Passive foundations on clay?

[zzPaulzz]

I’m keen to have a passive foundation with in-slab UFH pipes. My ground conditions are boulder clay over sand & gravel, and the site has lots of trees. Does anyone know if these conditions are likely to make a passive foundation anymore expensive than other methods?  Thanks.

[MikeSharp01]

Passive slabs are more expensive than strip foundations but your soil conditions are the same as ours, although we have no large trees close, so no reason not to. You need to get a proper structural design based around a few cores taken by a soil mechanic as we did. The rest was just hard work - but worth it.

[Alan Ambrose]

Hi Mike, possible to say a bit more about you slab? Did you, for instance have to pile down through the clay to get some load bearing on the gravel?

[G and J]

[WARNING - DUMB QUESTION ALERT!]

 

As I understand it, a passive slab foundation is basically a big lump of concrete cast inside a polystyrene tray.  I think such can have a good u value as the polystyrene can be nice and thick, and thick edges to the tray help reduce cold bridging at the bottom corners.  I suspect they are also effective for a passive house due to the simple large size of said slab.

 

We haven’t got room for the tray and we probably haven’t got the budget, but I can’t see why I can’t get close to the key attributes with a strip foundation, beam and block floor, and enough insulation in the right places.

 

Have I missed something fundamental?

[Dave Jones]

we have a passive equivalent slab using block and beam. Marmox perimeter to eliminate any thermal bridging, 150mm PIR or if you have depth 300mmm XPS for same price.

 

Edited August 26 by Dave Jones

[IanR]

3 hours ago, zzPaulzz said:

I’m keen to have a passive foundation with in-slab UFH pipes. My ground conditions are boulder clay over sand & gravel, and the site has lots of trees. 

 

It will come down to the bearing capacity of your ground at the dig depth, ie. 600mm - 700mm below finished floor level (likely 450mm - 550mm below ground level). There's no reason why your ground wouldn't achieve the required bearing capacity of, typically, 70 - 100 kPa/m². There are also pile supported insulated rafts for ground that doesn't achieve those values, but cost goes up just as it would for a traditional foundation.

 

3 hours ago, zzPaulzz said:

Does anyone know if these conditions are likely to make a passive foundation anymore expensive than other methods? 

 

If you price up what goes into an insulated raft foundation v. a tradition foundation, and you are comparing like-for-like, ie. a 0.1 U Value with a potential 0.04 W/mK Psi value and include the insulation, UFH and Screed that a traditional foundation requires in order to be equivalent, then materials, labour and Engineering costs of an insulated raft are lower than a traditional foundation.

 

However, an insulated raft foundation remains a niche product in the UK and the lack of experienced ground workers that will install them and the lack of Structural Engineers that will Engineer one correctly (optimised) pushes the self-builder to a packaged insulated raft where margins may be higher and travelling ground-works teams incur higher costs etc.

 

It's a product that needs a lot of researching to get to a good value. If you are willing to take the Installation on yourself, with a couple of ground workers then it can be very good value.

I'm happy to recommend Advanced Foundation Technology Ltd. My experience with them was excellent. Their advantage is that the business owner Olof, has been designing and installing insulated rafts in Sweden for +30 years, where they are not a niche product as 70% of houses use them.

 

https://www.advancedfoundationtechnologylimited.co.uk/our-products/ 

Edited August 26 by IanR

[Dave Jones]

1 minute ago, IanR said:

 

It will come down to the bearing capacity of your ground at the dig depth, ie. 600mm - 700mm below finished floor level. There's no reason why your ground couldn't achieve the required bearing capacity of, typically, 70 - 100 kPa/m². There are also pile supported insulated rafts for ground that doesn't achieve those values, but cost goes up just as it would for a traditional foundation.

 

 

If you price up what goes into an insulated raft foundation v. a tradition foundation, and you are comparing like-for-like, ie. a 0.1 U Value with a potential 0.04 W/mK Psi value and include the insulation, UFH and Screed that a traditional foundation requires in order to be equivalent, then materials, labour and Engineering costs of an insulated raft are lower than a traditional foundation.

 

However, an insulated raft foundation remains a niche product in the UK and the lack of experienced ground workers that will install them and the lack of Structural Engineers that will Engineer one correctly (optimised) pushes the self-builder to a packaged insulated raft where margins may be higher and travelling ground-works teams incur higher costs etc.

 

It's a product that needs a lot of researching to get to a good value. If you are willing to take the Installation on yourself, with a couple of ground workers then it can be very good value.

I'm happy to recommend Advanced Foundation Technology Ltd. My experience with them was excellent. Their advantage is that the business owner Olof, has been designing and installing insulated rafts in Sweden for +30 years, where they are not a niche product as 70% of houses use them.

 

https://www.advancedfoundationtechnologylimited.co.uk/our-products/ 

 

 

do they provide an indemnity policy for their design and product  for the main build warranty provider ?

