Green Building Forum - Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... Mon, 17 Aug 2026 03:16:32 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221458#Comment_221458 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221458#Comment_221458 Wed, 18 Mar 2015 10:54:11 +0000 kentishgreen
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.]]> Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221462#Comment_221462 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221462#Comment_221462 Wed, 18 Mar 2015 11:30:57 +0000 ringi Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221465#Comment_221465 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221465#Comment_221465 Wed, 18 Mar 2015 12:05:07 +0000 kentishgreen 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.]]> Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221468#Comment_221468 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221468#Comment_221468 Wed, 18 Mar 2015 12:21:04 +0000 tony Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221474#Comment_221474 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221474#Comment_221474 Wed, 18 Mar 2015 15:30:42 +0000 ringi
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.]]> Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221478#Comment_221478 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221478#Comment_221478 Wed, 18 Mar 2015 16:41:34 +0000 tony Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221864#Comment_221864 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221864#Comment_221864 Fri, 27 Mar 2015 17:51:04 +0000 djh
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.]]> Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221871#Comment_221871 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221871#Comment_221871 Sat, 28 Mar 2015 07:57:44 +0000 DarylP ]]> Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221885#Comment_221885 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221885#Comment_221885 Sat, 28 Mar 2015 10:48:31 +0000 fostertom Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221887#Comment_221887 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221887#Comment_221887 Sat, 28 Mar 2015 11:28:15 +0000 Ed Davies = ground level) unless the French drain is very effective which would require nearby daylight (somewhere downhill) to empty it to or very reliable pumping.]]> Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221890#Comment_221890 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221890#Comment_221890 Sat, 28 Mar 2015 12:34:56 +0000 fostertom Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221893#Comment_221893 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221893#Comment_221893 Sat, 28 Mar 2015 12:51:01 +0000 djh Posted By: fostertomIf 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.
Hi Tom, I agree about the advisibility of perimeter insulation, but it isn't frost heave that the OP is concerned about. It's heave due to trees, which is the same issue I had.

In our case we put a French drain in just lower than the below-slab insulation but there's no way to put it any lower (e.g. under strip foundations) because of the winter-time water levels like Ed mentions.

I'm also not sure that your proposed solution is shown in the NHBC guide, which seems to be the bible for easy approval by the various interested powers-that-be. Of course, we should be all about exploring better alternatives here, but it's best to start with your eyes open.]]> Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221908#Comment_221908 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221908#Comment_221908 Sat, 28 Mar 2015 15:26:44 +0000 fostertom
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.]]> Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221929#Comment_221929 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221929#Comment_221929 Sat, 28 Mar 2015 20:59:41 +0000 djh Posted By: fostertomDef not in NHBC - in fact I thought I was the only person in the world doing it this way.
I thought it was quite common in North America?]]> Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221936#Comment_221936 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221936#Comment_221936 Sat, 28 Mar 2015 22:51:59 +0000 tony Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221939#Comment_221939 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221939#Comment_221939 Sun, 29 Mar 2015 00:06:21 +0000 fostertom Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221942#Comment_221942 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221942#Comment_221942 Sun, 29 Mar 2015 06:44:09 +0100 tony
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.]]> Piled slab foundations: what anti-heave options to go for? Cold bridging, insulation issues... https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221945#Comment_221945 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13221&Focus=221945#Comment_221945 Sun, 29 Mar 2015 10:49:55 +0100 fostertom Posted By: tonyclay swells up progressively until it is saturated with water and as water is drawn out of it it shrinksWhat if the route by which water enters or leaves the clays under the building, is intercepted by a perimeter french drain, which diverts any horizontal inflows to the drain, as well as draining the clays under the building?
Posted By: tonyThe process can happen from the surface to several meters down
What if the building prevents any vertical inflow down from the surface?
Posted By: tonydrying can be as a result of tree roots pumping water out which happens seasonally or as a result of prolonged dry weather
What if there are no live tree roots under the building because they've been cut by the perimeter trench, except deep down maybe, below the level that you normally have to go with founds to avoid danger of heave?]]>