How do you get floor heat in the middle of a slab?

rhl

John Siegenthaler talks about embedding floor heat pipes in the middle of the slab. How do you actually do this? If your using wire mesh do you first lay the pipe on the mesh on the ground and then somehow raise the mesh? Also does matter much for a 3” slab ?

Jamie Hall

A 3 inch slab is going to be pretty thin. Yes, in principle, you can lay the pipe on the mesh rebar and support the whole thing on chairs -- but if you use say 5/8 inch pipe, you're only going to have an inch or so cover -- which is very very likely to crack. You could also support the pipes under the rebar on 1 inch or so chairs; at least the cracking wouldn't be visible.

Lots of insulation under the slab. Also, in a slab that thing, the subgrade must be very well compacted. And furthermore, I'd use fiber reinforced concrete...

rhl

I’m planning 3” of XPS foam, For R-15. laid out in a picture frame. The minimum recommended (based on John Siegenthalers book) is R-9.

hot_rod

The concrete folks suggest 3 times the aggregate size above the tube. Most red-mix is blended with 3/4" minus rock unless you ask for other size. So ideally you want 2-1/4" of concrete above the tube to minimize cracking and provide adequate strength, if they mix with 3/4 rock. Ask them.

The only way to end up with 1" over the tube is to go with a smaller rock in the concrete mix. Around here they call pea gravel mix a 3/8" minus aggregate, so 1" over the tube would work. Smaller aggregate does end up being a bit weaker mix.

The 3" slab makes this all more of a challenge as Jamie mentioned.

Either keep the tube down on the mesh on the insulation. Or go with a smaller rock in the mix and support the mesh on some 1-1/2" chairs type chairs. Or a thicker slab.

There is "ideal" installations, and practical systems. Trying to keep the tube up high in the slab to maximize performance can lead to floaters. Be sure to have water in the loops to help keep it from floating in a thin pour.

rhl

I see, so just sticking it on the floor insulation is probably a reasonable tradeoff here.. trying to raise things up 1/4” doesn’t seem super practical.

Zman

I have never seen a concrete crew actually pull up the mesh, so it ends up worthless as reinforcement and functions only as a tubing holder.

3" is too thin unless you are using a very special concrete mix.

Tubing at the bottom of the slab is not ideal. Your 3" insulation will help would downward heat loss.

I think the best installation would be a 4"or 5" slab and real rebar on chairs. This give you a solid floor and the tubing in the center. It will cost a bit more but perform much better.

Jamie Hall

Could I digress very slightly, and enquire why it is that you want a 3 inch slab in the first place? That really is pretty thin. Unless you have a really good crew to place the mesh -- and keep it where it belongs -- and use the finer aggregate as @hot_rod suggested -- and use fiber reinforced concrete with a plasticizer in it, as @Zman implied, you will have cracking problems, tubing or no tubing. Even with all that, over compressible insulation (such as the XPS) it may crack if there is any significant load on it. You must not -- with that slab thickness -- exceed the rated compressive strength of the foam with localized loads.

hot_rod

Where is the slab being installed? Basement, shop? If it is a non-structural slab you can get away with a thin pour. The sub grade or framing below needs to be stable, compacted base rock or joists sized for the load.

I have poured 1-1/2' slabs over wood floors with a beefed up mix. Extra fiber, plasticizers, a few other additives to slow curing. It did have some hairline cracks but the plan was to put a engineered wood flooring over it, so as long as the slab doesn't shift you should be fine.

Gypcrete flows down to 1-1/2" for radiant also, but it is not a finished surface, and needs to stay dry

rhl

It’s for my basement floor. We dug down about 14” to add crushed stone and drainage pipes. I can probably ask him to do a 4” slab instead of a 3” slab. My excavator gave me the name of the concrete company so I can inquire about the aggregates and whether it’s fiber reinforced.

We also are doing the garage, which will be much thicker. I think like 6” or 7” currently the entire basement garage included is a 1” slab from 1929.

