Styrofoam under concrete?

[64dragnwagon]

I am sure it has been discussed and I know many people do it BUT, I don't understand how having a product witch is flexible under a slab of concrete is a good thing. How does the slab keep from cracking when it is floating on top of styrofoam? Also it seems like when you lay it on top of the gravel base if there are any small air gaps (which I am sure can't be avoided) what keeps it from settling over time and cracking the concrete when heavy cars are driven on it. I understand the benefits of the styrofoam as a vapor barrier and insulation but it just seems like a problem. Please explain why I am wrong.

 

[oilslick]

good question!

 

[ForceFed70]

It's not styrofoam but another foam board product.

The board is actually very stiff and not flexible. For example: You can walk on it without leaving footprints (unless your boots are dirty).

 

[64dragnwagon]

What happens to it when you apply 3000 psi to it?

 

[Ramthisone1]

I'm listening too...

 

[64dragnwagon]

I just had a thought, is it primarily used or only used with in floor radiant heat? I see more negative issues than positive with using it. What is the real value of it? Also I don't think it is cheap.

 

[Zick]

I don't have any scientific answers for you except that it's been done thousands of times without any issues. You have to get the right kind of foam board as they are not all the same.
http://building.dow.com/na/en/applications/building/foundations/foundationslab.htm

 

[pop pop]

The foam board will never see 3000#/ft. Probably heaviest load will be 100#/ft, maybe a little more. The stiffness of the concrete will distribute the load over a large area.

 

[Beaumont67]

In our part of Canada, post holes (for fense & sono tubes) have to be dug to 4 fy. deep, below frost line.

Co-worker's dad had an implement shed poured in 2 sections, years apart.
- original slab on gravel & all wavey cracks
- remaining slab of concrete poured over foam...never cracked

Swimming Pool installers put foam down the inside of the footing, to keep concrete from cracking...due to frost expansion.
When concrete wants to lift from underneath soil pressure, the foam contracts and saves the substraight.

In 1991, I built a 24x30' garage with packed chips & dust / 1" foam / rebar & wire mest / concrete polished & saw cut.
- in the spring, my floor does not sweet
- still zero cracks
Foam edges not taped and no 6mil vabor barrier plastic used either.

I talked my neighbor into using foam, as well...he thought I was nuts (at first), but finally took my suggestion. His 1991 garage floor is free of any cracks / and no moisture problems.

I am sold on using foam under a garage concrete floor.

 

[DanarchyCustoms]

I actually have seen what you are talking about on the History Channel (Modern Marvels if I am not mistaken). They used large pieces of styrofoam to support roadbed, railroads and airport runway. The advantages slip by me at the moment but everywhere I searched said that it was too limit stress because of freezing and thawing.

Just a quick link:
http://www.marinij.com/ci_6151642

Its actually pretty neat, just too bad this stuff sticks around for a gazillion years...

 

[formula_pilot]

4 inches of Dow & OC 25psi foam under my new floor (June 2011). Where the 2 post lift columns are going to go, I used 100psi sheets, but that is likely overkill. Also went with 50 psi foam right at the door openings, but that may also have been an unnecessary expense. Because of being concerned about having the slab foat over foam, I went with 6-8" thick and 4000psi concrete for the floor. No cracks yet and I had a heavy lumber yard delivery truck drive right in to unload an entire unit of shiplap pine. None of my vehicles will ever load the floor like that, so it was a good test, no cracks. A few extra yards of concrete is cheap insurance when you look at the total cost of a new building.

 

[gatchel]

From what I have been told by an "engineer" friend of mine...You don't need any more than 25psi foam board under concrete in a residential garage. If you are parking a 747 or a tank in the garage then that would be a different story. It has to do with the way concrete displaces the load across the whole slab and the slab presses down on the foam. I don't have the math or the formulas to support this at this time.

 

[dirttracker18]

The answer is very simple.

Your slab is supported by the compacted gravel based not the foam board (do not use styrofoam, get proper foam panels).

The panels are between your concrete and your base and the compression of the panel is minimal. There should be no "flexing" of the panels as that would mean your base was not compacted correctly.

As stated, your pad may be heavy but the actual pressure on one spot is not that high. The weight of your pad is spead over it's entire area.

 

[K'ledgeBldr]

Styrofoam- is a trade name for polystyrene by the Dow Chemical Co. And when most people think of 'styrofoam', they think of white beads compressed together into the shape of a coffee cup. However, there are all kinds of different polystyrene.
What is being referred to here is better known as XPS.
Here's a product sheet from Dow:
http://msdssearch.dow.com/Published...foam/pdfs/noreg/179-04454.pdf&fromPage=GetDoc

 

[tmars]

I too had the same thoughts, I did it though. I put down a 6 mil vapor barrier then the 25 psi dow insulation boards. I did pour a 5 inch thick pad and as of now not a single crack. Not even in the stress cuts. I also poured a foundation first, so I have concrete foundation then foam then concrete on top of it. I am very pleased so far. I didn't use rebar, I used fiber in the concrete. The only reason I used mesh was to make sure the pex didn't float and it didn't cost much

 

[Highbeam]

Ack, I had this big thing written up and then it went away. Anyway, short and sweet.

Typical soil is about 2000 psf strength. That's only 13.8 psi. Gravel is also very flexible and only as strong as the subgrade. As such, 15 psi rated foam is plenty strong for beneath the slab.

