EPS?

Extruded Polystyrene (XPS) as its closed cell & waterproof.

Not Expanded Polystyrene (EPS), this is open cell & water permeable.

I think the floor detailing inwards of the wall is key to suitability of a vapour permeable material.
With a dpm tucked under the dpc I cant see 'breathability' being a problem, but would look to mitigate any moisture retention through a french drain to the building perimeter.
Would 'the panel' also suggest the below dpc ins. should continue fully down to founds ?

Some info on rigid board ins. here (US site)
http://www.greenbuildingadvisor.com/green-basics/rigid-foam-insulation

how far below dpc level are we talking? does it need to be insulated? you can use the same idea as the actual render system build up using eps but with a specially formulated render product that is suitable for below ground level.

fostertom,
I agree with your comments on using render to keep out moisture. if in constant contact with the ground it probably will find its way in as all render does need to breath a little. The products used at this location is not the same as the above though, it is more of a cosmetic solution i think - a plinth detail to make the job neat, non permiable but i cant see why it wont work if taken deeper. It is broken at dpc from the above system. I have not seen any ewi system that is continous through to foundation but by my thinking I dont see why this could not be achieved as you say different products could be used below ground and dpc for backfill protection etc and another above to achieve a render finish.

I have a cutaway drawing of this detail if you would like me to post it up

Hi , any thoughts on this detailing for floor to wall junction
finish: thin coat render
internal : wet plaster
Block: fibolite
Currently blocks laid up to dpc , just wondering the best solution for dpc position and below dpc EWI insulation.
UFH in slab, so would 25mm-50mm PUR upstand between slab and block make sense.

thanks Jim

Looks good to me --- even very good --- I prefer the first one but that is more about engineering and style than anything else.

thanks , no. 1 is it then ,
now on to the next problem , what to do at the top of the wall !
Always a bit dangerous, the 'making it up as you go along' design strategy

I'm still getting low temperatures at the skirting when we renovate like that, by putting insulation in the floor and dropping the external insulation 500mm below the finished floor level. I've even tried dumping excess heat from solar panels at the footing to improve matters. Better to use a "Passive Slab", you get the same temperature 18-19 degrees on the wall, the floor and at the junction in cold weather.
The junction you detailed above gives temperatures of 15-16 degrees in practice at the junction, similar to using 1.8 U-value windows. You should be aiming to build a structure with no cold spots, no convecting currents and free from condensation.

VH , yes passive slab seems the best solution ,
building over a main drain run and poor ground made the traditional foundations the simplist solution without getting a designer/engineer involved. Next time I'll see if I can find a engineer/designer who doesn't do the usual huffing and puffing when I show them the passive slab detailing .
Your right , getting it right at the design stage is the way to go.

On current project, the EWI supplier and experienced fitter are quite relaxed about running ordinary EPS 200thk uninterrupted down to bottom of found of existing 500thk rubble wall built direct (with slight wall thickening) off quite gd subsoil. There is a galv base rail stepping up and down somewhat above varying ground level, but its purpose seems vague and unnecessary. No requirement for 'impermeable' XPS below GL, no protection against backfill, no bellcast or visible change above GL, render taken down a bit below GL, no finish reqd below that. So it can look just like the rendered original.

The trench we dug, down to found bottom, we've made about 700 wide, leaving about 500 after EWI. Installed french drain on the bottom, which slopes v slightly away from the wall (careful no over-dig) and backfilled with Leca (instead of no-fines aggregate) to within 150 of GL, Terram wrapped and topped with topsoil. Leca is pretty cheap, light and easy to handle, and can be stacked into the trench in unopened bags, voids filled with loose Leca.

The effect is that the EWI is guaranteed free of standing water (except when the stream-side site floods!), in fact is contacting mainly extensive airspace, between the Leca granules. EPS (and Leca) do absorb a small amount of water when saturated, which reduces their insulative value (but not greatly) - but do not degrade physically. When saturated conditions clear, they are free-draining and will dry out again. This seems the robust approach, rather than relying on supposedly closed-cell insulations and/or waterproof coatings or membranes, which IMHO will let some water in but never let it out again.

The Leca-filled trench amounts to a kind of wing insulation, added outboard of the EPS downstand. The Leca's lamda value of 0.11 is 3x worse than the EPS's 0.038, but it is installed in 'thickness' at least 3x the EPS's. The path length of heat going down into the uninsulated extg floor slab, down out under the EPS downstand + Leca wing and up to surface, is considerable - and can be readily increased by widening the trench and laying in more Leca spreading outboard. Topsoil is a lot happier put back on top of Leca, than on a wing of EPS.

I'm doing the AECB Carbonlite Thermal Bridging (using Therm) course http://www.carbonlite.org.uk/carbonlite/course_detail.php?cId=6 in Feb, so will be interesting to see the effect of playing with dims of such installations.

Posted By: jamesingramit seems the concern is future problems with airtightness at this junction

The ground below a ground bearing slab is bound to contract when no longer exposed to rainfall. So a ground bearing slab is bound to settle to some extent & the DPM is liable to fail when this happens. By bearing the slab on the inner leaf you are guaranteeing that there is no relative movement between wall & floor & no undue pressure on the DPM. This seems like a more robust solution.

I don't think upstand insulation to the perimeter of a ground bearing slab does very much as it does nothing to help the temperature at the base of the wall. So the edge of the slab may be a little warmer, but the base of the wall will be unchanged or cooler if anything.

The best way to address the thermal bypass of the floor insulation by the inner leaf is to use foamglas in this location. You can minimise the load on the foamglas & the cost of this approach by placing the insulation over the slab & under screed. That way the foamglas doesn't have to take the weight of the slab. I'd prefer to do it without screed as your second diagram, but you'll need to do some calculations & the foamglas may need to be wider than 150mm to take the load.

David

i think it more the fixing cement behind , rather than the eps , i agree eps seems pretty water proof to me .
.Who are you using ?

fostertom wrote (2 days ago)
EPS (and Leca) do absorb a small amount of water when saturated, which reduces their insulative value (but not greatly) - but do not degrade physically.

Any way of putting a value on the loss? Also if the EPS did absorb some water would it then be subject to frost damage?

I once knew a dingy sailor who wanted to test some EPS prior to using it for buoyancy as he heard that it absorbs water so he weighed some and then put it in a bucket of water with a brick on it for one month after which he weighed it again and found no difference. So how much would it absorb anyway?
Peter

Therefore I presume that vapour permeable does not necessarily mean water absorbing. But what about frost damage when EPS continued down to foundations as suggested above
Peter