Russell griffiths

Insulated foundation ( passive slab )

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Evening, I know many on here have used an insulated slab, I tried not to call it a passive slab as even though it may be insulated the house you put on top does not necessarily have to be to a passive standard. 

 

So question,  how thick was the vertical element to the insulation, not how much you have under the slab, the bit that stands vertically and held the concrete in place, I don’t want to pay for two much insulation if it is not needed, looking at various companies and they all have different specs. 

 

Cheers. 

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Thanks @HerbJ very helpful, quick question. 

 

Your slab  is quite complex with lots of thickening beams throughout. 

Looking back could you see a benefit of having a thicker slab all over rather than all the associated work in forming the thickening beams. 

 

My last house had beams incorporated into the slab on top of piles but that was due to sloping ground, I’m thinking know all the effort in creating these beams you could just cast a thicker slab all over, more concrete yes but less labour. 

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Posted (edited)

I believe my slab is a fairly standard design for a MBC  foundation, which are usually designed by a very practical and experienced SE with a long track record.  I am sure that he woudl have designed the most effective technical solution, taking cost  into account.

 

My house is quite large ( I think the slab area is 214m2, including  large double garage) and I suspect having thicker slab would not have been cheaper and may have caused other technical  complexities.  A smaller slab may yield a different solution and discussion withna SE would be helpful for you.  From memory, the  slab was constructed by MBC in about 5 days.

Edited by HerbJ

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We have the same type of slab. The thickening in many places is due to point or line loads. Thickening the whole slab would add an awful lot of concrete.

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8 minutes ago, jack said:

We have the same type of slab. The thickening in many places is due to point or line loads. Thickening the whole slab would add an awful lot of concrete.

Yes it would add an awfull lot of concrete, but what is cheaper 1m square of concrete or 1m of eps. 

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30 minutes ago, HerbJ said:

I am sure that he woudl have designed the most effective technical solution, taking cost  into account.

 

Im afraid I disagree with this, he may be a super smashing engineer the absolute dogs danglies, but I don’t believe for a minute he designed it with your wallet in his mind, he would have designed it to meet all the relevant criteria and to completely cover his back,  but he would not have given a monkeys if you couldn’t afford it. 

 

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Having a look at this pic just for an example. 

Lets say for argument that all the insulation in the pic is 100mm thick, so we have 300mm under the main slab with 100mm of concrete over the top, so if we require thicker sections through the slab for point loads or supporting walls why not get rid of the top layer of eps and replace with additional 100mm of concrete, so in effect the picture would have 200mm of eps and 200mm of concrete. 

 

Tell me why this is silly/stupid

im hoping that I’ve preempted your answers in my head, but need some clarification. 

Cheers. 

79552D80-34B5-41CC-BE74-7E9A623877D6.jpeg

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14 minutes ago, Russell griffiths said:

 

Im afraid I disagree with this, he may be a super smashing engineer the absolute dogs danglies, but I don’t believe for a minute he designed it with your wallet in his mind, he would have designed it to meet all the relevant criteria and to completely cover his back,  but he would not have given a monkeys if you couldn’t afford it. 

 

 

FWIW, I have a spreadsheet somewhere comparing at least three insulated slab systems, from when we were looking at options.  One was Kore (the system than MBC normally use), one was Supergrund and another was IsoQuick.  I think there was also another company, too, but I can't recall their name.

 

Some used a lot more concrete than others, in essence they used a simpler design without local reinforcement for any internal or edge loads.  IIRC, one use 250mmm of reinforced concrete.

 

The key thing with all of  sethem was that when you add up all the costs (and they are not easy to compare, as they all quoted on a different basis) they all cost about the same.  some had more cost in labour and steel, some had more cost in concrete, but all were within about 10% of each other in total cost.

 

In the end our decision was based on the tie up between Kore and MBC, that meant the foundation would be installed by MBC, and for me that was the clincher, as it removed a massive amount of risk.  Had we used a different foundation company to the frame company I could see that we may well end up with an argument if the accuracy wasn't spot on, and it just seem a lot easier to use one supplier for both.