Gordy

"practical" Is the key word. You will be trying to put a lot in a confined space, while trying to maintain clearances. Don't forget the contraction cuts if any with tube location.

I would do fiber mesh, or wire pins instead of woven mesh. If the finish will be just a generic basement slab. One less thing for the concrete crew to worry about maintaining during the pour. Also one less thing to inflict damage to the tube.

You won't find less than a 1" slab bolster for a continuous chair for mesh. 1" chair, plus mesh, plus 1/2" tube leaves an 1 1/4" over the tube. In a perfect world. Now the pour begins. With people walking all over, the mesh is going to pump up, and down, and you really won't know where the tube ends up. Until you see it sticking out of the concrete............

Good compaction of the sub base, and properly located contraction cuts goes along ways in preventing slab failure.

hot_rod

3-1/2 is a typical slab, as that is the dimension of a 2X4 used for forming. Will the basement slab be a finished surface, or do you plan on covering it?

I'd be impressed if a concrete installer could get the slab the exact thickness you request across the entire area.

Also 4" should be plenty in a shop, unless you work omg heavy equipment. The thicker the slab the slower the response, both up and down in temperature.

rhl

Right, I confirmed that we can add the fiber reinforcement to the slab. Based on what the concrete people said, and what you guys are saying we can easily increase the thickness of the slab by about 1”.

If we “plan” for a 4” slab does it still not make sense to try and lay the pipes in the middle?

Just so I understand what if it was a 6” slab? Would you do two 3” pours or something?

hot_rod

These are rail type "chairs' for holding the mesh, or rebar in the center of the slab. Put these down on the foam, lay the mesh on top and zip or wire tie the tube to the chair and mesh.

They come in different sizes, this is a 2" and would work for mesh on a 4' pour.

If you use rebar get an 1-1/2" chair as you will need to cross over the rebar in spots and that is the thin part.

IF the slab is going be the finish surface I would power trowel it to burn off the fibers that poke through. Otherwise it looks like dog hair all over the slab, hard to clean or polish.

If you really need a 6" slab, just a single pour is fine with the tube suspended on tall chairs. Keep in mind the extra cost of going from a 4 to 6" pour. A 20X30' slab goes from 7 to 12 yards when increasing from 4-6" thickness. Adding another 12,000 lbs of mass also.

Just know concrete guys hate chairs as they are trip hazards when they pour. But they also hate lifting the mesh into the pour. Not an easy "sell" for the guys doing the pour.

RetrosPex

Use 25 psi rated foam. Tape the joints. If you are going 3" thick of foam (a great idea) use two layers, stagger the seams, and tape them. 4" of fibered concrete. Either use the chairs, as people mention, or as a alternative, get some 3/4" 20' sections of cheap pvc tubing and staple it to the foam with plastic staples with barbs. Put it in a cross hatch pattern so your PEX tubing runs across it at 90 degree angles, and zip tie the pex to the pvc. They are going to walk all over it, and like guys said they won't pull up the mesh. It rusts anyway...why use it?

Pressurize your tubing with water prior to pour, and have a pressure gauge so you can watch for leaks. You can buy a setup for $20 to do that. Have a ball valve on it so you can shut off the water quickly if a leak occurs. In about 20 slab pours, I've never once had a leak. Oh, and don't put any joints in your tubing. Figure out your loops to stay at about 300 feet max length for 1/2" tubing, and put the loops so that the hottest water goes the places that are the coldest. Like in front of a outside door, or basement window.

Make sure the dirt is compacted carefully before doing any of this. Do not put in a huge amount of fill and compact it all at once...do it in lifts of a few inches at a time to get proper compaction.

Hook up your floor when it is cured, and have a nice cold beer on your warm floor.

Zman

I like chairs much better than continuous PVC suggested above. The slab will be weakened for sure with the PVC technique.

Jamie Hall

25 psi foam? That would be permissible under a 4 inch or 5 inch, fully reinforced slab. However, as I noted above, under a 3 inch slab with pipe in it, the floor load rating will be the compressive rating of the foam; the slab cannot be used to contribute. You could check with your building code, but I'd not be comfortable with anything less than 50, and preferably 100 psi.

