# Installing PEX



## Virgil (Sep 29, 2009)

A question to those you have experience with installing PEX radiant floor tubing in slab concrete... What seems to be the normal method is to fasten the Pex loops on top of the rebar and wire mesh prior to pouring the concrete... However, this places the tubing closer to the final surface of the concrete floor.... And therefore, more likely to have a concrete nail/ lag bolt pierce the tubing later during interior wall construction... If the PEX tubing was fastened instead on the extruded poly.... insulation board beneath the metal reinforcing grid work the tubing it seems would to be better protected from damage during the concrete pour operations and afterwards.... With the PEX tubing placed directly on multiple layers of overlapped poly...insulation board between the ground and the concrete mass and an equal amount of insulation along the perimeter edges a couple of additional inches of concrete on top of the PEX tubing I think would not have that much of a detrimental affect on the heat distribution.... If anything it would increase the amount of thermal mass within the heat distribution path upward ....and increase the thermal retention cycle... I recall back in the 60s the than building professionals saying that 3 1/2 inches of fiberglass insulation between the 2 by 4 studs would be all of the insulation a house would need in this life time and next.. Well.. they were wrong about that... The plan would be that once the PEX loops were installed air pressure would be retained on the tubing while the reinforcing grid work was being installed and of course during the concrete filling operations... That way if a leak or damage to any of the plastic tubing occurred it would be immediately known.... So... What is the pros and cons of installing the PEX tubing directly on the insulation board beneath the reinforcing metal grid work???.... Thanks... 

Virgil...


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## biggkidd (Aug 16, 2012)

We tied it on with zip ties. Biggest problem I see is walking across the wire with pex under it. Might pinch the pex. Wire is small not much area to distribute weight. When you roll a wheel barrow across it the tire gives or you shoe gives and no damage. But with it under the wire whats going to give? I honestly don't know if its a good idea or not.

Good Luck

Larry


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## Virgil (Sep 29, 2009)

Thanks for the input.... Our initial consideration is a slab approximately 6" thick... The actual metal 1/2" rebar would be raised above the rigid poly....board insulation and the PEX tubing by 3" height stanchions, commonly called rebar chairs.... The rebar chairs would be placed at each square crisscross point of the rebar @ 16" o.c.... The upright supports we are considering are Hercules brand 3" rebar chairs... model # 912 sold in 100/pack.... Using these stanchions it is indicated that workers can walk directly on the metal grid without it collapsing .... Than additional 6"x6" wire mesh would be placed on top and tied to the rebar network... I have seen concrete installers while attempting to pull up the mesh wire from the bottom back into the concrete mix with metal rakes also pull up pieces of insulation.... That's scary..... Therefore,,, if these upright supports are indeed that strong the likelihood of tubing damage would be greatly reduced,,, I think... and definitely provide a more uniform rebar placement and strength within the floating concrete slab.... Eventually the PEX radiant floor tubing would be tied into a solar thermal system.... Any comments..???

Virgil....


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## SolarGary (Sep 8, 2005)

Hi,
Certainly not an expert on this, but just a thought...

If you apply heat to one side of a concrete slab, it will take it some time to work its way through the concrete to the other side. Google Trombe Wall to get some numbers on this, but it might be hours.

I think that putting the pex on the bottom of the slab may increase the time lag between when you turn the heat on and when you see actual heat at the top of the slab significantly?
Radiant floors already have a pretty long time lag, and you may not want to add more?

Gary


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## nosqrls (Jun 9, 2012)

The way I have seen pex laid out for under floor heat is. Foam insulation under slab same stuff they use for road base. Then wire mat looks like cattle fence. then pex in a pattern for zone staying 1 foot from walls so possible nails when putting in walls. then float a wood sub floor only glued together no nails . then nail/glue floor to that or carpet. pressurize pex with water if fear of collapsing.


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## Virgil (Sep 29, 2009)

