# Styrofoam under concrete slab



## [email protected]

Hi all, I have a customer who has asked me to do something that we usually dont do here in east Tn. He wants me to have styrofoam laid down under the basement floor when we pour it. I have asked around locally, but it doesnt seem to be real popular here. I have heard that the slab will most likely have lots of cracks in it.
If I were to do this, I would at least put 1/2 rebar on 16" -2' grid. 
What are some concrete finishers experience with this.
Thanks
Randy Belk
Athens, TN


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## genecarp

Putting styrofoam under the slab is fairly common here. If the base is preped and tamped properly, and the correct concrete and wire is used, you will crack no more or less than you would without the foam. use plastic VP under the foam, and you will end up with a nice warm dry slab, GMOD.


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## neolitic

Agree with Gene, 
the prep is the thing. :thumbsup:
10-10-6-6 is good for most basements.
The #4 16oc seems a tad much to me.


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## CONCRETE MIKE

genecarp said:


> Putting styrofoam under the slab is fairly common here. If the base is preped and tamped properly, and the correct concrete and wire is used, you will crack no more or less than you would without the foam. use plastic VP under the foam, and you will end up with a nice warm dry slab, GMOD.


:thumbup:I agree,why rebar? Are you going to drive a tractor trailer on it? If u are going with wire no10 wwm is good,it comes in 5'x150' rolls.


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## griz

A more common detail I have seen is using extruded foam. Same principle as Styrofoam. This is a very common detail in a commercial freezer. If you are intent on using rebar go with a #3. Although like the others have said wire will work just fine.


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## Rockmonster

Agree with gene here as well....we use 2", it's got that T&G going, we tape it some spots to keep it tight...if the base is nice and level, it is a walk in the park....wwm, run the radiant tubes, and you have a warm, dry, easily heated space....


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## BuildersII

We had a really interesting job in an old school converted into a halfway house back when I was working for a drywall company. It was a second floor auditorium with a sloped floor down to a stage where they needed a flat and level concrete floor. Obviously adding all that weight to the floor would cause structural issues and cost a fortune in concrete, so we used a foam filler before having the concrete poured.

The way it worked was we had to survey out the auditorium and figure our finish floor height, and then we sent the numbers to a company that pre-cut all the foam with the proper slope. The thing is that they sent us a bunch of 16" wide foam pieces with a sloped side to be glued to the existing sloped floor, and we installed each row of foam pieces 4" apart. The idea is that the concrete would fill those 4" gaps and act as solid support for the layer of concrete going on top of all the foam.

I just thought it was a pretty ingenious way of saving weight and cost, while giving a solid and durable surface.


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## Cole82

Doing it right now. I agree with every thing every body is saying. The only thing different is I use expanding foam on the seams. It really glues everything together and makes it a uniform insulattion value.

Cole


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## stuart45

It's part of our Building Regs to insulate under the floor slab, and round the perimeter as well.


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## TimNJ

I put 4" styrofoam under my slab back in 1992. No cracks or problems.


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