Gordy

Xps Formular 150 has a compressive strength of 15psi, and the 250 is 25psi. Plenty sufficient for under a 3-4” slab in a basement.

15psi is 2160 lbs a sf, and 25 psi is 3600 lbs sf.... that’s a pretty heavy 1 square foot point load in a basement.

Jamie I think you are incorrect. The concrete even at 3” which will have mesh will bridge any point loading. So the slab is not limited to the psi rating of the foam.

What does limit it is poor compaction of the sub grade., and a poor quality concrete mix design.

nibs

Have not read all the comments, but here is what to do.
Staple the pex to the insulation.
Skip the steel mesh, If you do use mesh, use plastic wire ties to fasten pex to the mesh
Use lots of fiber.
Use superplasticizer and a very low slump, Very little water.
cast in your control joints, you can use plastic drywall corners, they do not have to go all the way to the surface. Do not cut your control joins.
Sleeve the pex at the control joints, a foot either side, plan your loops to make minimal crossings of the control joints ( we usually got away with 3 crossings per loop).
If you are mixing your cement on site, use 1 cement, 2 sand, 2 aggregate by volume use superplasticizer and fiber in abundance, just be careful that the aggregate does not drop out.
If you are ordering in a truck load, specify 6000 psi mix. I would use fly ash as long as you are not pouring over brass/copper, it has been reported that fly ash will attack copper.
There is a ton of info re cement and mixes on the internet, spend a few hours to learn. Concrete is an amazing material.

Sorry I did not chime in earlier, but as we are in Mexico we have to go to restaurants to use the internet.

rhl

Thanks everyone for the education. It sounds like a 3” ish slab is mostly ok from what everyone is saying. It seems easier to do this particular job with something like uponor fast trak just right on the ground..

It does sound like adding fiber is a good idea, but will require my contractor to finish the surfaces more than he may have otherwise been planning. I’ll discuss that with him.

I’ll look into plastiscer. I believe they were planning ~4000psi for the basement areas that aren’t the garage, where they were thinking 5000-6000psi.

hot_rod

Do your best to lift the tube into the pour, at the bottom, stapled to the insulation is absolutely the worse location from a heat transfer point. Too deep and not encased in the pour to afford best transfer. Slower recovery and higher SWT when the tube is deep. Plastic (pex) is an insulator, so give it the best chance of moving that energy at the lowest SWT..

nibs

@hot_rod, can't believe am gonna argue with the mayor of the wall.
A three inch slab is pretty thin, my 2" slab responds beautifully to the heat input, with the pex stapled to the insulation. Your knowledge of hydronic systems is legendary. Concrete is a whole other discipline with solid science backups.
What, in the practical world is the diff between pex up 1.5 inches in cement compared to pex under 3 inches?.

Gordy

Now how would that look with all the studies done that prove different? The mayor knows better, and should rightly so say what is scientifically proven.

Concrete with out pex is a waste. Pex not in concrete is a waste

Key point is make sure the tube is completely encased. , and at a reasonable distance from the top surface.

Jamie Hall

OK, OK... I'm conservative... and always look at a concrete slab in structural terms...

Just don't drop anything heavy on it.

Zman

It doesn't sound like there is a limitation that restricts you to 3".
A 4" slab is a safer bet and will probably cost the same as adding fancy additives.

hot_rod

I> @nibs said:
> @hot_rod, can't believe am gonna argue with the mayor of the wall.
> A three inch slab is pretty thin, my 2" slab responds beautifully to the heat input, with the pex stapled to the insulation. Your knowledge of hydronic systems is legendary. Concrete is a whole other discipline with solid science backups.
> What, in the practical world is the diff between pex up 1.5 inches in cement compared to pex under 3 inches?.