Thanks folks for the feed back.... The Trombe Wall system is very susceptible to a critical coefficient of heat coupling unbalance... Reference to Solar Thermal Engineering Space Heating And Hot Water Systems, by Lunde... etc.. pgs 472 etc... Too little thermal mass exposed to direct solar radiation during the day resulted in over heating conditions within the living area followed by high reverse btu lost by convectional air movement, glass surface conduction heat transfer, and radiation paths back through the glazing with nonexistence or poor insulation... Think in terms of these very poorly designed, at least in terms of energy efficiency, houses with huge expanse of south facing windows with inadequate summer shading overhangs, no useable window night time insulation, and primarily wood interiors, (low interior thermal mass structure)... Trombe Walls composed of earth materials or water that worked well needed very good planning... The optimum coefficient of heat coupling is indeed a tricky thing to determine... We had a very good discussion on this subject awhile ago in a previous posting entitled, "Solar Linked To Concrete Slab", you may want to refer back to those postings... From the discussion I think the overall agreement was that the plus 36 tons of thermal mass concrete floor in a typical 1400 square foot well insulated house would reduce the likelihood of interior overheating when coupled to a 300 square foot thermal solar array heat source... Yes, indeed, the increase of the concrete depth would surly increase the hysteresis effect of heat migration through the floor mass to the living environment... But, it eventually adds some percentage to the base line temperature that is achieved... In this type of large thermal mass system I prefer to think in terms of several day averaging of what base line heat is maintained as opposed to being to concern with the daily swings caused by the ever changing weather conditions... With a large enough thermal mass a smoothing factor is achieved... We live, as I have stated before, in a very poor solar gain region... And, it would be unrealistic in terms of cost and size to ever consider that we could obtain 100% annual heating from solar energy.... So, back to the practical aspects of my original question, I do 99 percent of all my construction projects alone.. And with that, how I can physically accomplish a particular phase of construction safely and maintaining good engineering standards is very important.... It would seem that these wider foot print and supposedly stronger stanchions would be ideal for placing the rebar grid above the PEX tubing... This would allow the multiple loops of tubing to be lay out in large sweeping turns covering the whole monolithic slab without concern where interior floor wall fasteners would be located.... The PEX heat distribution network would be more uniform through out the slab... The rebar chairs that I have used on occasions have been the typical flimsy wire type.... One does not stand on them to long before it collapses... It is also much easier to increase the size of the solar collector array later, if needed, as opposed to try to increase the amount of thermal mas within the building .... I think in a few weeks I will order a bag of these rebar chairs and see how strong they really are.... If, anyone is interested I will post my findings here later.... Any Comments....???? 

Virgil...


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## dirtman (Sep 15, 2011)

When I was in the house building business we did a fair number of radiant floors. We always installed treated sleepers to secure interior walls to. I can't think of a bigger mess than trying to find and repair a punctured line. Also there are fasteners to secure pex to foam. I know you can get them at Menard's if you live near one. I prefer to keep the tubing about two inches below the surface. You get a better response time and the mass below the tubing will still soak up heat.


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## Virgil (Sep 29, 2009)

Thanks Dirtman for the feed back comments... For a guy that has first hand experience with installing PEX radiant floor heating systems.... if you could, I have a few follow up questions for you.... Have you ever used the Hercules Brand rebar chairs that I spoke of in the previous postings...??? If so,,, how did they hold up...??? Did you use the Foamular 250 2" thick extruded pink foam board or equiv.. blue board under the concrete slabs and along the footing perimeters...??? If so, did you ever use multiple layers of the foam board to increase the over all R-value of the ground contact insulation..??? I am considering using three layers of 2" foam board for the interior concrete thermal mass floor.... R-30 range... And lastly,,, Where any of your radiant floor installations tied into thermal solar collectors...??? If so,,, how effective were they...???? 
Thanks... 
Virgil....


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## dirtman (Sep 15, 2011)

Sorry for taking so long to answer. I forgot about this thread. We never used more that one 2" layer of extruded foam board. It's expensive and I believe that the benefits of more weighed against the cost wouldn't be cost effective. The purpose of the 6x6 mesh in a concrete slab is to hold the slab together if, when, it cracks. When we used reinforcement we use the ridgid mesh, not the rolled kind which is a pain in the butt. I also got fond of fibermesh in concrete and found it worth the extra money. None of our houses were solar.


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## Virgil (Sep 29, 2009)

No problem Dirtman....Again thanks for the replies... Yes, I been there with the stiff wire rolled 6x6 mesh.... Particularly, if your rolling it out by yourself... That stuff is a real pain... I also prefer the sheet 6x6... We are considering all the factors for a super insulated house envelop.... The standard construction around here is also 2" in a double layer of 1" .... or of course, the single layer of 2" extruded foam board....Pink or Blue it isn't cheap...!!!... A link from Gary's website showed a Super Insulated house in Mass... That used 3 layers of 2" poly-board for the slab insulation... The house was designed for a net zero energy use.... That's not bad thing.... I did order a bag of the Hercules Brand rebar chairs.... I am getting them tomorrow.... So, I will see later this week if these chairs girded with 1/2" rebar are indeed as strong as advertised.... 
Thanks again.... Virgil....


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## dirtman (Sep 15, 2011)

Because the slab exposed to more expansion and contraction in a radiant floor we have found that they are prone to more cracking. Of course all concrete is prone to crack. The fiber mesh helps some but putting ceramic tile right on the slab is not a good idea. It will crack right along with the slab. We overcame some of these problems by bedding sheets of durock in thinset over the slab and then tiling over that.


