Floor heat under slab questions
I’m installing floor heat in a concrete slab, in zone 4a. we are planning on insulating to R-15 both horizontally and vertically, on top of 6″ of crushed stone and a network of 4″ PVC drain pipe. Following the basement articles here we would put 3″ of EPS/XPS foam, and then polyethylene. The slab will be about 3″ or so.
I am looking to minimize floor heat errors, by, getting something that the pex tubes can stick into. Many products for this purpose are EPS foam products: https://www.creatherm.com/documents/prod_spec_sheet_s45_t45_u45.pdf
I know the advice here is for the polyethylene over the foam, not under it, to avoid issues of trapped moisture, so my question is, are these products really ok to use? They claim that if you make them thick enough, they act as a vapor barrier (as is the case with thick enough EPS foam), but, in reality, it wont be as good due to the many seams that will be created.
Can i just use it? Is there a polyethylene version of these products? I can’t seem to find one?
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Replies
Ryan,
The reason that the poly is suggested to be placed over the foam is to:
- Minimize the bleed-water that may accumulate under the foam when the concrete is poured, which can take years to dissipate.
- Keep that water available to aid the concrete while it is curing.
If the sheet underlayment you are contemplating does these things - which it appears to - then having the poly under the foam is fine.
Thanks for the explanation. Just to clarify, your saying its ok to use these products directly on top of the polyethylene vapor barrier?
So from top to bottom the sandwich is:
3" concrete
floor heat system + pipes
polytheline vapor barrier
3" of EPS/XPS Foam
6" Crushed stone + pipes
Earth.
Yes, but I'm going further than that and suggesting you could move the poly to below the foam if you wanted to.
I see. To me, it seems safer to put the polyethylene over the foam, as this site + the great lstirubek generally recommend, as long as it’s still safe to use this material above it. Ideally someone would just make a polyethylene version of these track systems..
Right, but Joe L and GBA recommend it for the reasons I outlined in post #1. If those conditions don't apply then the poly can go where ever you want it to.
Why do you think it isn't safe to use the creatherm above the poly?
Usually the pipes for an in-slab radiant heating system would be embedded within the concrete slab, not placed underneath. You want to suspend the piping within the slab. There are preformed wire supports normally used to support rerod that can be used for this purpose.
Bill
With only a 3” slab I highly doubt it will matter that much, also this is much less labor intensive. Also a worse mistake is getting the layout wrong, leading to cold spots.
Leaving it under the slab might increase the risk of failure in the future. I’d try to at least get the piping all inside the slab, even if it’s not all in the exact center.
You could probably cut pieces of remesh (a welded grid of reinforcing wire) cut into strips and used on edge in rows to create a set of evenly spaced supports. Use steel wire to tie everything together.
Bill
Ryan,
There is nothing wrong with putting the pipes under the concrete, it is done all the time. There is also foam supports with pipe grooves you can get and replace some of your EPS, might be cheaper than EPS+creatherm. There are staples for attaching the PEX to standard foam but they are more labour to install then the panels, I would only recommend it for DIY.
Akos,
Thanks, What is an example of the product your discussing ? I’ve seen uponor fast trak..
I’m thinking about laying the pipe myself to save $$. I already have layout software..
I’m considering how much effort it would be to tie down if there is rebar raising the thing up 1.5” or so.
Ryan,
For the foam PEX tracks:
http://www.nudura.com/divisions/nudura-products/integrated-series/hydrofoam
Stapler:
https://www.pexuniverse.com/foamboard-staplers
The hard part with the staper is getting a good grid, lots of chalk lines.
>>“I’m considering how much effort it would be to tie down if there is rebar raising the thing up 1.5” or so.”
It’s nothing you won’t be able to handle. You’ll need something sturdy to anchor the end of the pex as you start, maybe a temporary concrete block or a short piece of rerod hammered into the ground. Once you’ve tied off the end, you can unroll and tie down as you go. Bends sometimes try to lift up, so you might need to anchor those spots better. Straight portions of runs are less trouble. The most important thing is to work in a pattern. As you put more pipe in, it gets in the way of new runs more, so try to work towards a side where you can get out.
Bill