I think it depends on the soil type you have and how much it costs to take waist away from the side.

I expect that if your subsoil will take a Viking hous type raft foundation without much MOT, it may be the cheapest option.

Posted By: TriassicI have a raft design and I told the SE I wanted insulation below. He called one of the manufacturers to discuss loading and has produced drawings showing the use of EPS 300 below the foundations.

Using EPS 300 everywhere sounds excessive, and AFAIK you can't get graphite in that rating. We have EPS 300 around our ringbeam and graphite EPS 70 elsewhere.

I don't know about relative costs.

Yes they do but they won't come to Devon to just do a foundation slab, they will sell me the eps in kit form but I still have to pay the SE about a grand for the design particular to my house.

P.s. Woodgnome did you see my pm to you?

Posted By: djh

Posted By: TriassicI have a raft design and I told the SE I wanted insulation below. He called one of the manufacturers to discuss loading and has produced drawings showing the use of EPS 300 below the foundations.

Using EPS 300 everywhere sounds excessive, and AFAIK you can't get graphite in that rating. We have EPS 300 around our ringbeam and graphite EPS 70 elsewhere.

I don't know about relative costs.

it is excessive to use EPS 300 under the whole foundation, the SE took the easy option in specifying it. I will do the EPS loading calcs myself in order to reduce the amount of EPS300 being used and use EPS100 under the majority.

Not sure why you used graphite EPS?

Interesting question regarding the relative cost of EPS300 and 100, anyone got the costs?

Insulated rafts are normally used with timber frame or EWI. If you have a masonry outer leaf then you need to think about how you will support that without bridging the perimeter insulation. Using an independent trench fill foundation for the outer leaf could lead to differential movement between outer leaf and raft/inner leaf. I have seen studies from Denmark where an outer ring beam is used to support the outer leaf and this is joined to the main raft with steel rods. To my mind, £1000 is money well spent to mitigate this risk.

David

What about lightweight aggregate (maybe aircrete) foundation blocks with some EPS on the outside?

(I think lots of insulation under the slab is pointless unless you are having UFH, or insulation so well that you can save the cost of installing a heating system.)

I talked to one ICF vendor a long time ago, they claimed that with deap strip foundations, the cost of using ICF below ground was less then the saved concrete from a full width trench fill.

Posted By: davidfreeboroughUsing an independent trench fill foundation for the outer leaf could lead to differential movement between outer leaf and raft/inner leaf. I have seen studies from Denmark where an outer ring beam is used to support the outer leaf and this is joined to the main raft with steel rods.

That's quite similar to our situation where the bales need to be supported over their width. The original design had a separate strip foundation to support the outer edge, but we finished up with a ringbeam around the raft tied with some steel and concrete ribs. Still OK for PHPP bridging.

Posted By: joe90Having already spoken to the B.C.officer about foundations for the garage the ground is medium shrinkable clay and they have no problems with strip foundations at 850mm deep and a little wider than minimum so I guess they will not insist on S.E. Input for a trench foundation.

That sounds similar to our ground conditions, though we do have trees as well, and was another reason we ditched the strip foundation in favour of the ringbeam. I couldn't get a design that both the SE and the materials supplier would sign off on for clay heave protection under the slab whilst still maintaining the integrity of the sub-slab insulation. The insulation needs to be pressed against the slab, but that's not how heave protection works - it crushes.