floor insulation construction suggestions

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Hello, im in the process of insulating the floor in my solid floor/wall cottage. I Have done similar work on other houses in the past but on this job I'm limited to depth so need some advice.

The room has a cellar below that takes up about 1/3rd of the foot print. The previous owner had screeded over the York stone floor by approx 50mm which I have removed. ive also carefully removed the Yorkshire stone flags (hoping that they can be re used) then dug down in the dirt below by approx 150mm to reach the cellar ceiling (huge stone slabs) ive then used this as a reference point to level the rest of the floor in the room to be level with the cellar ceiling slab.

I have calculated that I have approx 250mm of depth to get back to the required height of the adjoining room floor. Thats 150mm dirt, 50mm Yorkshire stone, 50mm screed above York stone. I considered the following floor construction

30mm sand blinding
visqueen sheet
100mm Celotex insulation
visqueen sheet (to prevent the boards floating)
65mm screed
remaining 55mm for the Yorkshire stone relaying on cement dot dab.

Because I cant get any deeper or come any higher, my options are a bit limited. Im not sure if I should reduce the insulation to 50mm or 60mm? so I can increase the slab/screed depth to 100mm. put the slab in first to help secure the walls where ive dug out.

Thanks
 

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100 Celotex is more than enough for building regs, but if you wanted more then ...

Is the 30mm blinding necessary? If the cellar slab is smooth or the hardcore fine, then the blinding can be removed or reduced. Or if relatively flat concrete slab then just put the DPM on it then the insulation board.

As you are laying slabs on the screed, the screed could be reduced to 50mm
 
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Are you putting the sand layer straight onto the soil?

Normally you'd have hardcore, compacted first, then as you have listed.
I've just done something similar, the owner didn't want 100mm celotex, and wanted to use 50mm instead, so reluctantly, I used this and added a celotex upstand, this was covered in 100mm concrete slab, then they are are going to screed/tile it.

You could probably reduce the sand blinding layer slightly.
 
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100 Celotex is more than enough for building regs, but if you wanted more then ...

Is the 30mm blinding necessary? If the cellar slab is smooth or the hardcore fine, then the blinding can be removed or reduced. Or if relatively flat concrete slab then just put the DPM on it then the insulation board.

As you are laying slabs on the screed, the screed could be reduced to 50mm
What do you suggest a minimum for the blinining? I originally read 30-60mm somewhere and went with with min. To be honest its mostly muck rather than stone. theres the odd rock but can be smoothed out.
 
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Are you putting the sand layer straight onto the soil?

Normally you'd have hardcore, compacted first, then as you have listed.
I've just done something similar, the owner didn't want 100mm celotex, and wanted to use 50mm instead, so reluctantly, I used this and added a celotex upstand, this was covered in 100mm concrete slab, then they are are going to screed/tile it.

You could probably reduce the sand blinding layer slightly.
yes I had planned to. the groud that the the Yorkshire stone slabs were sat on is a kind of earth/dirt stone mix. nothing like MOT hardcore though but quite compact. Because its been fine for 200 years as is under the York stone, I though it would suffice as is without the hardcore though I appreciate it could sink and cause issues.?

Ideally id like 100mm Celotex if I can fit. but I may have to compromise to 60 or 70mm to get a decent slab. without a screed.

Would that be acceptable? what are you thoughts on the slab being at the bottom to help support the walls from where ive removed the dirt. of are you not concerned about that because ive only gone down a small amount..

As you suggest an upstannd of 50mm to prevent a cold bride to the outer wall. I have in the past used a roll of 10mm foam with a skirt on that was taped to the visqueen but I think 50mm might be more effective this time
 
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Blinding is intended to prevent sharp stones puncturing the DPM. The smaller the sharp edges the less blinding required, and it could be zero.
 
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Blinding is intended to prevent sharp stones puncturing the DPM. The smaller the sharp edges the less blinding required, and it could be zero.
non of the stones are sharp. they are all rounded and compact into the dirt. I think your right, illl reduce the blinding to incorporate more insulation
 
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It doesn't look like you've undermined the walls (on your photo), so I wouldn't worry about that.

70mm is probably 'lean' as a slab would go, but would be doable, you could chuck some mesh into it, as there will be a crack when it dries
You could reduce celotex to 80mm and make the slab thicker? Just fill gaps with expanding foam and tape the joints in aluminium duct tape.

Out of interest, are you mixing ballast or having ready mix? What m² are you working with?
 
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Yeh I thought the slab could do with being thicker. It did make me laugh when taking up the previous screed that whoever laid it and incoperated chicken wire to strengthen it. It had pretty much rusted away.

I’m on with finishing the floor dig today and tidying up ready for sand. I don’t think I’ve undermined the walls. it was loose earth / dust at the edges and the walls look sound. It was more that the York stones / screed was pushing back at the walls or at least holding them in place. Nothing the new floor won’t be able to do.

I’d planned to use a local firm who come with a wagon of ready mix and pipe it in. Area is approx 25m2
 
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just to come back to this

ive wacker plated the rubble that surrounds the cellar ceiling and levelled to it. ive then added a sand binding layer, a DPM and then cellotex insulation. I reduced my cellotex from 100mm to 80mm as discussed as im short on space to get back to a level with other rooms, I have about 80-100mm left for concrete / floor covering. Sadly I dont think ill be able to re lay the stones.

My question is, should I go with a thick screed with some rebar or thinner concrete and then a screed? I understand that concrete is stronger because of the aggregates used but I would then need a screed on top to make a smooth floor for the floor finish. so logic would say go screed with rebar but of course im not sure.

Thanks for any advice.
 

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