> @nibs said:
> @hot_rod, can't believe am gonna argue with the mayor of the wall.
> A three inch slab is pretty thin, my 2" slab responds beautifully to the heat input, with the pex stapled to the insulation. Your knowledge of hydronic systems is legendary. Concrete is a whole other discipline with solid science backups.
> What, in the practical world is the diff between pex up 1.5 inches in cement compared to pex under 3 inches?.

I'd consider it discussion , not argument

I am of the opinion a tube completely surrounded by concrete will move heat energy better than on that has a portion of it sitting on the insulation. How could this not be true? Concrete moves heat energy better than foam me thinks.
I'll bet if you were to cut open a slab with tube on the bottom you would find air pockets along the bottom portion as the 3/4 or larger aggregate doesn't flow under the tube very well without some vibration to move the "paste" around the tube.

Similar to when they pump tall foundation walls and you sometimes see a lot of exposed rock when the forms are removed.

I've shown some numbers above and some FEA data, let's see you numbers or analysis to prove you points.

rhl

Ok so a 4” slab with 1/2” tubes about an 1” below the top is not a bad plan. Quite frankly I still don’t see how we are going to get someone to the lay the pipes out at this high elevation without tripping over everything.

Jamie Hall

rhl said:

Ok so a 4” slab with 1/2” tubes about an 1” below the top is not a bad plan. Quite frankly I still don’t see how we are going to get someone to the lay the pipes out at this high elevation without tripping over everything.

Speaking as an engineer who used to design some of this stuff... there were times when, for various reasons, I had to design slabs with some rather complex rebar patterns which were, I agree, difficult for the concrete crews to pour correctly. As a result, the price for the pour was higher. However, it is the designer's task to create something which he or she can endorse, which he or she knows will work -- and it is the installer's task to do the job correctly. Quite frankly -- to use that phrase -- if they are unwilling or unable to do so, they shouldn't be on the job.

hot_rod

Just depends on how badly you want it right, I suppose. I've tube 12,000 sq ft slab with rebar grids up on rail chairs and the tube attached. The engineers were on site to assure the bar was in the slab, as their name was on the permit.

Certainly can be done on a small residential slab.

Concrete guys are a different breed, I have seen them take a rake and perforate the 6 mil vapor barrier because it holds water and slows the cure. Hard to argue with a team on surely concrete finishers.

Things go very quickly when the trucks arrive and the mud starts flowing. Not much time to change horses mid-stream.

The key is to let the concrete crew ahead of time about radiant.

Boxes of donuts in the morning also go a long way to getting crews on your side.

nibs

Once you start dealing with a thicker slab 4" etc, then the rules change, of course you would like to get the pex up into the middle of the mud. In a thinner slab raising the pex is going to increase the stress points on the concrete and exacerbate cracking. Unless the cement is poured very carefully there will be air pockets around the pex, especially since we need to use water reducers, and minimal water in the mud, this applies to all slabs and columns.
My point is that the difference in heat absorption by pex on the insulation or in mid concrete will be very difficult to measure in a 2 or three inch thick slab. Unless we are dealing with huge areas. My experience with concrete spans many years, in the field and in theory. Have had to fix errors and design difficult pours,
PS; I do not have my books here in Mx, and have only a little time at the restaurants we use for WIFI.Cheers,

rhl

Thanks guys. I went over all of this with my guys. I think we are developing a plan..

Gordy

Concrete work is very competitive. Most times it’s about getting multiple pours at multiple locations in one day to make money.
Things like vapor barriers do hinder production depending on weather at the time.

Biggest thing is communication. making sure the concrete contractor is on board with all the details on the radiant so he can bid accordingly. It does add labor, and probably a pump cost to a job that may not need one with out radiant. Expect to pay more.

nibs

Read a sad tale her a year or so ago about a cement crew running a cement buggy over wire tied pex, the pex wound up with a number of leaks,
We poured our floor in 5'wide strips, and hand carried the cement so as not to harm the pex.
Good luck and don't forget your expansion joints.

hot_rod

never underestimate the limitless ness of incompetency 😲
most all concrete supplier have or have access to pumpers, even small pumps pulled behind a pickup

Gordy

Jamie Hall said:

rhl said:

Ok so a 4” slab with 1/2” tubes about an 1” below the top is not a bad plan. Quite frankly I still don’t see how we are going to get someone to the lay the pipes out at this high elevation without tripping over everything.

