength of this

me neither. you need a structural engineer to sign off on that

[Internet Know How]

1 minute ago, Thorfun said:

doesn't digging down to just have a void increase the costs? why not dig down to the basement floor level and then dig the pool area out a bit further? seems counter intuitive to do it the way you suggest and more costly! not only the increased depth but then also paying for block and beam flooring and supporting walls etc.

 

seems crazy to me.

 

Also, you'll need some form of sump and pump to pump out water from around the basement.

We did consider having a split level slab, so you are correct that whilst it has cost more to dig out, the engineering aspect putting back is a bit easier. the pool area takes up pretty much half of the basement footprint too. It's also made water management so much easier. As for the sump pump, not planning to have a pump running externally around the basement once we backfill, but we are having external tanking, sika in the concrete and maybe even an internal 3rd line of defence too...such as slurry to the walls or the internal drainage with sump pump

[Internet Know How]

5 minutes ago, Thorfun said:

me neither. you need a structural engineer to sign off on that

Yeah I will ask the SE to double check it, thanks!

[Thorfun]

15 minutes ago, Internet Know How said:

As for the sump pump, not planning to have a pump running externally around the basement once we backfill, but we are having external tanking, sika in the concrete and maybe even an internal 3rd line of defence too...such as slurry to the walls or the internal drainage with sump pump

I'd be more concerned about the water pressure lifting up the house than water ingress! but I'm not a structural engineer or an engineer of any kind but I do know that I have an external sump and pump with a land drain around the base of my basement and there is a constant run of water (and it's above the water table!!) where water runs beneath the surrounding rock 2m below ground level. I'd hate to know what would happen if I let all that water build up with the upward pressure on the basement and house above.

 

I'm sure more intelligent people will be along soon.

[Bitpipe]

11 hours ago, Thorfun said:

I'm sure more intelligent people will be along soon.

 

Well, not so sure about that...

 

11 hours ago, Thorfun said:

I'd be more concerned about the water pressure lifting up the house than water ingress! but I'm not a structural engineer or an engineer of any kind but I do know that I have an external sump and pump with a land drain around the base of my basement and there is a constant run of water (and it's above the water table!!) where water runs beneath the surrounding rock 2m below ground level. I'd hate to know what would happen if I let all that water build up with the upward pressure on the basement and house above.

 

I'm sure more intelligent people will be along soon.

 

This is a real risk - friends built a basement near us and found an underground Thames feeder stream running through their plot during survey. Had to de-water during build (expensive) and SE designed a very meaty slab to prevent the hydrostatic pressure pushing the basement out of the ground. As it was their rainwater harvesting tank popped out twice during the build.

 

11 hours ago, Internet Know How said:

We did consider having a split level slab, so you are correct that whilst it has cost more to dig out, the engineering aspect putting back is a bit easier. the pool area takes up pretty much half of the basement footprint too. It's also made water management so much easier. As for the sump pump, not planning to have a pump running externally around the basement once we backfill, but we are having external tanking, sika in the concrete and maybe even an internal 3rd line of defence too...such as slurry to the walls or the internal drainage with sump pump

 

I can see the logic here, we had a small area where our external basement stairs come to ground level and while it made sense for the excavation & retaining wall to follow the line of the stairs, was much simpler to just chop out and cast a square box, what you loose on the additional muck away and concrete you gain on speed and simplicity of construction.

 

12 hours ago, Internet Know How said:

Architect specified 150mm Recticel Eurothane® GP PIR board (thermal conductivity 0.22 W/m.K) or equal approved. im not sure on compressive strength of this

 

From their data sheet, https://www.recticelinsulation.com/sites/default/files/downloads/Eurothane GP Technical Datasheet.pdf the compressive strength of 150mm board is 140KPa for 10% deflection, so almost equivalent to EPS 150 grade. 

 

By comparison EPS 200 which our SE specced has a strength of 200KPa for 10% deflection. EPS 250 and 300 is often specced for sub slab applications also.

 

The calculation will depend on the weight of the whole structure above the insulation and point loads will factor also - one for your SE.

 

WRT insulation thickness, you need a thicker EPS sheet than PIR to get the same R Value - we used 300mm for under the slab and 200mm (of EPS 70 grade) for the outside basement walls - so the trade off is more excavation & muck away vs cost of PIR / cost of EPS. You can obviously stack up individual thinner sheets of insulation to get the thickness you need or buy as a single block. 

[Internet Know How]

4 hours ago, Bitpipe said:

 

Well, not so sure about that...

 

 

This is a real risk - friends built a basement near us and found an underground Thames feeder stream running through their plot during survey. Had to de-water during build (expensive) and SE designed a very meaty slab to prevent the hydrostatic pressure pushing the basement out of the ground. As it was their rainwater harvesting tank popped out twice during the build.

 

 

I can see the logic here, we had a small area where our external basement stairs come to ground level and while it made sense for the excavation & retaining wall to follow the line of the stairs, was much simpler to just chop out and cast a square box, what you loose on the additional muck away and concrete you gain on speed and simplicity of construction.

 

 

From their data sheet, https://www.recticelinsulation.com/sites/default/files/downloads/Eurothane GP Technical Datasheet.pdf the compressive strength of 150mm board is 140KPa for 10% deflection, so almost equivalent to EPS 150 grade. 

 

By comparison EPS 200 which our SE specced has a strength of 200KPa for 10% deflection. EPS 250 and 300 is often specced for sub slab applications also.

 

The calculation will depend on the weight of the whole structure above the insulation and point loads will factor also - one for your SE.

 

WRT insulation thickness, you need a thicker EPS sheet than PIR to get the same R Value - we used 300mm for under the slab and 200mm (of EPS 70 grade) for the outside basement walls - so the trade off is more excavation & muck away vs cost of PIR / cost of EPS. You can obviously stack up individual thinner sheets of insulation to get the thickness you need or buy as a single block. 

Thanks for the advice! I see what you mean by the thickness of insulation. The trade off for us is not digging any deeper to accommodate a 200-300mm insulation board. I have raised the use of the Eurothane sheet with my SE for input. We installed a water tank to capture all water run off, and we cast the tank in the ground with concrete so it will not move. Our slab is 400mm thick and our walls will be the same. We have some 25 tonnes of rebar going in as part of the basement build, which is mixed between 16 and 20mm bars.