I'm building a slab home in Michigan, zones 5-6. Sand base with 4" EPS foam, vapor barrier, 5" slab. I'm trying to figure out a good way to install pex tubing, sleepers, and bottom wall plates where the concrete contractors won't move things as they pour. I was curious if anyone has poured a thin slab first (2" in my case), attached the pex tubing, sleepers and bottom plates (2- 2 by's, 3" thick) and then poured the "topping" slab after. The thin slab would allow me to accurately layout walls, and securely anchor the bottom plates and sleepers in exact locations....and hopefully not move during the final pour. It would also put my pex about mid slab. Any thoughts would be appreciated.

We think getting the PEX at mid slab is worth the additional effort. Most slab designs require the reinforcement to be at mid slab anyhow.

We like to tie the PEX to the rebar/mesh and just lift this assembly to mid slab elevation during the actual pour. This does take a skilled concrete crew to accomplish this. Some contractors like to use chairs to pre-elevate the rebar/mesh, then tie the PEX to the elevated rebar/mesh, and then accomplish the concrete pour. This also takes a somewhat skilled concrete crew to work around this pre-elevated assembly. Many contractors just staple the PEX to the EPS and then let the concrete crew place the rebar/mesh and pour the concrete. You do lose some emitter efficiency in doing this, but this efficiency loss decreases as you increase the R-value below the PEX relative to the R-value above the PEX. So what you do is largely a function of your comfort/skill level and how much emitter efficiency you are willing to lose.

We only place PEX directly in either thin slab or in standard slab. We never use sleepers or plates, but there are others who frequent this forum that should be able to provide you with some good guidance/recommendations.

I would certainly agree with sailor, if you are hell bent on doing things the hard way, but yours might be an even greater waste of time money and elevated risk.

Go for it.

Thanks for the reply...sleepers are for flooring attachment, plates are for wall attachment, couldn't tell by your response if you thought the sleepers and plates were for the pex. With a 2" initial slab, I have a good surface to snap chalk lines for wall layouts, a good way to attach the sleepers and the bottom wall plate (or not, could just attach the wall plates to the finished concrete later) and a good way to layout and securely attach the pex. It puts the pex mid slab and (hopefully) eliminates any movement of sleepers, etc by the concrete crew. I am concerned with attaching everything to the foam ( even if chairs are used to elevate the pex and reinforcing) and the concrete crew keeping everything in the correct location and depth. The foam does not seem to be a stable enough surface to attach anything but pex lines to it (which would put the pex almost 5" deep anyways). Yes I add a little cost with an extra finishing, but its minor compared to working around any members that get moved during the pour.

Posted By BadgerBoilerMN on 28 Sep 2015 10:50 PM
I would certainly agree with sailor, if you are hell bent on doing things the hard way, but yours might be an even greater waste of time money and elevated risk.

Go for it.

I fail to see the waste...same amount of concrete, slightly more labor with an extra finish ($500), less risk because I'm attaching everything to a stable surface while keeping things at proper depths, spacing, etc. No concerns of the concrete crew moving or inadvertently altering locations or depths. If I'm overlooking something or missing an issue, please let me know. Concrete contractor agreed with this and is only charging me a little more for an extra finish.

Hope you plan on keeping your house at the same temp all the time, I mean all the time. No setting back the temp at night for better sleeping. A 5 inch slab takes hours to change the temp in. With spring fall winter seasons trying to find a setback time/temperature for the thermostat is not a easy task. You are going to also have over and under shooting all the time also.

Guessing that you will have to keep the temp constant, then the whole slab will always be at the same temp so it really doesn't mater where you put the pex. I have 1.5 inches of concrete and have all the above problems. I couldn't imagine 5 inches. And I am using a programmable controller. This is on an average insulated house also so I have lots of heat loss. A well insulated house is going to be even worse, you will be opening windows when it is 40 degrees out.

Is this going to be your only source of heat?

I think I would add a wood sleeper floor over a 3 inch cement floor with no concrete in top, but I am not an expert. So the layers would be: 3 inches cement then one by lumber with pex weaved in-between and then plywood top. I am sure there are other ideas, options.