Speaking as an engineer who used to design some of this stuff... there were times when, for various reasons, I had to design slabs with some rather complex rebar patterns which were, I agree, difficult for the concrete crews to pour correctly. As a result, the price for the pour was higher. However, it is the designer's task to create something which he or she can endorse, which he or she knows will work -- and it is the installer's task to do the job correctly. Quite frankly -- to use that phrase -- if they are unwilling or unable to do so, they shouldn't be on the job.

Here is one for you Jamie. Factory built in 1918. There is a free span 8” thick slab over the basement level that is 100’ longs 20’ wide. Columns every 20’ down both sides of the bay. The fella that bought the abandoned factory for 600k is a machinery mover. So this slab was taking a beating of 80,000 pound semis being loaded, and parked over night with a 15 ton fork lift doing most of the loading, and unloading.

The double mat #5 rebar 10” on center both ways was tied diagonal instead of transverse, and longitudally to the bays length. Not designed for shear at all. The slab was starting to fail, and the bottom mat rebar was reflective cracking on the bottom of the slab. Some spots the concrete spalled off the rebar which showed it didn’t have clearance which is usually 1”.

The rebar is what I call 3rd generation. First being the spiral rebar, second the square rebar, third the round rebar with less amounts of ribbing in it, and present day being 4th generation until epoxy coated, and now stainless steel rebar.

Anyway we ended up installing shoring scaffold 12k rating four sections in each bay with 16’ long 12” tall x 6” wide I beams transversely in the 20x20 bays tightened everything right up..

The funny part is there were signs posted on the main level that
Read “ 70 ton load limit per bay. With the no more than 350lbs a square foot kicker.......which basically means you can have 70 tons in a 20x20 bay, BUT it had to be spread out over 400 square feet.

I’m the end what got me, and the structural engineer was the thought of installing the rebar diagonally. He said that was the spookiest design he ever seen.

Jamie Hall

Steam piping isn't the only place where one sees things which get one to saying "What were they thinking?"!

nibs

Did one remediation on the ceiling of underground parking, prime contractor left out the concrete beams.
We drilled in thousands of anchors with epoxy, tied the rebar and used shotcrete to stiffen the floor.
Never did get back to see the finished work.

hot_rod

Concrete when used for residential radiant in most cases is for heat transfer. Footings, foundations and thickened slabs handle the structural loading. So we want to maximize the ability of the slab to move heat. Studies and analysis have been done showing the optimum way to place the tube for best heat transfer.

Certainly tube at the bottom of a 6" slab will still perform, but the steps to make installation the best performer are really not that tough.

Most all of the concrete installers I have worked with agree that fiber in the mix provides better crack reduction than re-mesh or even rebar placed the bottom of the pour.

The only time I see concrete installers actually lift the mesh and tube into a pour is when it's on their own home me included

Gordy

Pulling the mesh up never works well when they do attempt it.
It just gets trampled back down to the bottom. Your standing on what you are trying to lift.

nibs

We all feel that the pex performs best when in mid slab, when practical.
Does anyone know of any hard data to support our belief?.
With modern technology an accurate test series would not be all that costly. 2" through 6" by the inch, comparing pex on the insulation and in mid height.

hot_rod

This article was talking about tubing depth about 18 years ago. Read it over and see if you agree.

I think infrared pics of various slab installations would be telling and may match these SIMs closely.

Along those lines, the same thoughts on the styrofoam "knob" type insulation where the tube is at the bottom and also surrounded by foam in portions of the loop. Be nice to see some modeling or testing on that application also.


https://us.v-cdn.net/5021738/uploads/editor/fd/xbnns80xn6mq.pdf

https://us.v-cdn.net/5021738/uploads/editor/fd/xbnns80xn6mq.pdf

hot_rod

Sorry about the double link, here is what I was after.