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Mesh or no mesh?

Sukhoi29SU
Sukhoi29SU Member Posts: 83
edited April 2020 in Radiant Heating
Planning 4” concrete slab with Uponor pex tubing - stego wrap vapor barrier with foamular 250 insulation. Have staples to staple the pex directly to the foam. However, I’m debating using wire mesh and zip tying pex to the mesh. I’m not concerned with pulling the tubes to the center of the slab. I’ll get plenty of heat in this room with the pex at the bottom of slab. I thought I read a thread here awhile back where I thought @nibs recommended not using mesh, but using a lot of fiber instead. I couldn’t find the thread though - wanted to make sure. I think I’ve read that elsewhere, though - that I could expect a better slab without wire mesh in it. Any opinions on the matter? This will be a lower level garage / workshop floor. The garage floor above it will be on quad lock (icf) engineered floor with a 5” slab- and for that I’ll be zip tying the pex to rebar. Thanks in advance

Comments

  • GroundUp
    GroundUp Member Posts: 1,495
    If you're not concerned with suspending the tube, I would absolutely staple it to the foam. Zip tying sucks, first of all, and mesh is never consistent. This may be an unpopular opinion, but in my heavy clay ground here, the only way to build a decent slab is with rebar. Mesh and fiber mesh still allow a lot of movement and fiber mesh typically leaves a really ugly finished surface. Every slab I pour gets a 12" grid of #4 bar and none of them ever move. It's cheap insurance IMO
  • Zman
    Zman Member Posts: 7,433
    If you don't mind wasting energy, heating the earth, and living with the response lag you will get with the tubing at the bottom, by all means staple it down. The mesh is theoretically a good option. In reality, you would have better luck winning the lottery than getting the concrete guys to pull up the mesh. The best practice is tubing tied to rebar on chairs.
    https://www.pmmag.com/articles/87540-depth-perception
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    STEVEusaPA
  • Sukhoi29SU
    Sukhoi29SU Member Posts: 83
    Thanks for the responses, guys.
    I’m expecting about a 25% loss in performance in this slab if I staple the tubes to the foam. (Reference attachments) that @Gordy provided me with awhile back.

    This particular room is approx 1700 sf of lower level workshop- mostly below grade, surrounded by ICF - very well insulated. In one corner I’ll have an 1800’gal wood gasification boiler that’s going to be putting off heat as well.

    I thought I could probably sacrifice some performance down there to make the ease of installation of pex easier and not have to worry about the guy sawing through the pex when he’s sawing his control joints. I was concerned that the mesh would float towards the top or the guy would pull it to close to the top of the slab - the risk vs reward didn’t make too much sense to me.

    However, if the recommendation is to use actual rebar and zip tie the pex to the rebar 2” down.. I guess I can consider that. I imagine those ‘chairs’ would have to be pretty sturdy to hold up rebar? How difficult is it to pour the concrete without knocking these chairs down and having the rebar fall to the floor during pour? Thanks again
  • Paul Pollets
    Paul Pollets Member Posts: 3,567
    The problem with foam clips is they tend to snap off the foam when the concrete workers are pouring and working the pour. If the loops are pressurized, they'll float and create a big hassle. All my slab pours are done with loops tied to 6x6" welded wire mesh that is secured and placed parallel and square. It's fine to use rebar, it's placed under the welded wire mesh when called for. I'll also use chairs in thicker slabs so the tubing sits in the middle of the slab. You can purchase a PexGun to tie the tubing to the rebar or wire mesh if you don't want to use zip ties.
    MikeL_2BillyO
  • Jamie Hall
    Jamie Hall Member Posts: 20,401
    I would do as above -- but I would always use fiber reinforced concrete. If it is properly finished, the finish is fine (you could not tell the fiber is in there). Of course, if you have the three stooges doing your concrete work...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 18,157
    Power trowels are the best way to finish a slab that has fiber. The heat from the trowel blades gets rid of then "hair" and can provide a glass like finish. Slabs in shops should be a bit rougher finish or they will be very slippery when wet. I had to take a big grinder to my shop slab to rough it up after the finishers got it to "perfect"

    Really no value to mesh or bar if it ends up at the very bottom of then pour. Mainly mesh helps hold tube spacing.

    In a 4" slab it costs you 10- 15° more SWT when tube is at the bottom to get the same BTU/sq.ft.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GroundUp
  • GroundUp
    GroundUp Member Posts: 1,495
    hot_rod said:



    In a 4" slab it costs you 10- 15° more SWT when tube is at the bottom to get the same BTU/sq.ft.