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I also had an MBC passive slab with thickening under line and point loads. The Structural engineer that MBC use is paid for by them and I am pretty sure MBC would be very interested in the engineer designing the cheapest solution for them to build. EPS is often used in civil engineering projects as land fill under structures rather than pouring masses of concrete. I assume this is for cost.

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Posted (edited)

@jackis correct. The foundation system, including the slab has to be designed to take account of the loads imposed by the house structure and the soil conditions. 

 

+1 @JSHarrisand@Alex C

Well @Russell griffiths, that's telling me. about my capabilities  and approach..... Actually, I am not sure that your strong assertion is correct and you have made wild assumptions about what my criteria were or how I executed my project.

 

The foundation slab was supplied, designed  and installed lump sum by MBC, who were bidding competitively against other suppliers.Just like @JSHarris I did a lot of competitive bid evalution and they were extremely competitive and I knew I had negotiated the best deal for me at that time (including exchange rates and other project risks).  In addition MBC took responsibility for the integration of the slab and the timberframe design and installation.

 

MBC supply and build dozens of these slab every year and, believe it or not, they operate a very successful busines and  have their microscopes on the costs and labour required complete their projects. If there is a better and cheaper way o execute installation of these slabs, then I believe MBC will have worked it out and  used it on the next project - as they have done with many other aspects of construction.

 

I didn't offer my input as the answer to your very specific requirements but only to give you some input to a specific question. For you, the challenge is to find YOUR most effective and economic execution plan taking your site and  requirements into consideration all the relevant factors, including budget.  Hopefully, you will share your experience for others to benefit from when you have completed your project.  There is nothing wrong with your thinking but it's  the result has to work for you and your project.

 

Edited by HerbJ
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++ to all that the other MBC customers' comments.  We are another one.  Our slab was different because it was also supporting a stone skin. Hilliard, the SE that MBC uses specialises in these and knows his stuff.  The SE calcs that he provided kept our BCO and warranty provider happy.  Another thing to note is that MBC use installation crews that spend their lives putting in this type of slab. In our case, I found that they really knew their stuff and were amazingly particular and professional in the details of construction.   I was extremely impresses with them and their work -- a league away from the usual building gangs.

 

MBC is just one case, but the main point to take home is that designing and implementing these slabs have a lot of subtleties that the specialist companies and crews have refined over the years.  Your house is going to be standing on these foundations, so you can't afford to get the design or the implementation wrong.  It's just not worth the risk.  My advice: use a specialist company with lot of installation experience, a decent track record with consistently good references from their customers..

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Posted (edited)
3 hours ago, Russell griffiths said:

Yes it would add an awfull lot of concrete, but what is cheaper 1m square of concrete or 1m of eps. 

 

EPS100 is quite a bit cheaper than concrete per unit volume.

 

3 hours ago, Russell griffiths said:

Lets say for argument that all the insulation in the pic is 100mm thick, so we have 300mm under the main slab with 100mm of concrete over the top, so if we require thicker sections through the slab for point loads or supporting walls why not get rid of the top layer of eps and replace with additional 100mm of concrete, so in effect the picture would have 200mm of eps and 200mm of concrete. 

 

Tell me why this is silly/stupid

 

It's not stupid, but if you're happy with the U Value that 200 EPS provides, and you have the bearing capacity in the subsoil that can take the higher pressures from the line and point loads, why put in the extra concrete if it's not required? The localised 200mm thick integrated beams are very easy to produce, you just leave a gap of the desired width in the top layer of EPS, so there really isn't any extra labour needed to produce them.

 

Since UFH is generally included within the insulated slab, there is a benefit to keeping it "thin" - to keep its heat-up time as low as possible.

 

Edited to add: I have an AFT engineered insulated slab, which is on similar principles  to the MBC version. 

Edited by IanR
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11 hours ago, Russell griffiths said:

Yes it would add an awfull lot of concrete, but what is cheaper 1m square of concrete or 1m of eps. 

🤔

2 sheets of of 100mm EPS can be carried by one man. That negates over half a m3 of concrete !

