If you use a product like Crete-heat the pex is walked into the hubs built in to the panel. no attaching to the mesh

The Crete-Heat looks cool, but doesn't it put the Pex at the very bottom of the slab? Like stapling to foam panels.

OP - you are getting yourself all worked up because some people get tied up with the last degree of effiency with no regard to practicality. If you search back through the radiant forum, you will come to a multi page document discussing exactly where the pex should set in a slab. It is interesting to note that ALL the radiant professionals who do it for a living (and have for many years) put the pex on the bottom.
While most agree that there is a statistical advantage to center to top placement, it would appear that the advantage works out to about thirty cents a month for each month of the heating season. That is for an average residential slab. Sailor - did you ever WAG at what that cost was??

Save yourself a lot of grief. Install the pex on a product like smartwall mentions (and there is lots of variations of the theme, just go to your local eps manufacturer and they probably have something similair) or just staple it to the eps.
Use 3/8 to 1/2" bar, 18 to 24 inches on center each way, set it on chairs and have it done with.

If you are going to wheel the concrete in place, put down 2x10 runways but you would be better using a line pump.

Posted By CairoBoy on 23 Feb 2014 06:20 PM
Thank you all-

I'm liking the #3 on dobies. rows 12" apart.

Great stuff on the EPS. Thank you. Is there a brand or two that is preferred?

The topic of PEX and control breaks is interesting. "Sleeve and hold low." I may have tee shirts made...

Brand doesn't much matter- there are plenty of vendors both small & large, but density does matter.  If you're planning to staple the PEX to the EPS use "Type-IX" EPS, which is ~2.0lbs/cubic foot density or you'll popping staples.  If you're planning to tie the PEX to the steel mesh, "Type-II" EPS (1.5lbs nominal density) is the ticket.  You'll pay a bit of a premium for the denser stuff, but it won't cost any more than XPS in most markets- usually less. Type-II is usually about 9-10 cents per R per square foot. Type-IX might hit 12 cents, XPS (even 1.5lb XPS) starts at 12 cents. 

If you're planning a finished floor (even paint or stain) put a 6-10 mil poly vapor barreir between the EPS and concrete.  If you don't plan a finished floor you could put the poly between the EPS and gravel, which makes it easier to install the PEX.  With the poly under the foam excess water during the pour/cure will puddle between the foam poly and take many months to dry through the semi-permeable foam, which can mess up your finish floor, but with the poly between the foam and concrete there are no pools/puddles- the capillary draw of the concrete keeps the moisture well distributed and dries comparatively quickly.

Raz panels and Crete heat are coated on the top of the panel

Yes, placing the PEX on the bottom versus the middle of a 4” slab having 2” of under-slab EPS insulation will cost you 2.5% more in BTUs for the operational life of the system. So a building needing say 50,000 BTU/H with the PEX in the middle will need 51,250 BTU/H. So placing the PEX on the bottom isn’t terrible, but if you factor in the number of operational hours the system will be used and what you pay for your BTUs, the increased operational cost can be quite significant. So one could do a ROI analysis to help make this decision. However, it really doesn’t take that much time or effort to place the PEX in the middle, so we just do it. The last time I checked, we were still living in a free country...so folks can do whatever they choose.

Very good advice Dana with regard to where to place the VB and perhaps another consideration for NOT placing the PEX at the bottom by stapling it to the EPS.

Hahahaha! Wow what a revelation! Technical vs practical payback. Should have been my first question. Glad that's straight in my mind now. Thank you FBBP.

Stapling to the EPS would be great, but the vapor barrier placement becomes an issue as Dana said.

The Crete-heat is cool in that it replaces EPS and the vapor barrier. Wonder what the $ premium is over EPS and Vapor barrier.

Getting the PEX at the top of the slab would be about 5% better than having it at the bottom. Again, a good ROI analysis will set you free and factor in your installation capability/practicality too.

OP - this is one of the products I have used and like a lot. http://www.plastifab.com/technicallibrary/docs/pdf/0406_Radiant%20Floor.pdf
http://plastifab.wordpress.com/2012/10/29/radiant-floor-heating-using-plastispan-hd-insulation/
It is almost the same price as plain eps. Many of the plastic boys have a similair product. They just take their big cube of products and drill holes through it every 8". They then harp cut it to the require thickness. I usually use it over one additional layer and glue the two together offsetting joints.

