Helpful? 0

Is there anything that needs to be added when covering up an existing slab with sand, XPS and 4" more of concrete?

See the attached diagram which shows (from bottom layer up) ground, existing slab, sand, XPS, concrete with heating PEX. The concrete will be polished and be the completed floor surface.

I can't find this scenario anywhere except someone said putting a second slab was ludicrous:-)

This is a remodel, where a one story residence will replace a 2 story house. So the new structure, so to speak, will be built on what was the garage/shop level with a slab 9" below the perimeter foundation. The goal is to bring the finished concrete floor up to level with the top of the foundation.

SLAB ISSUES.jpg340.27 KB
Asked by Caroline Di Diego
Posted Fri, 12/13/2013 - 16:23


4 Answers

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If I were you, I would include a layer of polyethylene between the rigid foam and the new slab.

I don't know how much rigid foam you are planning to install, or what your climate zone is. But it's often a good idea to include at least 4 inches of rigid foam under a slab with radiant tubing.

You also need to include vertical rigid foam insulation at the perimeter of the slab -- but I see that you have raised a question about that insulation in another Q&A thread.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 12/13/2013 - 17:03
Edited Fri, 12/13/2013 - 17:07.

Helpful? 0

Thanks for getting back to me so promptly ~ that's impressive and really appreciated.

I've only seen 2" XLS with 4" concrete in the NW Zone (rarely gets below freezing) The project in question is in the San Juan Islands (Western Washington Sate) ~ however I will investigate the layer of poly ~ would that be placed over the XLS and under the pex pipes for the heating?

My concern with the question was any layer of something required on the old slab, or will sand just be sufficient?

Answered by Caroline Di Diego
Posted Fri, 12/13/2013 - 17:24

Helpful? 0

I assume the sand is primarily there for filling & leveling?

If yes, since have 9" of room to play with, go with less sand, and instead of XPS (pink/blue/green board) instead use 4" of Type-II (1.5lbs per cubic foot nominal density), which should cost about the same as 2" of XPS, but deliver ~R16 instead of ~R10.

Furthermore, you will have saved the planet of some global warming, since the blowing agents used for the XPS have a global warming potential about 200x greater than those used for EPS. And after 50 years when most of the blowing agent for the XPS has finally left the foam, it's R value will have declined to about R8.5, whereas the EPS would be still be delivering R16 performance.

If some ignorant engineer or inspector balks at the lower compression deformation ratings of Type-II EPS (15psi) relative to most grades of XPS (25psi), bumping it to Type-IX EPS (2lbs nominal density) give it the same psi rating. But under a 4" slab and fully supported by another 4" slab, there is no WAY you are going run into problems with the lower rated foam. You could literally park a bulldozer on that slab without breaking it, once the concrete has had some time to cure.

Answered by Dana Dorsett
Posted Fri, 12/13/2013 - 19:31

Helpful? 0

Thanks Dana this is interesting and I will certainly take it under consideration; especially the recycling aspect.

I found this link which adds a little to what you said ~ for others to get some further clarification.

Answered by Caroline Di Diego
Posted Fri, 12/13/2013 - 19:55

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