Good luck.

You are describing what is called a Rat slab,
An under pour. I have done this in the past when I use to install, actually touched some 20 miles of pipe.
A 5 inch slab is thick, will take some time to cool/heat but that said outdoor reset, slab sensor can assist in management, slow response time though.
If you staple to the foam thats good, you can likely accomplish what you want in a 4 inch slab. Stapling pipe and placing wire mat on top is a fools move, It is similar to placing 10,000 sharp knife edges over the pipe. Each cut on the wire mat is a sharp pointed edge wanting to stick the pipe. If you use wire mat good, tie the pipe to the mat then you can see the edges as you go and secure pipe as needed to prevent accidental damage to the pipe. Not so with rebar though I like this on top of pipe, 18x18 grid, 24x24 , ends of rebar are also sharp but easily managed.
Nail down floor is a lot of sleepers, alternate attachment is glue floor to slab. But with the rat slab capturing the pipe and bar you are free to level glue and sleepers for flooring, sounds like a lot of effort. Check out some of the quality engineered wood floating floors. There is some nice product out there. Any internal walls can easily be pinned in place using red hat nails and glue also bolts can be place at time of pour simply set string lines on perimeter foundation for centers, stretch them out after pour during finishing phase and place bolts as needed.
Dan

Thanks for your replys.. It will be a well insulated home, total heat loss (including garage) was calculated at 61,436 Btu/hr . I understand what your saying about reaction time with a thick slab, but wouldn't zoning play a role in that as well? I'm hoping that once habits are relearned we could program a comfortable setup. We'll also have a wood stove insert for the shoulder months and/or quick(er) heat when needed.

I'll be doing the rebar, I work around ironworkers and rodbusters so it'll be a repaid favor. Floor is hardwood that was dropped from a tornado on my property 2 years ago. Sawed it, let it air dry for 14 months and is now at the kiln getting finished. If I didn't have that I would have done the engineered floor. Old farmhouse is our style so face nailing, ship lap, maybe even just plank is what we'll do...point being that we aren't looking for perfection in the finishes. Thanks Again

BOYS, BOYS I've been trying to learn something here and all the bickering and insults make it very hard to get through a thread and gain any worthwhile knowlege, I was under the impression that this was a helpful website for DYI info. How much needs to be contracted is relative to the knowlege, skill sets, determination and desires of the individual. all this petty back and forth undermines your own credibility and that of this site! JUST MY OPINION

Jumping into this thread to ask a few questions during my research

1) mention of putting the rebar up on 'chairs', then tying the pex to the rebar. Are the 'chairs', small clay blocks of about 1.5" height? I have discussed with another builder how to assure the pex to be placed mid-slab, and rather than trying to get a concrete crew to lift it as its poured, he recommended using some sort of clay blocks to rest the rebar on to assure your pex tubing is set up.

2) sand base..... do people use a sand base? drain rock base? some sort of special compacting sand that wont settle and allow your slab to crack after a few years? What is the actual product that is going under the insulation that you are leveling and compacting?

3) how does the layering go, or does it matter.... does the vapor barrier go UNDER the insulation, or on top of the insulation as this poster suggested

4) 4" EPS.... most people standard use 2" XPS, right?

5) regarding the posters concern of having the sleepers placed into the concrete....if you are installing framing and baseplates directly onto the concrete, how does one go about assuring no damage to the pex? only using adhesives? short concrete fasteners? arranging your PEX lines so they aren't anywhere near your framing and baseplates to assure you aren't going to ding one with a powder activated fastener?

6) concerns of a 5" slab not being responsive enough, being too much of a heat source, and not being able to cool off. Is this more of a problem calculating and overestimating your heat loss and putting too big of a boiler on? of does a 4" slab not also have this concern? If one is going to use a hydronic system in a 4" slab, how does one assure you aren't going to find yourself overheated and impossible to cool the slab off midday?

the more I read, the more that comes up I feel I need to research.... but ill be waiting to make my posting until I have a well thought out message and questions.
Dan

1. Yes, the rebar/WWM(woven wire mat) can "sit" on chairs. they make plastic ones that snap on, there are the blocks with tie wire iembedded, or some even break up concrete paver blocks to set under the re-enforcement. With a slab and WWM, chairs are not always very practical as the concrete crew has to walk out there on top of hte WWM when placing the concrete. It is more typical that the crew just pull the mat up into the concrete as it flows into the slab area.

