We are considering in floor radiant hot water heating in the renovation of our 1930s house. I’ve already torn out the old concrete basement floor in order to also replace the old drain lines. I am concerned about the new concrete floor rising or otherwise moving once the heat is on. My dad recently had a slab on grade home built with the infloor heating, and the slab has raised over 3/4 inch in some places, in relation to the foundation. The slab does not overlay or tie into the foundation walls, but is contained within the perimeter. The contractor thinks the problem was due to overcompaction of the subgrade beneath the slab. I don’t want the same thing to occur in my basement if I go with infloor heat. Also, should I isolate the floor from the walls with expansion joint material ( which I could use as a screed), or should I pour the floor right up against the basement walls, perhaps tying into the walls with drilled rebar dowels tied to the floor reinforcement? If so, any ideas on what to use for screeds around the perimeter? Thanks in advance!
Replies
Heat can't make a floor rise and fall. There has to be something wrong with the base on which the floor was placed, and the heat has nothing to do with it.
There is no such thing as "over-compacting" a base. If the base is the right material, crushed stone, or almost the right material, sand, it can't expand after compaction. The only things that can expand are materials like clay, which expands when wet and should not be used under a slab, or organic materials like peat moss.
You should isolate the floor from the walls for two reasons. First, the slab should be able to expand and contract independently from the walls. Second, the slab should have a thermal break between it and the walls. For a basement, half-inch or one-inch expanded polystyrene foam makes a nice break.
Thanks, WayneL5. Now all I have to do is engineer a sluice to convey the mud from the upstairs landing down the basement stairway, to be further distributed via wheelbarrow! No room for concrete pumps here, and besides, this was an old gold mining town, so building some sort of sluice would be historically appropriate.
I can just picture this contractor trying to cover his sins, "I guess I just tried too hard and did too good of a job compacting that base under there, just can't fight Mother Nature, She heaved it right up there!"
BS
Either he used poor material under that slab, and didn't pay attention to drainage, or the footers under tha foundation were poured on disturbed and uncompacted soil so they settled, leaving the impression that the slab lifted.
I do know of an instance where the slab lifted when the soil under it was right. Since it was built in an area of wet clayu soils, the builder dig out all the infill to four feet deep, and laid in structural fill, compacting it well. He did a fine job with soils, but ignored drainage, so all the surronding wet soil drained to his gravel hole inthe ground, and in under the slab. Since it was unheated, it froze in winter and lifted the slab.
your Question seems to be more about whether the PEX tubing can tolerate minor heaving and shiftiung of crete. the answer is a qualified yes. There are few synthetic materials touger than PEX. It is even used under exterior driveway slabs to facilitate ice removal/prevention.
using separation screeds at the perimeter of the slab is a standard item that has lots of advantages.
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