Basement - transition from above ground insulation to below ground

[Ed_]

I am building a walkout basement, this means that down the sides the ground level will slope, from top of basement at front to floor level of the basement at the back.

 

I'm struggling to understand how to detail the above and below ground insulation. 

 

i think I need to use EPS below ground and PIR is not suitable. As it's below ground, I need less, so 150mm EPS is ample, but that won't work above ground, so I thought I'd use 150mm PIR above ground and that works about roughly equivalent.

 

The issue I'm struggling with is how to detail the transition given the slope? Can I just butt them up, render over the top to just below ground level and put in a french drain? Should I cut the insulation to match the slope (with lots of wastage) or just get as close as practical using blocks? I was planning on just adhering directly to the concrete.

[JohnMo]

Easy way is to think that the underground and overground insulation has to meet to give a thermal bridge free design. It's no use having external insulation below ground and internal insulation above ground.

 

So all depends on how you are constructing. Stick build, cavity, internal or external insulation, ICF etc. so a little more info needed from yourself. A drawing often helps instead of words.

[Thorfun]

yeah, maybe with a sloping basement like yours the best route would be all internal insulation providing you can maintain the thermal bridge as @JohnMo said.

[Russell griffiths]

Why won’t eps work above ground, I would have thought you could use eps all the way up and render over it, you may need a specific render product for below ground level. 

[Ed_]

33 minutes ago, Russell griffiths said:

Why won’t eps work above ground, I would have thought you could use eps all the way up and render over it, you may need a specific render product for below ground level. 

It would, it just needs to be thicker obviously. I'm limited to 100mm insulation where the timber frame joins the slab at ground floor level, to give a drip edge to the timber frame insulation. So 250mm EPS for example is quite a step out and probably won't look great, hence why I was thinking PIR as the thickness needed for PIR above ground is about the same as EPS below ground.

[Ed_]

1 hour ago, Thorfun said:

yeah, maybe with a sloping basement like yours the best route would be all internal insulation providing you can maintain the thermal bridge as @JohnMo said.

I've been round the internal v external debate so many times. You are right but I can't work out what to do about the cold bridge through the ground floor slab. How effective is a bit of external plinth insulation going to be to stop that? At least external is continuous. Maybe I am worrying about nothing.

 

Will add a drawing later to help!

[Thorfun]

25 minutes ago, Ed_ said:

It would, it just needs to be thicker obviously. I'm limited to 100mm insulation where the timber frame joins the slab at ground floor level, to give a drip edge to the timber frame insulation. So 250mm EPS for example is quite a step out and probably won't look great, hence why I was thinking PIR as the thickness needed for PIR above ground is about the same as EPS below ground.

i'd be concerned about how having so much EWI will affect the condensation risk and any sort of airtight membrane internally? unfortunately i'm not clever enough to know the answers to that! 🤣

[Ed_]

My understanding is that having the insulation on the outside is better for condensation, as it means the concrete is warm. Insulation on the inside and the concrete is cold so the condensation point moves to inside the wall, potentially.

[Russell griffiths]

Can you not go 50/50 as if you built it out of icf blocks, 

[JohnMo]

37 minutes ago, Russell griffiths said:

Can you not go 50/50 as if you built it out of icf blocks, 

Or do Ecobrix (Durisol) 95% external, with insulation in place from factory, do whole lot in same material, tank membrane the underground bit. 0.14 U value out the box with internal service void. Screw battens for external cladding direct to blocks, if rendering can be direct to blocks and/or stone or brick slips direct to blocks.