[IanR]

4 minutes ago, Dave Jones said:

do they provide an indemnity policy for their design and product  for the main build warranty provider ?

 

I can only speak for AFT, who provide a fully insured foundation (by Lloyds of London) and have all the European Technical certificates required to satisfy NHBC etc.

Edited August 26 by IanR

[zzPaulzz]

1 hour ago, Dave Jones said:

we have a passive equivalent slab using block and beam. Marmox perimeter to eliminate any thermal bridging, 150mm PIR or if you have depth 300mmm XPS for same price.

 

Nice! Who did the design for you? 

 

Edited August 26 by zzPaulzz

[zzPaulzz]

I dug a deeper hole. Sharing for your enjoyment if you like that kind of thing. 

[Dave Jones]

2 hours ago, zzPaulzz said:

Nice! Who did the design for you? 

 

 

me. its normal build, nothing special.

[Gus Potter]

18 hours ago, zzPaulzz said:

I’m keen to have a passive foundation with in-slab UFH pipes. My ground conditions are boulder clay over sand & gravel, and the site has lots of trees. Does anyone know if these conditions are likely to make a passive foundation anymore expensive than other methods?  Thanks.

At the end of the day my gut feeling if you have trees is to go for a strip found if you can. You compensate by using more insulation elsewhere. The trees are a big thing. Can't say much more with the limited info you provide.

 

14 hours ago, IanR said:

It will come down to the bearing capacity of your ground at the dig depth, ie. 600mm - 700mm below finished floor level (likely 450mm - 550mm below ground level). There's no reason why your ground wouldn't achieve the required bearing capacity of, typically, 70 - 100 kPa/m².

The raft designs I do are nearly always goverened by the compressibilty of the insulation.. and I've been doing that for decades like Olaf.

 

Tanners (TSD Ireland) also know their stuff, working with them just now. Look folks this is nothing new. Like Olaf and the Canadians putting insulation under concrete is not that hard. In Sweden and Canada the frost heave is a big thing.. in the UK we assume the frost only goes 450mm down tops.

 

For insulated rafts we need to know how much each layer compresses by. If you take an insulation that states it has  a 150-300 (tops) kPa compression strength.. that on the face of it looks like much more than a soil with a kPa of 75 -100.. but the fine print says that it will achieve 300 kPa at 10% compression. Now say the insulation is 300mm thick. You load that up and it needs to compress by 30mm to achieve its design strength.  Now 30mm compression will play havoc with the concrete slab.! It won't work!

 

When desgning these things I look at the soil first to get a handle on that, often I can put that to bed and just look at the insulation, it's U value and compressive strength at 10% compression.. Then the loads and flexibility of the slab.. see how much it can bend by without cracking and not needing daft amounts of rebar. I do a bit of juggling to balance the loads and the job is nearly done in terms of checking the structural strength. Unless there are point loads!

 

Now the nightmare starts. You look at buildability, how much you can pour in a day, where you need joints for shrinkage etc. These are actually the hard parts! This idea that folk have.. you put insulation under a slab (to make it a passive slab) and now you need to be a specialist is a bit off the mark.. the insulation is just another layer above the soil with a different elasticity.

 

Semi flexible passive rafts (a nuance but designed slightly diferently) tend to work down to soils with a bearing capacity of 40 kPa if the ground water is well down.

 

18 hours ago, zzPaulzz said:

My ground conditions are boulder clay over sand & gravel, and the site has lots of trees.

 

The insulated raft is not often technically a challenge.  The trees are important as these are the things that can add value, ammenity and don't forget.. things like shading! it looks like you are going for the PH concept.. so you have maybe lots of glass.. try if you can to look after the trees and not consider them as an obstical and take advantage of them. Can they provide shading when considering over heating in the summer.

 

As an SE/ deisgner I look at the site.. what makes it attractive.. say the trees and then try an find a solution that preserves the character, ammenity and then see if we can say use the shading effect and greening/ colour contrast to enhance the surroundings.

 

 

 

Edited August 27 by Gus Potter

[zzPaulzz]

Thank you Gus for your insights and reassurance. I’m also talking to TSD, along with a few local SEs. I agree with you about the trees though sadly I’ve had to take out a lot to make way for the house.

[MikeSharp01]

20 hours ago, Alan Ambrose said:

Hi Mike, possible to say a bit more about you slab? Did you, for instance have to pile down through the clay to get some load bearing on the gravel?

No piling, just three cores to check the state of the clay below the slab. We scraped off the surface  buried the soil pipes, put in the peripheral drain and then back filled with type 1 - rolling and levelling in layers, then covered in pea shingle before putting down the insulation modules, put in the dpm, made and installed the reinforcing , attached the UFH, poured the concrete. 

[FarmerN]

We had a meter of clay over wet sand with trees close by. Was more or less forced into piles down 4.5 M by structural engeneer , who was worried about clay shrinkage in summer due to tress.