2. Yes sand can be used, Pea gravel is also used as a capillary break, crushed rock is also used to fill level and compact if you have really poor load bearing soil quality. Clear sand actually compacts pretty well, especially with some water allowed to flow down thru it. If it didn't, we wouldn't make sand castles and have sculpture competitions with it The slab really isn't all that heavy a load on the soil. ConCrete = 150# per cubic foot. A 4" slab would be 1/3 that or 50# over 1 square foot of ground(50#/144SQ/IN = 0.35PSI). Compacted sand wil support in the neighborhood of 1500# per square foot. That is a typical building department design criteria, that the foundation is designed to, unless a lot of more pliable soils(clay/plastic) are found in the build site.

3. Yes, the layering matters. For an open pour(not undercover) ACI(American Concrete Institute) reccomends from bottom to top, base soil/sand or gravel, vapor barrier then insulation if used. Some engineers spec a capillary break(sand/gravel) between vapor barrier and insulation, but all this does is trap moisture, either from the weather or the pour, in that layer that takes a long time to escape up thru the insulation layer then thru the concrete. Any flooring put down over the concrete can suffer for quite a while as the slab moisture content takes a long time to stabelize. That layer robbing moisture from the concrete during the pour can also effect how the slab cures. ACI has a flow chart to tell you where it should be placed. The vapor barrier can also go on top of the insulation, but if you are using insulation it will provide some protection from damage by the concrete re-enforcement and the crew that pours... Many foams are considered vapor barriers by themselves if sealed properly. I would be concerned with how well taped and sealed seams would hold up as a barrier long term, especially after the CC crew spends a day dancing all over it.

4. I think 2"/R10 is code minimum under a heated slab. If floor heating, more insulation under the slab means less heat lost to mother earth and more into your toes. In some climates/soil conditions, the added cost will be recouped, especially as energy proces seem to go nowhere bu up. It is also pretty hard to put more in there later EPS over XPS. EPS is I think a little "Greener" to produce, but some of he XPS manufacturers are claiming cleaner processes also...

5. Everything you described... Adhesives might not pass inspection though, and with an incorrect slab makup, moisture content of the slab might cause them to not adhere long term.

6. The good news, Concrete is stable. The bad news, Concrete is stable... They can take hours to warm or shed heat. This is great if you want that heat, but in cases like the bedroom, what is comfortable to walk thru and on during the day, can be opressively hot when trying to sleep. Also if you are away any length of time, it can be quite a while to get the slab up to a stable temp, so many just leave the heat system on, or use timers/programming to restart it early enough to stabelize before your expected arrival. As to calculating the heat loss, well the slab response is a factor, but only in that it is a delay to heat transfer THRU the INSULATION barrier. The main factors are the insulation and air exchange values of the envelope(and possibly solar heat gain). The heat loss calculations should drive the size of your heat source, especially considering the slab will only absorb so much heat so fast... You can hit that slab with a bigger hammer, but since the tube area and spacing is fixed at the pour, it is probably not going to warm much faster, and it may even overshoot your desired temp considerably. Ever try and adjust the water temp while you are in the shower The delay of heat transfer with the slab makes it a little like the delay between the hot water valve and the water from the shower head landing on your skin...

Yes, research is good...

http://www.ccinetwork.com/p-324-ltm-concrete-1-snap-rebar-chairs-carton-of-500.aspx These are the chairs I used. I have a 5" slab and rebar is supposed to be in the bottom third of a slab, so these worked well. We also embedded #4 rebar into the stem wall when we poured and then bent them over and tied them into the rebar. This ties the slab into the foundation and locks everything together. Downside is it made installing the vapor barrier very time consuming and tedious (working around and taping and sealing penetrations). A lifesaver, though, was the rebar tier http://www.maxusacorp.com/product_detail.html?productcode=RB90392 Made a difficult, boring, and tedious job easy. I was lucky enough to know someone that had one. Took about an hour and a half to tie the whole grid.

