How to connect an insulated slab to an old stone cottage with minimal foundations?

[Jess Shannon]

We're building an extension on to an old cottage, stone footings are about 300-350mm below floor level. I've been talking to a couple of companies that supply these (KORE and Isoquick). They're saying the engineer needs to detail the junction and the engineer is asking for details from them.

I've sketched this up as how I think it might work. Isoquick said to just cut their foundation to fit. I've added some cork in at the edge but I'm wondering if that should continue down further so if there's any movement it's not straight onto the rigid insulation, or does that not matter?

I'm not quite sure how it would work at the thickenings on the out edge of the slab, should we slope the insulation up?

[Nickfromwales]

As you’ve drawn it seems fine, but unless this is a 200mm raft because load bearing intermediate walls are landing on it, then go for more insulation and a thinner slab? 
 

I assume you’ve omitted DPM/DPC for simplicity?

 

Lose the cork, and replace with 30mm PIR insulation. Cork is hygroscopic and friable. 

[Mike]

If there is no DPC in the wall - I'd guess there isn't - then if you block the moisture in the ground from escaping due to impermeable insulation or a DPM, it will tend to escape through the wall where it may cause a build-up of salts in the wall / plaster. I'd therefore be thinking about choosing the right hand side of your sketch but using limecrete in place of concrete. This isn't something that I've been involved with, but there are some pointers & further links at https://www.greenspec.co.uk/building-design/limecrete-floors-for-old-buildings/

 

You'll also need to convince your BCO.

[saveasteading]

I don't understand your drawing, lovely as it is, because i don't see how the old cottage relates.

Or are you digging out the old floor?

 

1 hour ago, Mike said:

You'll also need to convince your BCO.

Quite so. You'll need a detailed design from an SE, otherwise the bco will want the new bit built to current regs.