Ended up with Beam and block foundations with 180 mm insulation under 90mm screed. Perinsula under perimiter walls at insulation level.

My dream when we started out was a passive raft, possibly on Foam glass aggregate ( Geocell ) as the site needed building up a bit.

[IanR]

6 hours ago, Gus Potter said:

For insulated rafts we need to know how much each layer compresses by. If you take an insulation that states it has  a 150-300 (tops) kPa compression strength.. that on the face of it looks like much more than a soil with a kPa of 75 -100.. but the fine print says that it will achieve 300 kPa at 10% compression. Now say the insulation is 300mm thick. You load that up and it needs to compress by 30mm to achieve its design strength.  Now 30mm compression will play havoc with the concrete slab.! It won't work!

 

While the EPS300 typically used under the integral ring-beams of an insulated raft is rated at 300 kPa @ 10% compression, it's not going to see those loads in a typical foundation for a timberframe structure. For mine, worst case is less than a 10th of those loads, on the top layer of EPS, that obviously spreads across a greater area as it goes through the the EPS layers. We're talking 1 - 2mm compression.

[zzPaulzz]

@FarmerN I’m struggling with the dream-to- reality transition too. Did you lose any thermal performance or have to accept cold bridges with that design? I have block and beam in my current house and can feel air movement from under the house into the walls: definitely want to avoid that! 

Edited August 27 by zzPaulzz

[FarmerN]

I suppose I was just not committed enough to the Passive house.

Also wanted to use local people and was just getting no support from two different SE or architect.

The theoretical u values of the floor are still good, but I have no faith in the whole beam and block construction, cold air beneath the floor instead of solid earth . Yes also worried about thermal bridging

It’s all built now so got to live with it. A thermal imaging camera in winter will be interesting.

We have GSHP, MVHR , PV and battery

Yet to live through a winter, so February will be interesting.

[TommoUK]

..beam and block also allows for heave. Clay can swell (as well as shrink) if trees have been felled nearby.  The void under the B&B will help accommodate this. 

[saveasteading]

1 hour ago, FarmerN said:

cold air beneath the floor instead of solid earth

Ground beneath an uninsulated slab has a surprisingly good U value in a very big building, and the insulation manufacturers have stopped mentioning it.

However there are diminishing returns if you have good insulation above it, as not much energy reaches the underside.

For  a typical house it is about 0.8 to 1.0, which isn't great.

Therefore a suspended slab should be selected for other reasons*, and then add 10mm to the insulation if you want.

 

* on clay, on hillsides, where access is tricky for vehicles,  for low skilled diy, to rise higher without lots of fill., to get a floor without weather delay risk.

[zzPaulzz]

12 minutes ago, saveasteading said:

Ground beneath an uninsulated slab has a surprisingly good U value in a very big building, and the insulation manufacturers have stopped mentioning it.

However there are diminishing returns if you have good insulation above it, as not much energy reaches the underside.

For  a typical house it is about 0.8 to 1.0, which isn't great.

Therefore a suspended slab should be selected for other reasons*, and then add 10mm to the insulation if you want.

 

* on clay, on hillsides, where access is tricky for vehicles,  for low skilled diy, to rise higher without lots of fill., to get a floor without weather delay risk.

Would you say an air tightness membrane within the suspended slab with lapping to the walls is essential as part of the whole fabric design? 

[zzPaulzz]

2 hours ago, FarmerN said:

I suppose I was just not committed enough to the Passive house.

Also wanted to use local people and was just getting no support from two different SE or architect.

The theoretical u values of the floor are still good, but I have no faith in the whole beam and block construction, cold air beneath the floor instead of solid earth . Yes also worried about thermal bridging

It’s all built now so got to live with it. A thermal imaging camera in winter will be interesting.

We have GSHP, MVHR , PV and battery

Yet to live through a winter, so February will be interesting.

Yep, theory only get us so far.  You will hopefully find all is well in reality. 

[saveasteading]

1 hour ago, zzPaulzz said:

air tightness membrane within the suspended slab 

You will have a dpm in over the insulation, to stop the wet screed from washing through between boards (it floats!). That also provides your air tightness and yes, lap it to the walls.

[FarmerN]

Yes

20 hours ago, zzPaulzz said:

Would you say an air tightness membrane within the suspended slab with lapping to the walls is essential as part of the whole fabric design? 

Yes, a pretty good air tight membrane laid on the blocks, lapped up wall taped to a wide draping DPM, one or two doorways left a bit to be desired on a perfect seal, but no sign of air leakage at airtightness testing. Also a second membrane above the insulation beneath the dry screed used. This however was well punctured by the staples used to hold the UFH pipes in place.

[Gus Potter]

On 27/08/2024 at 07:58, FarmerN said:

My dream when we started out was a passive raft, possibly on Foam glass aggregate ( Geocell ) as the site needed building up a bit.

I've used this in one of my designs..The Client was keen on it and wanted to make it work ..  good results so far!