Insulating an existing slab prior to adding PEX

Phil_8

Working on a home that has one room (the family room that is slab on grade construction). The remainder of the house has a basement. The room which is slab on grade construction will have porcelean tile as the final floor finish. Thus it is necessary that the slab be insulated before installing the PEX. Should I lay down pressure treated sleepers onto the exiting slab and then 2" rigid foam between the sleepers? Is 2" foam depth adequate? Is there a recommended manufacturer or type of foam?
I am concerned about heat conduction through the 2 x 4 sleepers that will be in direct contact with the cold slab floor.
I assume that I can attach the PEX to the 2 x 4 sleepers? I assume that it is it best to pour mud atop the PEX rather than to install cement board on top of and directly in contact with the PEX. Then the porcelean tile with thin set.
If this has been previously discussed, I apologize and perhaps someone can direct me to that discussion.

Thanks

nibs

Use Extruded styrofoam, usually pink or blue, lay it on the existing floor and tape the joins. I would not use wood sleepers, as they are not necessary. You can buy staples to secure the pex directly on the foam. Google (Malco FBSN2 Extra Sharp Dual-Barbed 2-1/2-Inch Foamboard Staple) I made inserter out of a short 2x4 scrap by drilling a hole the same diam as the outside of the staple, then cross cut the 2x4 so that you have 1/2 the hole. if you did it right, the staple will lodge so you can push it in to the foam.
There are also vendors of foam with various methods for holding the pex in place, I acquired some that was left over from other buildings and used it to good effect for U turns at the walls.
If a big area you should make control joints, rather than cutting them in afterwards. Let the cement cure for 28 days min, and if possible turn up the radiant temp slowly over a week or 10 day period. Have written a few times about concrete design mixes for thin slab pours over pex, if interested you can look em up on the wall.

Larry Weingarten

Hi, I do think this has been discussed before, but I'd be inclined to put your 2" foam down and no sleepers. Then tie the tubing to whatever steel you're using to prevent cracking. I think @hot_rod has mentioned ways of doing that connecting. I'd also pour mud rather than cement board as that board would interfere with heat transfer.

Yours, Larry

nibs

On a thin slab, I would not recommend reinforcing steel. If you lay it on the foam, then tie the pex to it ( use zip ties not wire)
on a 2" slab, you cannot raise the steel mesh as you do not have enough cement depth, most engineers require 2 inches of cement, minimum surrounding any metal reinforcing. On a 4 inch slab, reinforcing mesh can work well.
Order 7,000 psi mud with fibers entrained and insist on minimum water content, if cured properly, you should not have a cracking problem.
IMO you should have 1 1/2 inches of mud above the pex.