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## Virgil (Sep 29, 2009)

Oh... That is a good tip to know.... Typically, How hot was the water initially, that flowed through the PEX tubing....???? Assuming the heat sources were from a low temperature furnace of some sort.... 
Virgil...


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## dirtman (Sep 15, 2011)

The installations we did all used a standard boiler to provide the hot water which was usually around 140 degrees F. I have heard of people using a hot water heater but the houses we built were usually over 3000 sq ft. A water heater would'nt have enough capacity. Even though you are dealing with more pumps we found that the more zones with shorter pex runs the better. It's been a while since I was in business but if I recall right you should not exceed 900 lin ft of pex per run. Keep you bends as tight as possible at the perimeter and under windows. You can go a little wider in the center of rooms.


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## dirtman (Sep 15, 2011)

Our heat sources were standard boilers that usually put out around 140 degree F.


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## Virgil (Sep 29, 2009)

Thanks Dirtman for the feed back... Your information has been very helpful.... If I come up with any other questions on this subject in the future I will drop it here for comments... It seems from other websites that 300 to 400 foot per PEX loop is the standard now depending on the tubing size... On a related subject I did receive those stanchion rebar chairs today... They indeed seem to be very sturdy with a wide foot print... I plan on snapping together some rebar with them tomorrow to test their strength... 

Virgil...


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## John_Canada (Aug 17, 2013)

This guy used the mesh on top of the pex and filled lines with water first because otherwise the lines can float to the top of the concrete. http://usonianredhouse.com/images/timeline-021.jpg

The only disadvantage I can see to this is the concrete may be weaker towards the bottom but he had control lines in a 4' X 4' grid anyway. He also had a backup Forced Air Gas system to take up the load with a high temp drop or in case of broiler going down (which it did).

The other interesting situation is he had two slabs on top of each other which allowed workers to build the plan on the bottom layer first and then use the top layer was later poured and finished as a colored concrete floor with the pex exactly in the middle of the two layers.


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## Virgil (Sep 29, 2009)

OK John, thanks for the input... I checked out the Usonian Red House... It has lots of thermal mass, but, also lots of windows in all directions, as it appears... Interesting about the slab over slab construction.... At our proposed building site we have a partially constructed concrete slab...of which, we are considering doing the same thing... It has been there for almost three decades and still in every good shape... The plan is to pour an additional 6" slab over it with multilayer of extruded poly-board and pex tubing sandwiched in between the two... Oh... one other point I forgot to mention... The pex tubing would be fasten to the foam poly-board with plastic clips or staples designed for such a purpose.... Hopefully,, these staples would help prevent the tubing from moving while the concrete is poured... However, as I plan on placing the pex tubing underneath the metal rebar grid work in the space created by the 3" upright stanchions even if some of the tubing does move upward it will not go any higher than the bottom of the reinforcing bars....


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## John_Canada (Aug 17, 2013)

Virgil, no prob. My wife and I visited that house and have had many conversations with the architect/owner as we are doing the same type of house with similar construction. He reported to me despite the windows (which we will have as well), the heating bills are well within reason. There are many reasons for this which I discussed here: http://usoniandreams.info/usonian-houses/usonian-house-windows-to-nature/ The problem we have is we must meet certain building energy requirements in Ontario now so we may have to cut back on window versus walls ratio. Still learning the requirements tho.

That is a super idea about the foundation adding to it and jacking up the pex. That part has always bothered me about the redhouse design.


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## Virgil (Sep 29, 2009)

Well,,, John that's interesting that you were able to actually visit the Red House... It looks very impressive... I was not able to link from this website but, did Google search the house... And was able to view the time line pictures and the laid out of the PEX tubing prior to the final concrete pour.... From what I could gather from the pictures...etc.. 3" of concrete sub layer was poured first, than a single layer of 2" extruded poly board was placed on top of that with the PEX tubing stapled to the rigid foam board.... It is curious that the 6x6 stiff reinforcing wire appears to had been placed directly on top of the plastic PEX tubing... No riser stanchions were used to hold the wire grid up into the final 3" concrete mix and away from the plastic tubing....??? .. If not.,, it seems to me that the reinforcing wire would be of limited usefulness to help strengthen the concrete... Though, I maybe wrong on that... Maybe I am not seeing the whole picture.... While you were there,,,, If, I could ask, did you notice any cracks in the concrete floor...??? If you feel uncomfortable with answering that question... no problem... I will understand... 

In the older passive solar books that I have read... If I recall right... For south facing windows the general acceptable ratio of glazing to wall area was about 12 percent window area provided that there was ample amount of thermal mass within the structure to absorb the solar heat... And I believe in the New York State building code regulations it is about the same ratio of between 10 to 12 percent window area... There are probably exception to this rule.... What further complicates this issue is the low-e emissions type glazing that cut down on the actual amount of solar gain through a window... 

Virgil....


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