The weight of the concrete itself only applies 0.5 psi to the foam. The rest is for the live load that you drive onto it.

Back to the soil. Drive on a gravel road, this road is only supporting your vehicle with 13.8 psi of bearing strength. Do you sink to China? No.

I propose that more EPS (styrofoam) has been placed under concrete than the more expensive XPS (pink or blue). Both can be purchased with similar strength ratings and both are adequate underslab. The only real difference is that the EPS is 20% lower inulative value. We build roads and railroads on top of huge amounts of the EPS as a lightweight fill. Pink and Blue is homeowner stuff.

Concrete is a rigid pavement. This means that the live loads such as tireprints that you apply to it are distributed over a large area. Much larger than the gravel road. You can pour a slab over a swamp and really put a big load to it without failure.

 

[DamienM]

Agree.. I ran blue board stright down below the frostline on a pre-existing slab and works like a champ. Also in for insulation I used 2lb closed cell foam to make a water tight seal and I seem to loose no heat at all in the floors or the walls when it -5 degrees.

On new home construction it is now code to do this on any foundation for energy loss.

 

[MDSPHOTO]

I too had the same thoughts, I did it though. I put down a 6 mil vapor barrier then the 25 psi dow insulation boards. I did pour a 5 inch thick pad and as of now not a single crack. Not even in the stress cuts. I also poured a foundation first, so I have concrete foundation then foam then concrete on top of it. I am very pleased so far. I didn't use rebar, I used fiber in the concrete. The only reason I used mesh was to make sure the pex didn't float and it didn't cost much

We just did the same thing in the basement of our new home.

 

[tdkkart]

Look at the data sheets, Dow Foamular 250 pink foam board is good for 25lbs/sq in, or 3600lbs/sqft. No issues whatsoever with putting it under a floor. The only conditions would be to have the fill under the foam very flat and well compacted. On a flat surface the foam board has more compressive strength than bare earth.

Is it important to have it under the floor?? You bet it is, and I have my own little bit of proof:

When I bought my place, the 30x40(1200sqft) pole building was divided in 2, 24x30 was totally unfinished with gravel floor and no insulation, while the remaining 16x30 was finished as an insulated room with in-floor hydronic heat, all done up nice with the exception of not putting any insulation under or around the perimeter of the floor.

I heated the 16x30 room as is for the first winter, then come spring I finished the rest of the building, including 2" of foam board under the floor, and 4" thick x 24" deep around the perimeter. R19 walls, R30+ ceiling.
I now heat the entire building for what it cost me to heat just the original 16x30 room that 1st season.

 

[CaptRick]

I used slab shield. It comes in 5' X 125' rolls. It's like huge rolls of sill seal with reflective material sandwich in between the layers. I think it came in 1/2" thickness. Easy to transport, Easy to cut, Easy to put down. (roll it out and tape the seams)

 

[thammel]

Let me add some technical points to this discussion:

1) as a prior poster stated, the 5" think concrete adds very little weight PER SQUARE inch to the 25 PSI capable foam board. Think of it this way, it's about 80 pounds per 0.5 cubic feet of concrete. That's about 864 cubic inches. So at 5 inches thick, that would cover 173 square inches, or 80 pounds for the entire 173 square inches, which is about only 0.5 pounds per square inch (at the 5 inch thickness. This means the foam is WAY over-capable.

2) For a small 20'x20' pad with 25 psi foam under it, the foam can support a total of 400 x 144 x 25 = 1.44 million pounds of weight. The pad weighs only about 30,000 pounds.

3) Don't get confused about 3000 psi or 5000 psi concrete. This is its strength and has nothing to do with its weight or mass.

Tom

 

[aar0s]

What thickness of foam are you guys using under your pads? Where i work we have it from 3/4 with a ship lapped edge to 2 inch S/E.

 

[tdkkart]

What thickness of foam are you guys using under your pads? Where i work we have it from 3/4 with a ship lapped edge to 2 inch S/E.

Most use 2" as I did, if I had to do it again I would seriously consider doing a double layer of 2", and 4" vertically at least 24" down all around the outside.

 

[OUTRIGGER]

Lift motor rooms often have a steel reinforced concrete floor over the top of the liftwell , then 2" of slab foam with a 9" + thick raised plinth of solid concrete. The motor , gearbox / traction sheave sits on top of the plinth , more to insulate sound & vibration from the building structure - still a fair loading. I have known some plinths over 40 years age & no problems with the foam or concrete.

 

[KELLHAMMER]

On my floor I used 2" thick dowfoam under the entire floor and 3/4" on the edges. The theory is the slab will heat up much quicker and minimal loss to the ground. It's been up and running for 5 years and is much more energy efficient than any other heat system for a slab on grade. The rising warmth heats the second floor as a bonus. We run the system at 65 deg at the highest thermostat setting.

 

[kdtjb]

This type of insulating is not just for in floor heat, but all types of heat. Forced air, over head radiant also. Remember this, warm or hot air rises. Like a hot air ballon. But heat moves from warm to cold, no matter which direction. up, down sideways. So insulation is a good thing where ever you put it.

 

[finn]

The same principle that allows you to walk on top of snow while wearing snowshoes applies to concrete floors and styrofoam.

My shop floor was used to park Michigan Train logging trucks in overnight. The 6” concrete over insulating foam survived that.

The surface is a little scarred up from parking steel tracked bulldozers, though.