Coldbridge between piles and floor slab.

[Russell griffiths]

So the picture is from a website for insulated floor slabs

now to me it looks completely wrong with the piles penetrating through the insulation and connected to the floor slab

reading the gumph on the website they explain that the cold area of the pile is the top part and the lower section is at a warmer temperature, so by insulating the top 600mm of pile will mitigate the cold bridge. 

 

Is this complete nonsense or does it make sense 

we are looking at 30 piles across the building so that is a huge amount of bridges if it didn’t work. 

Cheers russ

[Jeremy Harris]

I did a load of heat loss calculations for a pile supported passive slab for an Ebuild member, as he was concerned.  If the pile diameter to insulated length ratio is high (i.e. a long length of the pile is externally insulated) then the heat loss comes down to just 2D, in effect, and is surprisingly small.  I probably have the calcs I did for this a few years ago, and might be able to dig them out from my old PC, but I do remember having a long phone conversation with the chap whilst he was driving back from Italy and being surprised at how small the additional heat loss through the piles was.

 

If you have all the dimensions and number of piles I could do the same for this arrangement if you like.

[PeterW]

As @JSHarris says it’s a bit of an odd one but they don’t have the thermal bridge you expect and if the top 6-800mm is insulated then where an ordinary slab would have a cold bridge to 0c or less at the edge, the piles are at 8c or ground temperature. 

 

30 piles with a 30cm square section have an edge loss area of the equivalent of a square building of 81sqm. As the piles are at a warmer temperature the losses are substantially less than this. 

 

 

 

 

 

[Jeremy Harris]

I suppose the simple way to try and get your head around it is to think of the pile as being insulated all around, so heat can only effectively travel downwards through the insulated part, then work out the U value for that section of pile and see what you get.

 

For example, say you have a 150mm diameter pile, that is peripherally insulated for the first 1000mm down.  The area of the pile is 0.0177m².  Concrete has a λ of around 1.2 W/m.K, so a 1m depth would have an R value of about 1.2 m².K/W, which gives a U value of about 0.833 W/m².K.  Sounds grim, doesn't it?

 

However, the pile cross sectional area is only 0.0177m², so with the floor at, say, 22 deg C (assuming low temperature UFH) and the ground at about 8 deg C (typical UK soil temperature at depth) then the actual heat loss per pile comes out at 0.0177 x (22 - 8 = 14) x 0.833 = 0.2058 W per pile.  Not a lot, is it?

[Russell griffiths]

Thanks for your offer @JSHarris but at the moment I’m just chucking some ideas about and wouldn’t want to waste your time, 

to maybe alter things what would the difference be between 

a concrete pile with reo inserted, or

a steel tubular pile with a concrete fill and reo. 

[Declan52]

Could you not cast a 150mm reinforced slab on top of the ring beam you will have when you tie all the piles together and concrete these up. On top of this put your 200/300mm of insulation and then finish with 75mm of screed. 

 

[Jeremy Harris]

33 minutes ago, Russell griffiths said:

Thanks for your offer @JSHarris but at the moment I’m just chucking some ideas about and wouldn’t want to waste your time, 

to maybe alter things what would the difference be between 

a concrete pile with reo inserted, or

a steel tubular pile with a concrete fill and reo. 

 

 

Steel is a far better heat conductor than concrete, so best avoided.  Better than concrete would be the type of piles that @recoveringacademic used, a vibro compacted column of stone, as that would have a lower λ than concrete, I'm sure, because of the air spaces between the stone.  The only difficulty I can see is how to sheath the top metre or so with insulation, but that's probably not an insurmountable problem.

 

Frankly, for the sake of an additional floor heat loss that's off the order of 6 to 10 Watts in total for 30 piles  'm not sure it's worth worrying about too much, though.

[Russell griffiths]

Thanks @JSHarris

i have been chatting with @recoveringacademic Regarding this just trying to get a lot of quotes in to try and evaluate between traditional pile and beam, and fully insulated slab with some sort of ground improvement underneath.