We used sand as our base, under the foam. We are fortunate to have an abundance of really good sand, if not I would have put a layer of stone down.

We put the vapor barrier over the foam, under the slab (open, outside pour). All of my research led us to do that, and after pouring, I agree with it. Vapor barrier under the foam can hold water, be it condensation or precipitation and then you have no way to get it out (other than naturally drying). Once the slab is poured, and if there's water trapped, it will continue to dry after the slab has cured, so you can never be assured of proper moisture levels until that drying has occurred, which could be a long time. If your pour is under cover, you might be ok. Any rain or condensation we had prior to the pour (it rained), we were able to dry or vacuum out.

EPS. Evidently EPS foam outperforms XPS over time, and is better at not retaining moisture. Its also much less caustic to the environment in production. It's also cheaper..... We used 5". 4" and then another 1" layer installed 90 degrees to the first 4" layer, taped all the seams...more to keep the foam in place than for any other benefit (very windy at our site). Design was for 4", but we made a elevation change after the sub base was graded and compacted and it was cheaper and easier to just add one more inch of foam.

2 pours for us. First pour (3") has the rebar and locked in the slab to the foundation, as well as covered the insulation and water, drain lines. This allows us to layout our sleepers, bottom plates, and pex lines where we need to and anchor to the base slab without any fear of drilling thru any embedded pex. Once everything is laid out and anchored (and passed inspection), we'll pour the final 2" slab. This will not occur until rough framing is complete and we can accurately lay out our zones, manifolds, etc, as well as we'll have a last minute way to embed a missed water line, change wall layout, whatever we can fit in a 2" slab if necessary.

My thoughts on the 5" slab are that, yes I can see where response time can take a hit, but I feel strongly that zone layout and control can make it not such a big issue. Each room having its own zone and thermostat as well as having some separation between rooms from the embedded wall plates (granted its only the top 1 1/2, but it has to have some effect on the whole setup), sprinkle in some habit learning, I believe we can be satisfied.

Good luck

CWebster, your overall approach should work fine. Your initial 3” reinforced concrete slab is satisfying the structural requirements and the subsequent 2” concrete slab is more of a floor finish that accommodates the PEX placement. Placing the vapor barrier on top of the insulation makes good sense for the reasons you indicated. You will always benefit by having a high mass HR emitter when you have a well insulated and well sealed building. The thermal lag time associated with a 5” slab versus 4” slab is not drastically different and you won’t have any controllability issues if you use slab temp and indoor temp PID feedback. If you have a poorly insulated and poorly sealed building, a low mass HR emitter starts having merit and outdoor temp feedback becomes more important too. Just be sure to accomplish a proper heat loss analysis and HR design analysis so you determine the correct PEX spacing and circuit lengths for each zone. Given your thoughtfulness and thoroughness, I am sure that you have that well in hand too.

great information. additional questions

1) How much downside is there to stapling the pex directly to the XPS insulation, then laying the reinforcement (or using fiberglass reinforcement additive) in the concrete? In a 4" slab, obviously the pex wont be floating mid-slab, and there will be some efficiency loss, but for a smaller slab project, its this a big no-no?

2) As far as substrate goes.... 'sand', 'pea gravel', if I call up the gravel pit or provider, is there specific terminology I am looking for? I would think pea gravel (which doesn't compact and is quite loose to walk on) would be very different than something say angular, like washed 3/8" crushed rock without fines.

3) in terms of layering, I would be completing it in a closed, finished space. Outdoors and trapping moisture would be problematic, however layering gravel/vapor/XPS/pex indoors shouldn't be an issue. Additionally, should one still tape and seal the XPS if you are taping off the vapor barrier (as an added seal) anyhow?

4) Trying to save headroom, Im going with 2" XPS. Ill recycle a little more at work to make up for it

thanks again!

For any below grade slab (probably not the case here), I'd bring the slab PE up the wall 12" so that it can be lapped on the interior side of wall foam.

