# plastic under slab...



## woodworkbykirk (Sep 17, 2008)

today we started a basement reno. the existing slab is really bad for level and the h.o wants in floor heat.. so were going to do a new slab over the old . 

basically did a 2" foam treatment, then plastic then the mesh. then the question came up. does the plastic go on top of the foam or below, be it new construction or for a reno to make for a flat floor

one of our guys says below the foam for dampness, i say over the foam to have it closer to the warm side, someone else agreed with me but added in new construction having the plastic on top prevents it from getting torn by the 3/4 stone. 

whats correct?


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## cleveman (Dec 28, 2007)

I think it will more likely get scuffed up by the workers, so I would put it under the foam. I don't think the condensation argument holds any weight, because the foam will slow down the difference in temperature either way.

Whatever you are most comfortable walking on.


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## WarnerConstInc. (Jan 30, 2008)

It is used mostly to help with Radon around here.


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## griz (Nov 26, 2009)

I can't recall a detail where the plastic wasn't under the foam.
Pretty nasty to walk/work on if it was on top.


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## cleveman (Dec 28, 2007)

I want everyone to know I'm in the middle of Radon alley and I installed a Radon Ejaculation System (RES) on the last place I did. I recommend this to everyone, because the cost of an afterthefact RES (ATFRES) is costly.


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## mudpad (Dec 26, 2008)

cleveman said:


> I want everyone to know I'm in the middle of Radon alley and I installed a Radon Ejaculation System (RES) on the last place I did. I recommend this to everyone, because the cost of an afterthefact RES (ATFRES) is costly.


Really?


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## WarnerConstInc. (Jan 30, 2008)

cleveman said:


> I want everyone to know I'm in the middle of Radon alley and I installed a Radon Ejaculation System (RES) on the last place I did. I recommend this to everyone, because the cost of an afterthefact RES (ATFRES) is costly.


They are not very cheap for what it really is.

The worst part is the noise from the motor.


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## griz (Nov 26, 2009)

I'm afraid to bite on this one.:whistling


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## mudpad (Dec 26, 2008)

I have yet to figure out the logic of excavating, proof rolling, compacting and grading a building pad + or - .10, testing for bearing capacity, and then right before you pour the slab you put 2" of styrofoam between the hard compacted subgrade and the slab. 

Just doesnt seem to make sense to me.


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## mudpad (Dec 26, 2008)

griz said:


> I'm afraid to bite on this one.:whistling


Wife says that must really shoot it out of there!:thumbup:


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## WNYcarpenter (Mar 2, 2007)

We just did two floors with plastic over the foam, then mesh, then radiant.....I believe it is recommended to have a 12" over lap of plastic for radon venting.

I don't know the science, but that's how we did it....it's also recommended to foam any floor penetrations after slab(radon)......


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## woodworkbykirk (Sep 17, 2008)

the plastic is lapped, and taped. were not dealing with excavation and tamping. were simply going over the old slab which is really out of level. doing a new slab for new in floor heat and to level the floor. 

headroom's not an issue as the ceiling is 11 ft right now


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## Winchester (Jun 29, 2008)

I really don't think it really matters as long as it's there.


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## greg24k (May 19, 2007)

If you using radiant heat, why do you need the foam under the slab. Before you pour the new slab, wet the existing concrete real good and cover it with plastic over lapping edges 6" and leave it over night. Then poor you new slab, this way the existing concrete will not suck the water from the new pour and your new slab will never crack.


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## Jackpine Savage (Dec 27, 2010)

greg24k said:


> If you using radiant heat, why do you need the foam under the slab.


Because it gets expensive pumping BTU's into the ground.


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## greg24k (May 19, 2007)

He already has a slab under it, and he is installing radiant heat. If anything, he would need under slab Insulation with radiant heat pex, they have a flexible insulation that is about 3/8 thick with closed cell polyethylene on both sides, which will protect the heat transmission foil from concrete chemicals. Flex insulation will contour to the floor and lay flat on it's surface. The OP said he has uneven floor, if you install 4x sheets of rigid foam, you will have voids between the existing floor and foam and your new floor will crack and it will defeat the purpose.


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## woodworkbykirk (Sep 17, 2008)

with the foam we put down one full layer of 2" foam then where the floor dives down by 6" we put down extra foam to take up that space, this way were not using up as much concrete and wont be heating that much more either.

the existing slab is 60 yrs old so its not insulated thus adding the foam


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## greg24k (May 19, 2007)

How high will you be lifting the floor? Old houses are not known for head room in the basements...


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## TimelessQuality (Sep 23, 2007)

woodworkbykirk said:


> headroom's not an issue as the ceiling is 11 ft right now


:whistling


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## woodworkbykirk (Sep 17, 2008)

11ft ceiling right now. we added 2" of foam with 4" at teh lowest point to fill the void. then pouring a 3" slab i believe we'll prob use 5/4 x 4 for strapping so the plumbers can staple to the joists. so were still going to have 10' of head room. if that isnt enough... someones gotta lay off the growth hormones


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