Replacing a Concrete floor in Victorian terrace

[TBreezy]

I have a Victorian terrace in London. We are retrofitting the whole thing. I took down the partition wall between a reception room and the small back kitchen. The back kitchen was a solid concrete floor straight onto earth. It was wet and there was damp up the back wall. There is no DPM. The floor under the adjacent room (now the same room) is a suspended wooden floor.

I've dug out the whole floor to 30cm (brutal job). Now I want to put a new floor in. I am thinking a layer of gravel to level it off. Then 150mm of insulation and then 150mm of concrete to bring the whole the level with the joists of the other room.

Some questions:

1) Is this the right approach?
2) Do I need to think about ventilation under the adjacent floor?
3) any recommendations on the insulation?


Any help much appreciated!

Cheers

Tim

 

[Ashville87]

I’ve had a near identical issue recently. People say not to put down a traditional insulated floor slab as this can push moisture out to the walls. I’ve ended up putting 200mm of Geocell foamed glass then a 100mm concrete slab on top. The Geocell is supposed to deal with any moisture before it reaches the slab. Some will put a limecrete screed on top instead of concrete but I was recommended by my supplier this could be done with concrete instead.

 

[TBreezy]

I’ve had a near identical issue recently. People say not to put down a traditional insulated floor slab as this can push moisture out to the walls. I’ve ended up putting 200mm of Geocell foamed glass then a 100mm concrete slab on top. The Geocell is supposed to deal with any moisture before it reaches the slab. Some will put a limecrete screed on top instead of concrete but I was recommended by my supplier this could be done with concrete instead.

 

[TBreezy]

Thanks for responding. Did you use a DPM? It doesn't look like my walls have a DPC so when I'm looking at the technical instructions for kingspan ground insulation I am not sure where to take the DPM to. I guess water can't travel up the geocell as it's gravel so no need for a DPM?

 

[Ashville87]

I am putting a DPM layer on top of the Geocell but only to stop my concrete falling through it. I was assured by my supplier that any moisture will not reach this high up so shouldn’t be an issue for pushing damp up the walls. I’m lucky enough to have slate dpc in the walls.

 

[TBreezy]

Yeah I am worried as no DPC.

I am tempted to dig down another 50mm, lay a bit of geocell and then insulate with kingspan on top. The R value difference is significant between 200mm of geocell and 15mm of kingspan thermafloor (6.82 compared to 2.5) and then it might deal with both the damp and give better insulation overall. Also 150mm of concrete will presumably hold more heat for longer.


But does mean a ton more digging....

 

[TBreezy]

Also worried I start to compromise my footings as they started to appear at 30cm. But not sure that is a problem

 

[Ashville87]

I’m no expert on this, I’ve just gone on what I’ve read but I’m sure the minimum staring point for Geocell was 150 compacted. I ended up putting 190 down as that’s what two full bags worked out at for my area so I had no waste.

 

[TBreezy]

I have just read this thread which is also very helpful:

Help understanding my floor

We have builders currently ripping out an old kitchen. When doing so, they took up some tiles; sound more red square tiles underneath, took those up and then said they’d reached clay soil. They’re saying we need to dig out the concrete completely, and then put a membrane and insulation down...

www.periodproperty.co.uk

I think I need to work out what I am doing about the DPC in the walls and then figure out the insulation technique. Also the resident building control guy said he would suggest some guidance once I came up with a full insulation plan.