Underneath, I'd use 3/4" gravel (no fines), not sand.

is this a big no-no?

Depends on the details, but perhaps 5% (seasonal heating $ due to higher source temp). Maybe almost 0%.

We also embedded #4 rebar into the stem wall when we poured and then bent them over and tied them into the rebar. This ties the slab into the foundation and locks everything together.

I thought that this encourages slab cracking and the best way was to allow the slab to float/shrink freely. For example, some people put a little gravel over the footing edge so that the slab can't bond to it.

Hydronic radiant floor emitter upward heat effectiveness is described in some detail in the instructions for our Hydronic Radiant Floor Heating Design software:

Borst Hydronic Radiant Floor Heating Design Software

Here’s the excerpt from our instructions:

“Before you get too far with hydronic radiant floor system design, you should also consider the upward heat effectiveness of the various floor emitters that you may be considering. Slab emitters have the highest upward heat effectiveness and below-floor plate emitters have the lowest upward heat effectiveness. Even the location of where you place the PEX tube in a slab may have a significant effect on the floor emitter upward heat effectiveness. The upward heat effectiveness percentage of a floor emitter may be calculated as (1 – R/2)/100, where R is the emitter efficiency loss ratio and is defined as the R-value above the PEX tube divided by the R-value below the PEX tube.

As a point of mental reference, the emitter efficiency loss ratio is zero if all of the R-value is below the PEX (i.e., a 100% effective upward heat emitter where all the heat moves upward). The emitter efficiency loss ratio is 1 if 50% of the R-value is above and below the PEX tube (i.e., a 50% effective upward heat emitter where 50% of the heat moves upward and 50% moves downward). The emitter efficiency loss ratio is infinite if all of the R-value is above the PEX tube (i.e., a 0% effective upward heat emitter where all the heat moves downward). In fact, once the R-value above the PEX tube exceeds 2, it is normally considered to be a 0% effective upward heat emitter when designing a hydronic radiant floor heating system (reference Siegenthaler).

Concrete is about 0.1 R-value per inch and EPS/XPS is about 5.0 R-value per inch. So if you place PEX tube on top of 2” EPS and pour a 5” slab, the emitter efficiency loss ratio is (0.1)(5”)/(5.0)(2”) or 0.05 and (1 –0.05/2)/100 is a 97.5% effective upward heat emitter. If you instead place the PEX tube in middle of this slab, the emitter efficiency loss ratio is (0.1)(2.5”)/[(0.1)(2.5”) +(5.0)(2”)] or 0.019 and (1 –0.019/2)/100 is a 98.8% effective upward heat emitter. So for this example, this is a difference of 1.3% in floor emitter upward heat effectiveness. As such, this may be totally insignificant or very significant depending on your energy efficiency design objectives/requirements.”

We would certainly concur that it is best to have a floating slab (not structurally tied to the stem wall), especially if this will be hydronic radiant heated slab in a cold climate.

"slab PE"? 3/4" gravel noted. THe slab actually will be below grade in my instance. its in an unfinished, walk out basement. As far as stapling the pex to the XPS, is it then feasible to skip rebar or mesh reinforcement if you plan on using glass fiber in the cement mix? Other forums tend to feel although it strengthens concrete, it does not prevent cracking, even if you are tooling the slab appropriately. insight into that product? Or just pony up and put some friggin metal in that slab!

"Slab PE" is the polyethylene sheet just under the concrete. The suggestion is to fold it 90 degrees upwards at the edges so that it channels any unexpected wall water below the slab.

Agreed, steel (non-tensioned) doesn't prevent concrete from cracking. But it can prevent small cracks from becoming larger gaps and prevent levels from shifting.

We use wire in all of our slab on grade heated slabs and heated driveways. A basement or walk-out is strictly optional, but as Jon correctly points out will control cracks and prevent most modest shear forces. Preparing the substrate is most important.
1.3%...I believe Shakespeare referred to this as "Much ado about nothing". We try to keep is simple. Stapling 1/2" PEX to 2" of XPS, wire and 4" of concrete. Your heat source should be controlled with outdoor reset.