Why on earth would you actively remove insulation from under your heated slab? 200mm of eps isn't exactly poor, but not as good as 300mm. 

Also, the additional weight of flooding to the full ring beam depth may push your groundworks and S/E costs up so I think this option is a 👎 😉

 

Ask yourself one simple question...."Should I be buying more concrete unnecessarily, or fitting more insulation that will serve me better over my time in the property".  

Fwiw, I'm constantly arguing with customers 'builders' over slab construction, and specifications of, when I'm fitting UFH and screed after they have left. For some reason they prefer to pour ridiculous amounts of concrete into sunroom / conservatory / extension slabs instead of using more insulation. I can only imagine it's because that's how their great gran-pappy used to do it "during the war" and he didn't get any complaints. That and the fact that they won't be contributing to the additinal energy consumption that the poor floor type will cause :(

 

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Posted (edited)

We have an Isoquick insulated slab which has 300mm Peripor under the 200mm thick concrete slab and a peripor upstand which is 200mm thick.

Edited by PeterStarck
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Evening. FWIW @HerbJ I was stating that I disagreed with a statement you had written regarding the structural engineer, it was by no means a dig at your capabilities or approach. 

 

From my point of view if we didn’t question the way things are done and look for various approaches then we would never progress and would still be building in 9 inch solid brick. 

 

I question everything untill im satisfied of the correct approach. 

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Posted (edited)

So looking at the pic I found, am I missing something, in a lot of the replies it looks like the more insulation you can cram under a slab the better, I fully agree. 

In the pic is a ring beam that the wall sits on, this is sitting on 200mm of insulation, the rest of the slab is only 100mm thick so you can add an extra 100mm of insulation underneath so it sits on 300mm, if you then put 2-3 thicker beams throughout the slab to stiffen it up these will allso only have 200mm underneath them, so looking at the pic have we not created a slab with many major cold spots all around it and through out the middle. 

This will require the thoughts of blokes far cleverer than me. @JSHarris @IanR

E40145AC-AD0A-4EDC-BA8C-2F1E1BD085C3.jpeg

Edited by Russell griffiths

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9 hours ago, Nickfromwales said:

Also, the additional weight of flooding to the full ring beam depth may push your groundworks and S/E costs up so I think this option is a 👎 😉

 

 

Why? 

I believe it would be cheaper,

if you have to excavate to a required depth surely it is easier and less labour intensive to pour a thicker slab than to have spent the time pre forming thickening beams and adding the additional re enforcement. 

A pour and finished slab takes the same time to lay regardless of whether it is 100mm -150mm or 200mm

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HI @Russell griffiths

 

No problem - your post generated a lot of useful responses, which hopefully will give you  (and others) food for thought on the design, supply and construction of your Insulated Foundation(passive slab). As you will realise there are various alternatives and designs, all with differing propietary materials and variation of designs,  to follow-up and think about.  As @IanR points out, your ideas  and questions are not stupid and you have been given two examples of designs/builds which do not involve a "complicated" design and use more concrete as you proposed.

 

As with all construction projects, you have to bring your ideas to a specific project considering all the relevant aspects - location, ground conditions, ease of access, closeness to supply chain (concrete plant for instance), house build type (timber frame, conventional, etc), contracting strategy ( managing contractor, self manage with subcontractors, self build), budget, planning constraints & requirements.... At the end of the process, your project like most others will probably be a series of compromises resulting from all of the above and mores

 

Best of luck and keep posting. I am sure that you will get lots of support

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So @IanR said the cost of eps 100 is cheaper than concrete. 
 
Unless my maths is wrong (which it could possibly be)  I would also disagree with this. 
 
How much is everybody paying for 1cubic m of concrete ?
how much is a 100mm thick sheet of eps at 1m square. 
How far does a cube of concrete go when laid at 100mm thick? 
 
This is assuming my previous question about cold edges to the slab would dictate that 200mm of insulation all over was ok. 

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2 minutes ago, Russell griffiths said:
So @IanR said the cost of eps 100 is cheaper than concrete. 
 
Unless my maths is wrong (which it could possibly be)  I would also disagree with this. 
 
