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Old 08-17-2010, 10:08 PM   #1
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2" foam under concrete floor - dips?


I have a basement floor with several inches of sand on the surface as a base for a 3" concrete floor. I have smoothed and leveled the sand as much as I could but once I laid the 2" foam sheets over the sand it was obvious that the sand was not completely smooth as the foam would sag in areas as I walked on it. My question is, will the weight of the concrete cause the foam to conform to the curves of the sand, or will the foam resist the weight of the concrete and cause cracking down the road?

Thanks!

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Old 08-17-2010, 10:17 PM   #2
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2" foam under concrete floor - dips?


ftp://imgs.ebuild.com/woc/C99C023.pdf

http://www.concretees.com/people/bruce/pubs/c980427.pdf

http://www.radiantpanelassociation.o...cfm?pageid=420

http://www.nrmca.org/aboutconcrete/cips/29p.pdf

Be safe, Gary

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Old 08-17-2010, 11:06 PM   #3
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2" foam under concrete floor - dips?


If you use several inches of sand it will move since sand cannot he compacted. It draine well but that is it and there can be some movement until it is restrained by the slab. - Why just a 3" slab? That is not allowed in many areas because it can contribute to the structural system system if it is placed against the wall on on the footing. - Enough of the rant!!

Your walking on the pink panther foam is a dynamic load that is applied to an unconfined base that drains well, so movement can be expected. Your concrete basement "floor" is there to spread out the minimal vertical loads and confine the base. I hope you put down a vapor barrier under the slab, because future uses (flooring, etc.) may require that.

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Old 08-17-2010, 11:33 PM   #4
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2" foam under concrete floor - dips?


Not sure what Gary's link's say, and I sure hope Dick isn't getting too worked up, but here's my suggestion:

Make sure to use foam rated at a minimum 25 psi and take a long board to screed the sand even flatter.
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Old 08-18-2010, 06:44 AM   #5
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2" foam under concrete floor - dips?


Thanks for the suggestions but I am still not sure I have received an answer to my original question? Will the concrete weigh the foam down?

Yes I have a vapor barrier down under the foam, which is partly why it is hard to get things perfectly smooth, as I walk on the vapor barrier my foot prints disturb the sand.

Yes I screed the sand with a 2x4 before laying the vapour barrier, but there are still dips in the sand.

3" slab is minimum code in Canada and is the most common basement slab depth poured in my area (4" only when using in-floor heating).

The foam is type II EPS (R-8) which is the most common foam to lay under slabs in my area.

I realize that sand is not optimal, but it's there and since this is just a residential basement floor, I doubt it will see anything near the stress caused by driving a forklift over it.

Thanks.

Last edited by axiom10; 08-18-2010 at 06:59 AM.
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Old 08-18-2010, 08:41 AM   #6
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2" foam under concrete floor - dips?


If you plan the screeding, vapor barrier and foam so you work out of the space towards the door, you should get it flat. You will need to place some sort of stable pathways over this undulating foam when the concrete is brought in or it will snap and break into pieces under the weight of the wheel barrows.
Why are you using a 2" foam with only an R-8 factor? When I did the rat slab under the kitchen addition, I used a 2" polyisocianurate board with a an R rating much higher then that. The manufacturer recomended plastic on both sides of the material.
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Old 08-18-2010, 07:23 PM   #7
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2" foam under concrete floor - dips?


EPS foam and Polyiso foam are poor choices if XPS is availiable. They absorb and retain water much more than XPS: http://www.inspectapedia.com/Energy/...urate_Slab.htm
http://www.greenbuildingadvisor.com/...oam-insulation
Compare: http://docs.google.com/viewer?a=v&q=...4EyM707m0gPdXQ

Sorry about the earlier posting with just the links. Good- no fork-lift!

Compare your degree days with some listed for the minimum foam suggested: http://www.espenergy.com/insulating_foundations.htm

Be safe, Gary
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Old 08-18-2010, 11:20 PM   #8
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2" foam under concrete floor - dips?


Quote:
Originally Posted by axiom10 View Post
Thanks for the suggestions but I am still not sure I have received an answer to my original question? Will the concrete weigh the foam down?

The concrete will definately weigh the foam down, but it won't help in leveling out the subgrade much. If the foam leaves 2.5" now, I wouldn't expect more than 2.75" when the concrete is poured. I would also expect the foam to snop break a fair amount when placing the concrete.

Yes I have a vapor barrier down under the foam, which is partly why it is hard to get things perfectly smooth, as I walk on the vapor barrier my foot prints disturb the sand.

Yes I screed the sand with a 2x4 before laying the vapour barrier, but there are still dips in the sand.

3" slab is minimum code in Canada and is the most common basement slab depth poured in my area (4" only when using in-floor heating).

3" id the min. per code here as well, but you should realize that's it' merely a minimum, not a maximum.

The foam is type II EPS (R-8) which is the most common foam to lay under slabs in my area.

I realize that sand is not optimal, but it's there and since this is just a residential basement floor, I doubt it will see anything near the stress caused by driving a forklift over it.

Thanks.

If you care what it looks like at all, you'll be sawing/tooling a very tight joint pattern.
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Old 08-19-2010, 08:35 AM   #9
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2" foam under concrete floor - dips?


Quote:
Originally Posted by GBR in WA View Post
EPS foam and Polyiso foam are poor choices if XPS is availiable. They absorb and retain water much more than XPS: http://www.inspectapedia.com/Energy/...urate_Slab.htm
http://www.greenbuildingadvisor.com/...oam-insulation
Compare: http://docs.google.com/viewer?a=v&q=...4EyM707m0gPdXQ

Sorry about the earlier posting with just the links. Good- no fork-lift!

Compare your degree days with some listed for the minimum foam suggested: http://www.espenergy.com/insulating_foundations.htm

Be safe, Gary
According to the article"
"
For comparison's sake, extruded polystyrene (see POLYSTYRENE FOAM INSULATION) has higher compressive strength than polyisocyanurate, but both have similar water absorption properties as measured by standard tests. Both are over 90-percent closed-cell foams."

According to this, both have similiar rates of absorbtion.
Since this material is wrapped, top and bottom in 6 mil plastic and sitting on who knows how many feet of sand, I won't worry too much.
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Old 08-19-2010, 05:10 PM   #10
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2" foam under concrete floor - dips?


I agree, as long as the foam board is covered by the vapor barrier, no problems. If any holes are punched in either board at installation, I'd rather use the XPS:

"Glass faced polyiso goes from
absorbing 14x more water than
XPS, to absorbing 30x more
water than XPS, when measured
using the same method. Foil faced
polyiso goes from absorbing
1.5x more water than XPS, to
absorbing over 7x more water
than XPS." From the "compare" site: http://docs.google.com/viewer?a=v&q=...4EyM707m0gPdXQ

Be safe, Gary

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