Hi All,

Local ground and trees dictate that I need a piled foundation for my extensions. I've had the usual soil tests and engineers input etc. I have now shortlisted two piling contractors who have differing ideas on anti-heave measures:

Contractor A: Voided space beneath RC suspended slab
Contractors B: Cellcore under RC raft

Costs are virtually the same, though I may end up with more spoil with A. Structure on top will be SIPs with UFH.

My original intention was to insulate ontop of the slab but with a voided option, insulation underneath may be a (better?) solution? Obviously I wish to reduce cold bridges where possible.

However the cellcore option underneath the slab would also give some insulation value, so I could go for that route with insulation on top of slab.

I'm awaiting on the architect to draft his foundation/wall details, but in the meantime would value any input into my slab options. Will upload detail when completed.

Posted By: ringiWhy not Cellcore HX Plus?

Not sure which one they have included in their costings, but yes, Cellcore HX plus is the one I will demand. And then I will have additional insulation on top of slab too.

Tony remember you need a void so the soil up expand without effecting the building.

As the soil will also shrink at times, if the insulation is not fixed to the underside of the slab, a gap may form between the slab and the insinuation.

I looked at using Cellcore under my house, although I don't think it was the HX product. The sticking point was what happens after the ground heaves, thus compressing the product, when the ground subsequently drops again. The compressed product does not spring back, so there is a void. How do you keep the insulation in firm contact with the underside of the slab rather than resting on the ground or even just drooping a bit and allowing air movement. Cellcore were unwilling to warrant anything; perhaps it is different with their new product. You also have the thermal bridge question at the edges if you use piles or strip foundations.

In the end I went with a passive slab (raft) and a load of hardcore underneath. If your ground conditions allow for it, I can recommend that approach. It was simpler and cheaper.

If you do downstand perimeter insulation and/or horizonral 'wing'/skirt insulation around your perimeter, instead of underfloor insulation, then frost heave is eliminated. If you make the perimeter into a french drain as well, then all will stay bone dry beneath, so no heave resulting from moistrure change. Then the whole block of subsoil beneath becomes a massive temperature-stable heat store, helping all to keep warm in winter and cool in summer.

True.

Def not in NHBC - in fact I thought I was the only person in the world doing it this way. Building Inspectors so far accept the principle for extensions and upgrades, but prob a bridge too far for newbuild.

Heave due to trees works by changes in moisture content of clay, which their cycles can cause. As far as I can see, french drain takes care of that by copiously draining the soil to a low water content whatever.

A French drain cannot stop clay heave! Come on...

Highly shrinkable clay swells up progressively until it is saturated with water and as water is drawn out of it it shrinks.

Clearly under foundations or floors this if not good and where structure are damaged we call this subsidence.

The process can happen from the surface to several meters down, drying can be as a result of tree roots pumping water out which happens seasonally or as a result of prolonged dry weather.

A French drain will not stop clay either shrinking or heaving.