Styrofoam Under Concrete Slab

Thread Tools Search this Thread Display Modes
Old 03-25-2010, 04:18 PM   #1
randybelk@comca's Avatar
Join Date: Sep 2008
Location: Athens, Tennessee
Posts: 50
Rewards Points: 25

Styrofoam Under Concrete Slab

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.
Randy Belk
Athens, TN
randybelk@comca is offline  

Warning: The topics covered on this site include activities in which there exists the potential for serious injury or death. DOES NOT guarantee the accuracy or completeness of any information contained on this site. Always use proper safety precaution and reference reliable outside sources before attempting any construction or remodeling task!


Old 03-25-2010, 04:50 PM   #2
genecarp's Avatar
Trade: LI,NY designer, new homes, renovation work, concre
Join Date: Mar 2008
Location: Long Island, NY
Posts: 5,425
Rewards Points: 2,000

Re: Styrofoam Under Concrete Slab

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.

genecarp is offline  
Old 03-25-2010, 04:55 PM   #3
neolitic's Avatar
Trade: carpentry/remodeling/"Yes M'am we do"
Join Date: Apr 2006
Location: Beech Grove, Indiana, Birthplace of the "King of Cool"
Posts: 11,706
Rewards Points: 2,000

Re: Styrofoam Under Concrete Slab

Agree with Gene,
the prep is the thing.
10-10-6-6 is good for most basements.
The #4 16oc seems a tad much to me.
Put your location in your profile!
( seems there really are dumb questions)
neolitic is offline  
Old 03-25-2010, 09:34 PM   #4
Concrete Mike
Trade: concrete
Join Date: Jan 2010
Location: ohio
Posts: 285
Rewards Points: 250

Re: Styrofoam Under Concrete Slab

Originally Posted by genecarp View Post
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.
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.

Last edited by CONCRETE MIKE; 03-25-2010 at 09:42 PM.
CONCRETE MIKE is offline  
Old 03-25-2010, 09:57 PM   #5
griz's Avatar
Trade: General Contractor
Join Date: Nov 2009
Location: CA
Posts: 13,585
Rewards Points: 4,334

Re: Styrofoam Under Concrete Slab

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.
griz is online now  
Old 03-25-2010, 10:01 PM   #6
Rockmonster's Avatar
Trade: Masonry
Join Date: Nov 2007
Location: Nantucket MA
Posts: 640
Rewards Points: 1,966

Re: Styrofoam Under Concrete Slab

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....
Rockmonster is offline  
The Following User Says Thank You to Rockmonster For This Useful Post:
genecarp (03-26-2010)
Old 03-25-2010, 10:47 PM   #7
BuildersII's Avatar
Trade: General
Join Date: Dec 2009
Posts: 323
Rewards Points: 250

Re: Styrofoam Under Concrete Slab

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.
BuildersII is offline  
Old 03-26-2010, 07:00 AM   #8
Cole82's Avatar
Trade: GC
Join Date: Nov 2008
Location: Cedar Rapids, IA
Posts: 4,065
Rewards Points: 2,122
Send a message via AIM to Cole82

Re: Styrofoam Under Concrete Slab

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's Custom Creations
Cedar Rapids, Iowa

The boss of awesome
Cole82 is offline  
Old 03-26-2010, 09:09 AM   #9
stuart45's Avatar
Trade: Brickwork
Join Date: Oct 2009
Location: Somerset, England
Posts: 807
Rewards Points: 570

Re: Styrofoam Under Concrete Slab

It's part of our Building Regs to insulate under the floor slab, and round the perimeter as well.
Styrofoam under concrete slab-detail-20-20gf01.gif
stuart45 is online now  
Old 03-26-2010, 10:42 AM   #10
TimNJ's Avatar
Trade: Remodeler
Join Date: Sep 2005
Posts: 3,088
Rewards Points: 2,754

Re: Styrofoam Under Concrete Slab

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

TimNJ is offline  

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
400$ for a 14X7 concrete slab? Coastal Pros General Discussion 7 12-13-2009 01:00 AM
how much to charge. concrete curb demo. tie new curd to 6" slab Evan03 Construction 12 11-28-2009 12:54 PM
Concrete overlay over existing garage slab daft_GC Concrete & Paving 10 11-19-2009 07:42 AM
site preparation for future concrete slab on clay soil RFTTTE Masonry 8 07-21-2009 09:32 PM
concrete slab on paved drive? durham Concrete & Paving 7 09-13-2008 10:28 PM

Join Now... It's Fast and FREE!

I am a professional contractor
I am a DIY Homeowner
Drywall Talk is for

At we cater exlusivly to professional contractors who make their living as a contractor. Knowing that many homeowners and DIYers are looking for a community to call home, we've created DIY Chatroom is full of helpful advices and perfect for DIY homeowners.

Redirecing in 10 seconds
No Thanks
terms of service

Already Have an Account?