    Is there some real data on that somewhere that I could look at? I'm certainly not arguing, but that seems rather excessive. I have a number of 4" slabs with tubing at the bottom that are running 70 degree ambient temps and 75-77 degree surface temps with 86-90 degree supply water. I find it hard to believe the 2" tube elevation difference would allow 75-80 degree supply water to achieve the same 75 degree surface temp. Response time will obviously differ, but the BTU/sq ft seems dependent upon the run time more so than the water temp.

  • hot_rod
    hot_rod Member Posts: 18,157
    This model has 3/8 hardwood, about .5 R-value, so a bare slab would be a bit better output.

    The BTU output of any radiant surface is really the actual surface temperature and ambient air temperature difference. A bare slab about 1.9- 2 BTU/ sq ft/ degree difference. Doesn't matter how it gets there or how long it takes the output is always baed on the delta T. see for fin tube panel rads, air coils.

    Assuming that entire square foot is a consistent temperature most all slabs stripe a bit.

    82° slab surface- 68° ambient= 14 X 1.9 btu= 26.6 BTU/ sq ft.

    On a shop slab you could run higher surface temperature and cooler air temperature

    85°- 68°= 17 X 1.9 = 32.2 BTU/ sq ft output
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GroundUp
  • GroundUp
    GroundUp Member Posts: 1,495
    Would you not agree that a longer run time would change those results by lessening the striping and evening out the actual surface temp, regardless of tube depth? Perhaps my IR guns and FLIR are all reading low, but every one of my systems I have checked are closer to 10 BTU/sq ft based on the above formula while actual fluid data shows closer to 15 in the same slab. I'm not buying that I could cut fluid temps 10-15 degrees to achieve the same surface temp simply by raising the tube 2 inches. Since I'm stuck at home anyway, maybe an experiment is in order
  • hot_rod
    hot_rod Member Posts: 18,157
    It really depends on the load, the analysis was done under full load conditions. As the space warms or load goes down you could expect slab temperature to even out, especially on a fixed temperature system. that's how slabs overshoot.

    With ODR I would expect that profile to remain consistent, SWT would hopefully drop in relationship to the load. Color (temperature) may change but the profile has more to do with the conductivity of the concrete and tube spacing.

    An IR camera or sensors in the slab and data logging across a month run time could prove out the FEA. Or not :)

    Remember downward loss increases with tube at the bottom, as the profile shows. And increases temperature increases that loss. Hot to cold always, rate of heat transfer changes with T.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GroundUp
  • nibs
    nibs Member Posts: 499
    In a 4 inch slab, I would likely use rebar or mesh, in a 2 inch slab I would not, I would use fiber. I have personally troweled slabs containing fiber, and have never had any fiber showing on the surface. Fiber being so low cost, I would use it as well as steel re inforcing. When I used pex with steel reinforcing I zip tied it rather than wiring it, wire ends have been known to puncture the pex.
  • Sukhoi29SU
    Sukhoi29SU Member Posts: 83
    @nibs

    I do believe the thread was in reference to a 2” slab - I was considering a 2” suspended slab on my main floor of house at the time. Thanks!

    Appreciate all the comments / responses. Learning a lot here.
  • nibs
    nibs Member Posts: 499
    A suspended slab needs engineering, Be careful, even the form needs to be engineered.
    Sorry I was so long in getting to your topic.
  • Sukhoi29SU
    Sukhoi29SU Member Posts: 83
    @nibs I’ve sort of given up the idea of a suspended concrete floor inside the house. Going to stick with the existing floor joists and put an engineered hardwood floor or something there. Instead of radiant floors on the main floor, I’m planning on radiant ceilings. The lower level of home will be a concrete slab with in floor heat.

    I’ve read a few articles on concrete this AM. The workshop for the 4” slab has a garage door and I will be able to drive vehicles in it. I’ll have a wood boiler in a corner and hope to be able to drive truckloads of firewood up to the boiler. Also, I might store a car , UTV , etc down there. So , rebar might be desired for this - Although I just read an article that claimed rebar doesn’t do much, if anything, for slabs of 4” or less.

    Now I’m a little torn on staple pex down to foam, or put some rebar in there and zip tie the pex to rebar. Want to do it right, but I don’t want to put 100% faith into the contractor I end up hiring for the job. Been burned too many times so far on this project.

    Seems like, once again, many opinions on the matter but no clear answer for how to do it.

    Maybe a combination of fiber and mesh or rebar is the way to go for this slab.

    The good thing is that the base is limestone. I’ll have some gravel in there and try and make sure it’s well compacted. Sounds like making sure you have a solid well compacted base is one of the most important factors.
  • hot_rod
    hot_rod Member Posts: 18,157
    Same goes for rebar, unless it is up on "chairs" it will not add any value at the bottom.

