Concrete slab Prep- Grading for 2" foam board help

[hellspcangel911]

Hi all,
A couple years ago I built my 40x60 steel building and finally now coming back to pour the slab. I'll be doing a 5" slab, with 1/2" pex for radiant, stapled to 2" XPS board, on 15mm steggo VP. What I need help with is how best to prep the subsurface for the 2" foam board. My concrete guys and contractor friends say they just take the skid steer and flatten it with the bucket and its good enough. On a regular concrete pour I would agree as any imperfections, valleys or ridges would just have the concrete accommodate it.
I'm working with a layer of dead sand, I rake it out, check it with the laser level, compact it with the plate compactor, in a grid pattern, softly rake it with a 36" rake to knock down the side tailings the compactor made. But its still not perfectly flat and uniform. I feel like the more I mess with it the worse it gets.....
How flat does it have to be? How forgiving is the 2" foam board? I saw a video of a guy setting up two mason strings as a reference and using a 2x4 to screed the top. I like the idea but for a 40x60 slab thats a lot of work.... I dont mind doing it if you all tell me its worth it.
Is there a better way?

 

[The Cobbler]

I put mine over compacted traffic bond stone . it was raked level, compacted, several shots with a laser level and the foam was laid on top of that . it's been 6 or 7 years with no ill effects . I was low 400sqft

 

[Innovate1]

I did foam under the concrete for heat tubing in the floor in the house and just insulation in the shop building. Both have forced air heat and AC so we don't use the house basement floor heat. We also did the attached garage and that works great. I did gravel under the foam and leveled as best I could. Still had some cracking of the board from walking on it installing tubing where it wasn't completely flat. I would think it would be better/easier to get flat with sand. The board has a little give to it but not much.

 

[ConCretin]

The insulation board will conform to gentle undulations in the base. Get it generally level with the equipment and then level it with the flat side of a loam rake. While the thought of voids between the insulation and the base understandably invoke images of cracked and broken concrete, it's done every day, often with far less attention than you are giving it. With a little diligence, you'll be fine.

 

[Mattilac]

I would definitely want to get it flatter than what can be seen in your photos. Perhaps those don't show what your final prep looked like.

I just prepped a 40x64 for foam too. I started by spending a day cutting down the 3/4 recycled fill to the level I wanted, re-compacted that, then brought in two yards of sand to level out any imperfections in that. I think the key isn't so much to get it perfectly flat but to make sure there are no local highs and lows that a 4x8 foam board can't conform too. The board will bend and press down some under the weight of the concrete.

Like yours, mine also sat for a couple years before I got around to doing the floor. I think the wait actually does us a big favor in that the ground naturally compacts/settles some more. I noticed mine had in areas.

Here's what my base looked like when I started prepping it. That's 8-12" of recycled fill (originally put in back in 2021):



I took about 2" out (to account for foam and 6" slab I'm doing), graded it as best I could (endless amounts of back dragging), spot checking with the laser, and compacted again:



This got it pretty good, and I assume a lot of guys would have just started laying down foam at this point. But there were still various spots where I could tell the foam board would be spanning small dips, so in came the sand. I distributed it with the Bobcat but then mostly just brushed it around by hand. Then ran the plate compactor one more time.







I don't know if this is the right way to do it but it's what I did...

 

[hellspcangel911]

THANK YOU! Seriously, thank you everyone, much appreciate the informative words of wisdom and encouragement. Back to it tomorrow!

 

[Rusted Nut]

Hit it with the plate compactor, rake ridges down - you'll be good to go.

 

[Mattilac]

A lawn roller works really well, possibly better than a rake, to remove the ridges left by the compactor too.

 

[JessieAMorris]

I was in a similar boat 6 months ago. I had road base as my grade and was worried that I wasn’t going to be flat enough. If I did it again I’d go
With some sand as the final inch or so to make it a little easier to be flat.

I had some ~1/2 low spots under the foam I think, but the concrete seems to have pushed it down prior to cure. 6 months in and no cracking on my 4” concrete and I’ve driven a 8k forklift on it a little bit.

 

[jack stand]

If that sand is excessively dry you might wet it down some. Don't forget the plastic under the foam.

 

[hellspcangel911]

A lawn roller works really well, possibly better than a rake, to remove the ridges left by the compactor too.

