Insulating a Log cabin concrete base

[wingit]

I have broken up the previous concrete base, that was in the same location into fairly small pieces and excavated down a little, the broken up concrete being around 100 mm deep, approx 100 mm MOT 1 added on top and compacted, then around 70 mm of sand compacted on top. The wood form work is in place.

The initial plan was to lay a DPM on the sand, then some PIR/EPS type of insulation and pour the concrete, wrap the DPM up the sides and over the top of the slab a bit, and build the outbuilding to the edge, effectively enclosing the slab and insulation.

Time being short, however I went for a log cabin instead, in kit form. Given the way that the walls of the building will not go to the edge of the pad, I wonder now if there is any way to insulate the concrete pad from underneath. I would have just gone with insulation on top, but the levels of my garden and the work so far leave 150 mm to be reached. Pouring a 150 mm concrete slab for a log cabin feels like complete over engineering.

In summary, for this style of building, is there a way I can insulate under the slab to raise the level enough to pour a more modest 100 mm concrete base?

The base recommended is 595 x 320 cm. I've gone 50 mm larger each way, i.e. 600 cm x 325 cm.

Thanks

 

[Mr Chibs]

Wondering what is the point of the insulated slab actually is? Thermally the slab is still next to soil around the edges.

Would it not make more sense to insulate the inside of the cabin (walls/floor) instead.

You could then just cast an uninsulated slab.

 

[^woody^]

Insulation below a slab requires the slab to be heated up before the insulation starts to work, and it may be unlikely in a timber summerhouse.

The slab edges should not project past the walls either.

A surface in insulation product would be better, even if thinner.

 

[wingit]

The slab edges should not project past the walls either.

A surface in insulation product would be better...

It has a decent roof overhang, I was hoping that would help. The only way for the walls to align with the slab would be for the log ends to overhang the edge of the slab.

I don't understand what a "surface in insulation" product is, google didn't bring any light to it.

 

[^woody^]

A roof overhang is irrelevant, when the slab will be a thermal bridge, which in turn defeats the insulation layer.

Surface insulation is just that, insulation laid on the surface and then the floor deck fitted on top. It's the same product that you may put below the slab. Foil faced PIR would be most efficient.

 

[wingit]

With reducing concrete in mind, I'm going to go with Filcor 20 EPS by Cordek, a lightweight structural fill material. 50mm thick boards placed on top of the sand and then pour the concrete, this will allow for a 100 mm thick concrete slab. Thermally I expect very little benefit.