Insulated slab joining with EWI

28 Oct 2013
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United Kingdom
I'm just ordering for building an insulated slab foundation for a garage/workshop. My plan is to lay XPS insulation in the slab and have it join to form continuous insulation with external wall insulation.

I'm just wondering whether it would be best to build an insulated form for pouring the concrete, and then add EWI down to that - or just insulate underneath the slab, pour into a concrete form, and then use EWI to insulate down to that. Also, curious if anyone has any opinions on where the DPM should go. I'm planning on adding a Vapour permeable membrane (red) between the concrete and XPS (help prevent leaks during the pour too). Should I also add a DPM course under the first layer of blocks?

Here's the two options I'm considering:

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Hold up.

You can't just build your blocks off the edge of a slab. Unless the slab is designed to take the load at the edges, ie: thicker at the edges with significant steel reinforcement.

You don't need such excessive insulation detailing for a garage/workshop.
Keep the wall on its own foundation, and just butt the floor insulation up to the inside.
I've got 120m of rebar to reinforce. But no, hadn't considered the thickened edge. Was going to just do a floating raft like this:

But from reading, you're right I really should have a thickened edge. I was planning on 150mm thickness for the slab, seems 300mm thickened edge should be sufficient for a single story of single skin blocks.

Basically like this:

I've already excavated the hole, so not sure I want to change the plans to a completely different type of foundation at this point.

I'd think it's worth spending a few thousand on insulation now, giving me the option to heat it and not having to deal with rusting in the future.
You have no dpc in your detail either, that outer insulation needs protection from the elements/the lawnmower/something to hide it, eg:


As ever, the devils in the detail .....
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I was wondering if my plan for DPM is sensible.

The rendering will cover the external wall insulation, and I'll just take that down to ground level. There's no point covering the insulation below ground level, right? I've been considering raising the slab a bit, so if there is a gap between the EWI and the slab insulation, I can either just render that, or cover it with something.

The red line in my plan is vapour barrier between the concrete and insulation. I read that is preferred over a full DPM. Looks like it's pretty much in the same place as the green line for DPM in that pic.

Not sure if I should put a DPM on the outside of the lowest row of blocks in some way though. Can you put DPM under rendering? Not sure that make sense.

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