Great idea, I'll give it a pass when im home and post some pictures, Thank you

 

[hellspcangel911]

If that sand is excessively dry you might wet it down some. Don't forget the plastic under the foam.

Great point, because I have all my stuff on the second floor I've been trying to minimize the moisture, but a sprinkle is a good idea. I have 15mil Stego VP ready to go down

 

[The Cobbler]

Don't forget the plastic under the foam.

my understanding was with foam you don't need poly if you tape the seams ?
I used it with mine, but I thought it was ok to miss that step?

 

[hellspcangel911]

my understanding was with foam you don't need poly if you tape the seams ?
I used it with mine, but I thought it was ok to miss that step?

From what I've read the consensus is "why not" add it in there as its your only chance. My attached garage was built circa 1990 and I believe they didnt use a VP. I didnt think of it when I tiled the floor shortly after we bought the house and theres so much water that comes up through the 1/8" grout lines it drives me nuts.

 

[The Cobbler]

From what I've read the consensus is "why not"

I agree, and I have zero visible moisture

 

[beatuptruck]

In 2020 I did pretty much the same thing.
42x64 - 5” slab with pex stapled to Type 2 EPS foam. I spent days leveling/raking/compacting Recycled concrete. Then using sand to fill in low spots. By the time I was done with it you could walk on the foam and it would not even deflect at all since it was completely supported by the Base material.
I chose to install vapour barrier over the foam so that I could adjust the base with sand while laying the foam boards.
4 years later driving my 12000LB Forklift over it all the time and no unwanted cracks.
Probably overkill but I felt good about the job I did!

 

[jack stand]

my understanding was with foam you don't need poly if you tape the seams ?
I used it with mine, but I thought it was ok to miss that step?

If you're site is absolutely high, dry and well draining your ok - from what I remember reading. The thought of ground water almost washing your heat away points me in the direction of poly.
Spending maybe another dollar on a hundred dollar investment seems like a pretty good idea.

 

[billconner]

Was consensus vapor barrier on top of foam? I know Joe Lstiburek insists on that.

If you do that, how do you anchor PEX? I may have read those punctures don't matter but just a little hard to picture.

 

[jack stand]

Don't know who that guy is, but why wouldn't the point be to isolate moisture/water from your complete ground insulation - radiant "system"?
To a very minor degree all of the foams are hydroscopic and whatever water that might be absorbed before the 'crete is cured is a one time event that will evaporate. Ground moisture is reoccurring if you're site is not bone dry.
I'm sure much of this (position of the poly) is splitting hairs and inconsequential, I'm just a common sense type with no science background and could be wrong.
Water is a great carrier of heat, why would you want it in contact with the insulation and heating the soil vs the concrete.

 

[billconner]

I believe the study and research shows the foam will stay wetter with vapor barrier below it than on top. Makes sense to me and I find a lot of support for that position. That's how I chose.

Here's one of Joe's papers on the subject: https://buildingscience.com/documents/insights/bsi-059-slab-happy

Here's what Stego says: https://www.stegoindustries.com/blo...b-insulation-above-or-below-the-vapor-barrier

I like Joe's opinion of why bother with chairs and mesh. Good enough job on base and you won't need mesh or rebar.

 

[jack stand]

IDK, both of these guys seem to be with a company with something to sell. Maybe the one guys reason is the lower the costs of radiant in slab to increase access ($$) for homeowner/builders.
My tubing is tied to a 2x2 rebar grid with 6x6 mesh sitting near the middle of the slab. It worked well for fastening that tubing especially in a 2 story area in the house that the radiant designer called for a 6" spacing.
Bill I don't understand his position on the poly above the foam but don't presume to be a scientist. How would this add water and what is the source ?

 

[billconner]

I don't presume to beca scientist. Joe is though. I've heard in speak and have talked to him. Based on what he has presented, I'd presume he's cut both floors apart and measured it. You have the bleed water to begin with, and any water through the slab, and no where for it to dry to. And like walls, the ground is colder so condensation is possible.

GBA posters agree; https://www.greenbuildingadvisor.com/question/sub-slab-vapor-barrier-above-or-below-insulation

 

[Tbender78]

Why not spray foam the floor. Then you wouldn’t have to use the plastic vapor barrier. Staple the pex right to it.