    Over the years I found a good compaction, consistent slab thickness, proper cure, and fiber in the correct mix provides the best result.

    All, concrete has the potential to crack, limit the conditions that can cause it.

    My pet peeve is the lazy concrete guys adding 5 gallons or more to the truck when it arrives. The truck leaves the yard with the proper mix, water, gravel, portland proportions. Every gallon added to the mix reduces the strength, psi.

    Admixes can be blended, hot windy days for example, the mix can be modified for special conditions.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Rich_49
  • Wellness
    Wellness Member Posts: 129
    Rebar and chairs is the way to go for 4" + concrete slab. It's a little more labor intensive but, as others have said, it's easier to keep in the middle of the concrete pour than mesh, which tends to compress at the bottom of the concrete where it does no good. Also, as Jamie Hall pointed out, it's very important to properly cure and finish fiber concrete if you are trying to maximize strength and avoid cracks.
  • hot_rod
    hot_rod Member Posts: 18,157
    The biggest challenge is rebar and pex. Unless you install a tight grid of bar, 12" on center or less you do not have a good way to keep the pex from floating.

    Also the concrete guys will be walking on that tube span across the bar when it is up on chairs, breaking ties or potentially kinking the tube.

    Almost need to use mesh and rebar to get protection and adequate tie. But it is a **** to walk on that assembly when pouring the mud.

    I suspect 80% of jobs, the tube is at or near the bottom of the pour with mesh, unless someone has spent a lot of time to "chair up" the mesh.

    Continuous strip bar chairs work but it takes a boatload of them to keep mesh up.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GroundUp
  • nibs
    nibs Member Posts: 499
    These days even our little cement batch plant knows that using superplasticizer makes the cement flow as if you added water, they know that the less water in the mix = much stronger cement. Seems kind of contrary that keeping cement wet once it is hard makes it much stronger. Even after 50 years bridge abutments kept wet keep getting stronger, even if very slowly.
  • psb75
    psb75 Member Posts: 629
    There is the whole issue of controlling cracking with control joints. Sawing after the pour can be problematic with pex pipe and rebar and mesh "in the mix" i.e. depth-of-pipe and re-inforcement vis-a-vis the depth of saw-cut. And how to protect the pipe AT the expansion joint.
    Then there is the choice of "forming" the joints in wet 'crete with tooling or using materials specifically for creating and maintaining control joints long-term.
    Rich_49
  • hot_rod
    hot_rod Member Posts: 18,157
    The joints mainly keep a crack from telegraphing, I don't think they prevent cracking. Saw cuts need to be 1/3 of the slab thickness to do much good.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Sukhoi29SU
    Sukhoi29SU Member Posts: 83
    edited April 2020
    Got a bid from a local concrete guy today. When I mentioned pex he said the way he likes to do it is staple pex to foam, then lay mesh on top of the pex.

    He also said I don’t need any fine stone for compacting, that the 3/4 clear I have now is perfectly sufficient. I was considering renting a plate compactor and giving this a go myself...
  • hot_rod
    hot_rod Member Posts: 18,157
    The sub grade is most important, what type of soil under that 3/4 washed stone? That seems to be the area that causes slab settling, cracking, water issues.

    Overkill for you maybe but on commercial jobs there was alway a compaction test performed before we could insulate and tube. The sub slab work didn't always pass the compaction test.

    At the end of the day it comes down to what you and the concrete guys are comfortable with.

    Tube on the foam, mesh over it? I'm not convinced 6X6 mesh adds a lot of value to a slab, even when it is in the pour. If the mesh isn't being used to tie tube to, why bother? I'd rather see rebar over the tube if you are looking for an actual structural component. Only if the rebar is encased in the pour does it do a lot of good. 1-1/2" chairs hold it up into the pour best, don't let them use rocks or bricks to elevate the bar.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Sukhoi29SU
  • Sukhoi29SU
    Sukhoi29SU Member Posts: 83
    @hot_rod
    Limestone underneath... so that’s a plus I would think. Was a pain in the **** to excavate this giant hole in the ground ...

    Thanks again for the advice!
  • psb75
    psb75 Member Posts: 629
    Hot Rod is right esp. in his last paragraph. I did a tube job just like that last fall w/o 6 x 6 mesh. Rebar on chairs. Tube stapled to foam w/ 4 in. pour.
  • nibs
    nibs Member Posts: 499
    For our slab, we formed the floor in sections and poured on alternate days, stripping the form left us with control joints.
    The advantage that we had was site mixing the mud, so were not dependent on a subcontractor. I hired a laborer to help with the heavy stuff and to allow me to stay fresh for finishing, as it was I was trowelling sometimes at 10pm.
    As noted in earlier threads we put down a 2" +/- layer of lightweight concrete on the sub grade and set the foam into the wet lightweight.
    Imo if you use rigid foam under the slab, it lowers the bar for compaction. The foam bridges soft areas very well. But over kill is much less expensive than repairs so compact to your hearts content.
    Sukhoi29SUCanucker
  • Sukhoi29SU
    Sukhoi29SU Member Posts: 83
    edited April 2020
    Excavator just recommended to me that if I’m not pouring the concrete myself, to let the flat work guy be responsible for grading. That way if something happens to goes wrong with the slab ...

