I am sure i have read somewhere about an approach to insulation from heat losses to the ground where the perimeter of the building is insulated vertically, but the floor slab itself is a large deep heat sink type construction laid on top of a damp proof membrane, without insulation. I'm sure it was a thread on here but can't find it. I have a site with a lot of spare crushed concrete to use, and also a need to build the levels up significantly, so i wanted to read up on this. Cant find anything on it so am starting to think I imagined it.

Great, found the mammoth thermal store thread. Thanks

I have a suspicion you might have problems convincing a BCO it meets regs?

AIUI
Heat loss is directly proportional to the Delta and heat losses through the floor are insignificant apart from at the perimeter when the exterior walls are not insulated on their exterior.

To my mind that insulation would be better used elsewhere. if you can incorporate EWI you will get some thermal mass which will stand you in good stead.

<blockquote><cite>Posted By: bxman</cite>AIUI
Heat loss is directly proportional to the Delta and heat losses through the floor are insignificant apart from at the perimeter when the exterior walls are not insulated on their exterior.

To my mind that insulation would be better used elsewhere.</blockquote>
Yes, agreed. Crucial point is from DJH above - I have no idea what Building Control would think. Ground water levels are also going to be an issue, though there is a handy ditch running the length of the plot that can take groundwater away. Has anyone used Geocell in this sort of application?? - looks like it could provide drainage and insulation at the same time.

Relevance is manufacturers claim Geocell can be used for perimeter insulation as well. I guess a bonded EWI would work better, but I'm trying to minimise number of trades on this design.

converse quoth "Like this"

That looks a lot like a diagram of a passive slab.

There's a PDF about the system we used at
https://www.glavloc.com/downloads/KORE-Passive-Slab-Design-Guide.pdf
We had to dig a big hole and part-fill it with loads of hardcore (recycled from some industrial process), then the EPS 'bucket' and then the concrete slab. French drain around the outside. I got one firm of groundworks contractors to do everything.

You can obviously extend the insulation out further from the edge of the raft. EPS is what they use but geocell should work as well. It's not normal/necessary to bond the sheets of EPS together AFAIK. Maybe to allow drainage?

There's an article that discusses both systems at https://passivehouseplus.ie/magazine/guides/the-ph-guide-to-insulating-foundations

Posted By: converseI appreciate there might be arguments with the Building inspector and probably wouldn't pass building regs in UK as I read them

I think you're mostly right, but the GD episode had no insulation under the floor AIUI so it's possible to get around the regs (or at least the Approved Documents) if you know what you're doing.

They like most people used EPS for the insulation they do have rather then Geocell. I'm not sure why we're spending so much time discussing Geocell? Do you have some particular reason to want to use it?

Posted By: converseI also have reservations about its longevity. It's not as if it could be easily replaced if it's underneath a building

Indeed. There's 400 mm underneath our building so I hope it lasts! There have been some tests of longevity and I think there are links somewhere on this forum. But the thing that made me relax about the question was when I realised that railway embankments and some motorways are held up by EPS 'hardcore'.

I don't like its environmental credentials, but it is a very useful material at a reasonable price with a lot of track history so I'm happy to live with it as I live with my concrete slab.

AFAIK EPS has very much nicer eco credentials than any of the other plastic insulations inclusing Extruded polystyreme (XPS).
There is a good case for using petroleum oil to make insulations, when their use saves many times their oil content, in energy that doesn't have to be burnt subsequently for space heating.
There's that aspect of eco credentials, in which EPS is about on a par with the other plastic insulations; AFAIK it's in the other kinds of chemical pollution, which is IMO more insidious and long-reaching, that EPS shines compared with the others.

