Work on my insulated raft starts on the 8th. So that's 150mm xps with a 250mm slab (more insulation on top). Through this slab are a number of drain connections and various ducts for power, ASHP etc. These have to be located accurately for the timber frame to be placed.

I need to form 200mm x 200mm square ducts for the ducts (if you follow). I'll make up some 19mm ply boxes to form permanent shuttering in the slab to do that. This is what the frame suppliers require. Now the query:

 

Do I just shutter the concrete depth or both concrete and insulation?

 

The latter has the advantage of being able to secure the shuttered duct accurately as it will be held by the thickness of the insulation and cannot be displaced by the concrete. However, that leaves ply in contact with the sub-base stone and basically a 200mm x 200mm hole through the dpm under the slab (although there'll be another ontop of the slab). It also allows some flexibility in moving the duct/drain a little if needed. I could then fill the whole thing with expanded foam before the screed and UFH goes in.

 

The former however cuts down on a cold spot plus the dpm can be tightly wrapped around the drain/duct. But you have to get a not very flexible duct positioned just right through the insulation first time and 50mm electric duct is a bu...er to control. You then have to make sure the concrete doesn't move the timber box, so that's some form of pegging.

 

Thoughts?

For me, timber box on top of the insulation, pinned in place either to the pir or through into the sub base.

Then it's worth filling with sand or bricks etc to keep it solid against kicks or the weight of screed.

If you make the boxes to suit, they can be unscrewed and come out neatly.

Cheers for the advice - will do.