How much is everybody paying for 1cubic m of concrete ?
how much is a 100mm thick sheet of eps at 1m square. 
How far does a cube of concrete go when laid at 100mm thick? 
 
This is assuming my previous question about cold edges to the slab would dictate that 200mm of insulation all over was ok. 

 

I have a quote from Kore that puts the cost of 1 cu m of EPS100 at £58 delivered.

 

I paid £96 for 1 cu m of C35 concrete delivered.

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16 minutes ago, Russell griffiths said:

 

Why? 

I believe it would be cheaper,

if you have to excavate to a required depth surely it is easier and less labour intensive to pour a thicker slab than to have spent the time pre forming thickening beams and adding the additional re enforcement. 

A pour and finished slab takes the same time to lay regardless of whether it is 100mm -150mm or 200mm

Over say 100m2 what would be the additional weight of the extra 100mm of wet concrete poured? I would think that would be factored in to the specifics for the ground preparation at the design stage, possibly requiring additional sub base preparation etc, plus the extra cost filling the volume with concrete would be significant. I think your maths may be out but I've been wrong before :/

3 minutes ago, IanR said:

 

I have a quote from Kore that puts the cost of 1 cu m of EPS100 at £58 delivered.

 

I paid £96 for 1 cu m of C35 concrete delivered.

So basically twice the cost, and an additional 50+ days to dry out.  

Plus it's lower insulation value under a heated slab. 

🤔

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Posted (edited)
1 hour ago, Russell griffiths said:

So looking at the pic I found, am I missing something, in a lot of the replies it looks like the more insulation you can cram under a slab the better, I fully agree. 

In the pic is a ring beam that the wall sits on, this is sitting on 200mm of insulation, the rest of the slab is only 100mm thick so you can add an extra 100mm of insulation underneath so it sits on 300mm, if you then put 2-3 thicker beams throughout the slab to stiffen it up these will allso only have 200mm underneath them, so looking at the pic have we not created a slab with many major cold spots all around it and through out the middle. 

 

Even at the localised 200mm thick EPS below the beams the U value is still better than Building Regs, so I wouldn't characterise it as cold spots, and certainly not "majour".

 

But it does need to be taken into consideration when calculating the whole floor U Value, just as timber fraction does when considering the whole wall U Value.

 

Taking my floor as an example, I have a 475m2 slab, of which 79m2 "only" has 200mm EPS underneath, due to integral beams. But 396m2 has 300mm EPS underneath. So 83% at 300mm thickness and 17% at 200mm. The overall effect on U Value is quite small.

 

Your suggestion is to remove the top layer of EPS and have the 100% of the floor with 200mm EPS. Still an OK U Value, but not as good as good as keeping the reductions localised.

 

59 minutes ago, Russell griffiths said:

if you have to excavate to a required depth surely it is easier and less labour intensive to pour a thicker slab than to have spent the time pre forming thickening beams and adding the additional re enforcement. 

 

It's really not a lot of work to set out the EPS formers. I believe AFT allow 3 days for 3 people to get from compacted sub-base to concrete pour for an average slab. There's really not a lot of work to do the whole floor with beams, so very little to save by not doing the beams. The additional reinforcement would still be required to strengthen the slab where the load bearing walls will be sitting.

Edited by IanR
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34 minutes ago, IanR said:

 

I have a quote from Kore that puts the cost of 1 cu m of EPS100 at £58 delivered.

 

I paid £96 for 1 cu m of C35 concrete delivered.

 So I think my mind is being distorted by costs. The prices I have seen for eps look to be higher than the ones @IanR is quoting, and the cost of concrete is cheaper. 

 

I was typing this as Ian did his last reply. 

What an excellent reply and just what I was looking for. Top man Ian. 

So in essence having 20% of a slab with a slightly lower u value than the rest of it will be better than 100% of the slab at the lower value. 

I was having trouble thinking a colder area all around the edges would be sucking all the goodness out from the middle so why bother trying to improve the middle if the edges just rob it back. 

 

Excellent as always thanks everyone. 👍👍

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