Posted By: djhYou can obviously extend the insulation out further from the edge of the raft. EPS is what they use but geocell should work as well

This is where I hesitate to use plastic insulations, as horizontal skirt outboard of the building perimeter. Here it really is a little or a lot or accidentally exposed to the elements, and to mechanical distruption due to imposed ground loads varying over time.
The idea of it lying what - 300mm? - below finished GL, covered with topsoil, paving or whatever sounds really poor.
If topsoil, roots will soon enough penetrate and disrupt, even with root barrier overlay. The thin topsoil layer, isolated from deeper massiveness, will scorch is summer, freeze in winter and struggle to retain moisture stability.
Much better to use inert mineral Geocell, or my preference LECA, tho 2x or 3x thicker (for given insulation) and more expensive - but less so if you order LECA not in bags like garden bedding compost with fancy silicone coatings etc, but by the lorry load (or dumpy bagged or even blown-tanker load) in civil engineering grade.
Then it becomes just a mineral geological stratum compatible with topsoil and subsoil, tolerant of mechanical disruption or root penetration, and leaves the topsoil less liable to temperature swings.

Good question. I have been trying to model this in a crude way, as I find Therm software too tiresome. I think of the 'wing' more as extending the wall insulation, not so much the floor insulation - in fact to me it's a way of not having any floor insulation, instead letting the floor benefit from direct connection with the great temperature-stability of massive subsoil.

This thread started as the latter but is tending to veer off into an insulated slab, perhaps supplemented with a perimeter horizontal 'wing'.

1m of downstand is worth 2m of 'wing', and the bulk of the loss through it is where it's closest to the building, so it needs to be thickest there, tapering off outward.

A horizontal 'wing' of perimeter insulation may only be necessary instead of a downstand 'coffer dam' of insulation, where depth can't be achieved because the foundation is shallow, maybe built off shallow bedrock, but even then some more or less vertical EPS against the wall or at say 45o against vestigal foundation, like attached. Make it up as you go along!

To calc, I postulated 21C at floor/inner wall surface, 0C exterior ground surface. Therefore 0C ground temp at the far tip of the 'wing'. Other ground temps at 0.5m intervals diminishing from 21C to 0C. At each interval, use the local delta-t and conventional U value to calc heat loss for that 0.5m strip. From that you can see where the 'wing' thickness needs to be thickened and where it can be allowed to diminish. Turns out, what's shown on the attached isn't really thick enough - but a lot better than nothing and prob works better than it should, as the floor is, as I say, heavily temp-stabilised by direct connection with subsoil - shattered rock/shale in this case. At any rate, the downward temp gradient of the floor is hugely flattened.

Familiarity with the principle should achieve convincing of an open minded Building Inspector, at least in an existing building.

Hi, I developed the Passive Slab used by djh and marketed by KORE so a few things;
Wing insulation increases the temperature of the ground beneath the insulation, I modeled this and found that increasing the temperature of the ground beneath 300mm of insulation by 10 degrees reduces the heat-loss of a 200m2 house by 30kWh/annum, about Ă‚ÂŁ5, so the return on investment could be about 100 years.
Last time I looked the Embodied energy of foamed glass on the ICE Database was 2-3 times higher than EPS.
Frost protected insulated footings were used as early as the 1930s by Frank Lloyd Wright in the Chicago area.
But since that time, the Europeans have taken the lead in applying this concept over the last 60 years.
There are now over 1 million homes in Norway, Sweden, and Finland with insulated shallow footings which are recognized in the building codes as a standard practice.

Just to add, we were wondering if we would not have to insulate the existing concrete floor slab in this building if we EWI and continue it down. Hoping to design out a heating system if possible!?!?

<blockquote><cite>Posted By: Dominic Cooney</cite>Just to add, we were wondering if we would not have to insulate the existing concrete floor slab in this building if we EWI and continue it down. Hoping to design out a heating system if possible!?!?</blockquote>

I don't believe you'll achieve what you hope Dominic, I believe the ground will never heat up enough and will remain cold, any heat you put in will conduct away through the ground. The closest we got to no heating was this house http://www.viking-house.ie/solar-enhanced-passive-house.html
The 40m2 Solar Roof reduced the heating demand from 15kWh/m2/annum to 1kWh/m2/annum and the hot water demand from 20kWh/m2/annum to 1kWh/m2/annum. It has a 50m3 insulated gravel Thermal Store beneath the house and has an annual heating/hot water and electricity cost of €120.

Hi Tom, I believe at the time we were just trying to avoid condensation at the skirting, yes you can always go further, the top of the footings was down 500mm so we stopped there. On a side note I never actually saw the benefit of improving the airtigtness below 1ACH@60pA.