Celotex under foundation

[John D v2.0]

OK interesting idea so don't take this at face value, but I'm interested why this isn't done more.

I was watching a video about a passive house where they laid thick polystyrene on a base, then poured a massive reinforced raft on top and built off that to avoid any thermal bridge at the base of the wall.
Today I was looking up the compressive strength of normal clay soil compared with celotex. Seems they could both be around 0.15N/mm².
So my question is why wouldn't you dig out to 1m or whatever, fill to 250mm below the top of your required foundation, chuck 100mm celotex on joined with both the floor and wall insulation, another 150mm of concrete and then start building as normal?
Sounds like a massive faff on face value but I'd be interested whether this kind of modified trench fill would have been easier/better than building an engineered raft on polystyrene.

 

[tomfe]

You don't normally build you wall on clay though.
The eps they use for a passive slab is eps 200/300 which has a much higher compressive strength (they use it under roads etc as infill).
But yes you could run your slab insulation into your cavity and then lay your floor up to the cavity insulation with the correct design etc. Not sure how you'd drain your cavity though.

 

[Doggit]

If I've understood the concept, then the EPS would extend further than the wall, so you'd be building on the slab on top of the EPS. Not sure how you'd handle all the services though, but it's an interesting idea.

 

[tomfe]

http://www.viking-house.ie/passive-house-foundations.html

 

[^woody^]

There is no thermal bridge from a trench.

And loading characteristics are different between strips and rafts.

 

[John D v2.0]

Thanks for the thoughts guys, interesting.

You don't normally build you wall on clay though.
The eps they use for a passive slab is eps 200/300 which has a much higher compressive strength (they use it under roads etc as infill).
But yes you could run your slab insulation into your cavity and then lay your floor up to the cavity insulation with the correct design etc. Not sure how you'd drain your cavity though.

Thanks, i think millions of houses are built on clay, ours is. Just the load is spread over a 600 wide trench.
Seems you can get standard celotex 0.12N/mm² or fi5000 which has 0.175N/mm². But maybe the lower would also be fine. The cavity would probably need to have a tray above ground level, but I notice in your later drawing they just rendered outside instead.

If I've understood the concept, then the EPS would extend further than the wall, so you'd be building on the slab on top of the EPS. Not sure how you'd handle all the services though, but it's an interesting idea.

Good question, I suppose you could just insulate them the first metre or so inside or outside the house.

There is no thermal bridge from a trench.

And loading characteristics are different between strips and rafts.

Maybe not a thermal bridge in a traditional sense but the psi value isn't unity. Apparently passive houses are so high standard they have to make them pretty box shaped to get the best surface area to volume ratio, so to them this is a thermal bridge!
And what would be the concern with strip loading? I assumed as long as the bearing pressure was OK then the house would be stable.

 

[tomfe]

Thanks, i think millions of houses are built on clay, ours is.

I think you'll find it's been built on either a brick or concrete foundation.

 

[John D v2.0]

I think you'll find it's been built on either a brick or concrete foundation.

Ah you thought I meant just build the wall directly on the celotex? Actually that's why I put a foundation on top of the celotex

100mm celotex [...] another 150mm of concrete and then start building as normal

 

[^woody^]

And foam degrades.

 

[John D v2.0]

And foam degrades.

I'm trying to find a reference to that but I can't. All I can find is the manufacturer says the u values have been determined by premature aging of the material, but nothing about the compressive strength. It also says the boards will have the same lifespan as the building, but if the foundation fails the building won't last long! I can also find loads of people creating threads saying they heard after 5 years celotex is the same effectiveness as rockwool