Can moisture from the sand and aggregate under a slab cause rising damp?
I want to do a similar job raising a floor by 1 foot 2 inches to the level of an existing kitchen floor. The floor to be levelled is a concrete slab floor and in good condition. My plan was to lay four inches of aggregate /crushed rock topped with 2 inches of sand with a sheet of plastic layed on top of the sand. On top of the sand I intend to put 4 inches of insulation foam with a radon barrier (plastic sheet) between the foam and the final finished 4 inches concrete slab. The question is: Due to the fact that the heavily moistured sand and aggregate is sandwiched between the slab floor and the 4 inch insulation will they both dry out and can the moisture from the sand and aggregate cause rising damp on the concrete block walls?
With proper vapor protection below a slab on grade or basement floor you can avoid rising damp and basement humidity. I don’t see the point of adding sand, what is your intention there? Unless I’m missing something, it would be easier, cheaper and better to just go with stone. Not that would have a moisture issue, but in general, fines such as sand will retain much more moisture than just clear stone. To be sure I’m understanding, I want to give that back to you and let me know if I’m reading you right - all this construction takes place ON an existing slab, correct? You are talking about laying stone, sand, radon barrier and insulation on an existing slab of concrete to raise it up to meet the level of another part of the house?
If so – I would skip the sand, lay a bit more clear stone, level it and run a plate packer across it, lay down 2 inches of EPS foam (which is a vapor barrier) as you mention, then your radon barrier membrane, then 2 more inches of EPS, pour concrete and forget about it. You can see images about what I mean on this page about how to build a slab on grade on problem soils, including a plate packer with foam on top and a radon barrier between sheets of foam.
If you do all that, then the original slab below becomes inconsequential. If it as no vapor barrier below then it will absorb moisture from the ground, but the best way to build a slab on grade shallow foundation is exactly as mentioned – drainage stone, insulation, a radon barrier sandwiched between foam is a great idea, then concrete.
Mr Reynolds thank you for your informative reply.
The reason for the sand is to ptotect the membrane from being punctured by the aggregate.
I will follow you instruction.
Many thanks again
Very happy to help Frederick, good luck with your project.