    Might be some good advice.

    I wish I had the experience or time to do it myself, but I’ll end up hiring someone for the flat work , for sure.
  • nibs
    nibs Member Posts: 499
    Does the subgrade get inspected? on all my commercial or govt work, they did compaction testing before the cement prep could proceed.
    If the cement guy will approve the subgrade it will help.
  • Sukhoi29SU
    Sukhoi29SU Member Posts: 83
    @nibs Good question.. but I don’t think it’s required for the compaction to be tested prior to pour for residential. For the amount I paid for this building permit in Illinois you’d think an inspector needs to come out for every aspect of the project. Perhaps for commercial applications here it’s required, but I’m not sure.
    Glad to read about the foam lowering the bar for compaction - I was really wondering about that. I have foamular 250 pink board I plan on using.
  • hot_rod
    hot_rod Member Posts: 18,157
    Screed the gravel as best you can, a long 2X4 with a couple people. Any voids can cause the foam board to crack. Prep work can make a huge difference in the outcome and durability of the pour.

    Doesn't sound like you are planning to run heavy equipment on the slab, so a compaction test would not be a big deal.

    Usually you "know" if the ground below has good weight bearing ability. Top soil, sod, clay, soft spongy sub-base= not so good and you should excavate down to solid ground. Hard excavating limestone sounds like you have an adequate base.

    Most experienced excavators understand the earth they are moving and how viable it is for structural support.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • nibs
    nibs Member Posts: 499
    @hot_rod , When I have heard foam crack when I walked on it, the guy who screeded the sub base (moi) gets an earful, about 2 or 3 times on this house.
    That is one reason we set the foam down on wet lightweight cement, never any cracks when we walked on it.
  • nibs
    nibs Member Posts: 499
    Speaking of compaction, if you have good drainage and are using import fill, you can compact sand with water, and of course you can level/screed with the same water, no puddles, no islands.
    That is the way we did our foundations which are all resting on
    3 or 4 inch 50 psi foam.
  • Sukhoi29SU
    Sukhoi29SU Member Posts: 83
    Met up with a guy today for another concrete bid and he asked me a question I didn’t have a good answer to: why 4in slab? Why not go with a 5” slab? He said he’s done radiant slabs and recommended zip tying pex to the mesh and he says his guys are good about pulling the tubes into the center of the slab. He said they pour some, then pick up the mesh with a hook , and then finish pouring. Lot of good reviews online...
    I expressed my concerns about him pulling the mesh to high into the pour and then cutting the pex with the saw. He said he’s never run into any problems pulling the mesh up into a five in slab.
  • Zman
    Zman Member Posts: 7,433

    Met up with a guy today for another concrete bid and he asked me a question I didn’t have a good answer to: why 4in slab? Why not go with a 5” slab? He said he’s done radiant slabs and recommended zip tying pex to the mesh and he says his guys are good about pulling the tubes into the center of the slab. He said they pour some, then pick up the mesh with a hook , and then finish pouring. Lot of good reviews online...

    I expressed my concerns about him pulling the mesh to high into the pour and then cutting the pex with the saw. He said he’s never run into any problems pulling the mesh up into a five in slab.

    I think you found the right contractor.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    Sukhoi29SURich_49
  • nibs
    nibs Member Posts: 499
    Could be a good go. Have not seen it mentioned lately but it is good to pressurize the pex with air we used 100 psi, that way the mud will not deform it, and if you get a break it you can find it and fix it while the mud is still soft.
    Sukhoi29SU
  • hot_rod
    hot_rod Member Posts: 18,157
    Take a video of that pour😉
    Possible on a fairly small pour like that they will lift all the mesh. And if you are watching.
    Are they pumping this pour or wheeling?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Sukhoi29SU
    Sukhoi29SU Member Posts: 83
    @nibs thank you for mentioning that. I have the manifolds and bought something from supply house that will allow me to pressurize the lines. Didn’t know to what psi

    @hot_rod I’ll try to get some video! Not sure about pumping vs wheeling. Awaiting his quote. If he does a good job on this first 1500sf slab, I’ll have three more slabs for him- all with pex.