As Dana mentioned, the issue is finishing the floor. If you don't plan on doing that for a while, it won't be an issue. Also using a water reducer in the concrete mix will less the amount of water you need to worry about. Make sure the concrete has at least cured a month and then slowly add the radiant heat. It will help drive of the moisture.

Another great product to check out. My life just got so easy. A great day for me.

The floor will be finished and the vapor barrier will contact the slab for sure. Good tip on turning up the radiant heat slowly. Thanks for that

I forgot to mention FBBP that for you folks up North using more than 2" of EPS, the percentages I previously indicated would be approximately proportionately less.  E.g, for 4" EPS, 1.3 and 2.5% instead of 2.5 and 5%.

The PlastiFab product requires the Vapor Barrier to be under the insulation. This is OK?

Having VB under insulation is OK if you are not planning to finish the floor for a while as others have indicated. However, the VB is likely more prone to getting lots of holes from sharp gravel during the job and consequently be far less effective as a VB when placed under the insulation.

I thought the VP also had a role in the location of the vapor point or some such?

I understand slower drying of the water due to lack of contact with the porous concrete. Fantastic piece of insight. I have 3/4" hardwood to nail down.

Posted By CairoBoy on 25 Feb 2014 06:13 PM
I thought the VP also had a role in the location of the vapor point or some such?

I understand slower drying of the water due to lack of contact with the porous concrete. Fantastic piece of insight. I have 3/4" hardwood to nail down.

I suspect the term you're looking for is "dew point" not "vapor point", and it's a temperature not a location.

For ground vapor barriers dew point is irrelevant. The moisture content of the entrained air in the soil is for all intents & purposes at 100% relative humidity- at moisture saturation for it's temperature, which is exactly at it's dew point, whether the vapor barrier is there or not. But the air in the conditioned space is warmer, and at a lower saturation, thus a lower vapor pressure than the air in the soil, which will cause moisture to migrate from the soil into the conditioned space, unless blocked by the vapor barrier.  Without the vapor barrier that constant moisture load diffusing through the slab will often require mechanical dehumidification or constant ventilation with drier air to keep the indoor humidity levels healthy for both humans and building materials (like wooden floors/sub-floors or rugs, etc.)

In a wall assembly it's useful when designing the stackup to estimate the layer at which the average winter temperature will be below the dew point of the conditioned space air (typically assumed to be 30-40% @ 70F RH, or 37-45F, depending on climate zone), and adjust the vapor retardency of the intervening layers between the conditioned space and any moisture susceptible materials such as OSB sheathing on a stud wall to keep the moisture accumulation to a save & sane level, or to apply exterior insulation outside the susceptible layer sufficient to raise it's average temp up to or above the presumed interior air dew point.  But the dew point in that context is still just a temperature, not a location inside the wall.

Thanks for that insight, Dana. You are very generous with your insights. Very much appreciated.

We put the pex tubes under the mesh here in sweden works fine imho

here a pic  http://wellmax.se/files/grunder30.jpg

What exactly is the advantage of placing PEX under the WWF? To use the PEX as a chair to raise the WWF? Seems like it would be difficult/problematic to walk on this setup and pour the concrete. Seems like it would also be more difficult to affect a repair should that ever be necessary.

Yes, Dana’s insight and explanation about ground soil and vapor diffusion into conditioned space garnishes an A+ from me too. If you want to understand the relationships between air temps (dry, wet, and dew), air relative humidity, building material R-value, building material vapor permeance, air saturation vapor pressure, air partial vapor pressure, and resulting effect on building assembly moisture accumulation rate, you might enjoy playing with our software:

Borst Psychrometrics Software

Borst Building Assembly Moisture Analysis Software

We put the pex tubes under the mesh here in sweden works fine imho

How much insulation do you place under the slab and what kind? Under the footings?

I  have poured alot of foundations with pex under and over the mesh .. and i have never had a failure with pex under the mesh but a few with pex over the mesh . Pex is under pressure when we pour so its easy to spot a leak if there is one.

"How much insulation do you place under the slab and what kind? Under the footings?"

300 mm  80-100 kpa eps  is pretty much standard for shallow foundations  and 100-200 mm 300-400 kpa eps under footings

We use prefabricated eps elements as a form for footings

heres some pics https://www.google.se/search?q=kantelement&client=firefox-a&hs=Uh&rls=org.mozilla:sv-SE:official&channel=sb&source=lnms&tbm=isch&sa=X&ei=kSISU-jjJYOShgefgIHIAg&ved=0CAcQ_AUoAQ&biw=1920&bih=919#imgdii=_