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Thread: in floor heat and cement thickness

  1. #1
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    in floor heat and cement thickness

    I'm in the process of converting an old barn into a heated shop and the old milk house into an office, we have the new rafters and false ceiling installed and the milk house is done except for the new ceiling and insulating, now for the tough questions, anybody use in floor heat in their shop where your putting in heavy equipment, I was thinking about 8 inches of cement in the floor but the cement guys are telling me 10-12 and not to put down the two inch insulation board under the cement period, they claim it'll crush due to the weight and cause the floor to crack over time and to just tie the heat tubes to rebar and lay it down first so its under the cement, the second problem is they are also telling me that in floor heat won't work good in cement that thick. I'm slightly confused on all this, first everyone recommends to insulate under the cement and thats what we did in the milk house but is it really needed if I only heat the shop to around 50-55 degrees, thats the temp of the ground anyhow inside a building thats heated so why would heat be lost into the ground if its already 50 some degrees and heat rises doesn't it? The next part of my question is why wouldn't the heat make it up through 10-12 inches of cement if heat rises? it makes it through 6 inches just fine, why not thicker? Would the cement and weight of what your running on the floor crush the 2 inch thick foam board under the cement? Is that thickness needed for heavy equipment? I plan on putting in a service pit to work on and service equipment and and want to make it all heavy enough to back loaded lowboys in and right now we weigh in at 110,000 lbs so I want to make it good enough to not worry about cracking the cement, my largest excavator weighs in at 60,000 and the cement people are worried about the vibration and weight in a short area. I don't want overkill but not to undersize it either. Someone out there has got to have experience with this and basically none of the cement guys have done anything with heavy equipment and don't know for sure, its kinda unique and not a big demand for like houses and driveways so I understand where they are coming from as far as experience goes. Tanks in advance for any input.

  2. #2
    Senior Member bill onthehill's Avatar
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    I used a product called slab shield under my floors with radiant tube heat. It is a lot like bubble wrap insulation but has a foil layer as well as 2 flexible foam layers. It is only 3/8 thick and does the job. As for the concrete thickness I would think you want at least 10 inches on the main drive path with minimal stone under it. I used wire and fiberglass reinforced concrete and have not had any breakage but my weights are more like 20,000 not 110,000. I did my center section as a separate slab and went 6 inch or more on it. You could also run the radiant around the rest of the shop and not do the center where the heavy weight will be concentrated. I strongly advise sleeving the tube where it passes from slab to slab with a bigger diameter plastic conduit. This will prevent pinching the pex as the slabs shift.
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    Bill on the hill ...Do you like that heat ??? A sawmill near me put floor heat in there shop and i heard one of his boys say it was great till you gotta get on creeper ?? Too hot ..

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    I'm a GC with several years of experience in pre-engineered steel buildings for shops, warehouses, offices etc. Although not, an engineer, I've had experience. Although I didn't actually install the floor heat tubes, I did the concrete and worked with the guys who did install the tubes in a few projects. To me, as far as the floor cracking or crushing, the biggest most important thing is the base. Make SURE you have a well compacted stone base. If the base gives, the floor will give, whether it's 4" thick or 15" thick.

    As far as cracking, all concrete cracks. Generally it's shrinkage cracks. Typically I'll see shrinkage cracks every 25-30' across a floor slab. the best method of controlling cracks is sawed joints. Saw 1/4 of the floor thickness. (Make sure you don't saw into the heat tubes.) An ideal way to assure the slabs don't misalign at the controlled cracks is by installing bars specifically to adress that. I'd recommend a #5 or #6 bar every 2' -4' along the controlled crack point. Grease half the bar so the floor can expand. Don't grease the other half.

    If it were my floor, I'd start with a stone base. A minimum of 6", probably more. Then I'd have an unreinforced mud slab, maybe 4" thick. Then a layer of 1" Blue Dow board, maybe 2" since your in Iowa. Then, #3 or #4 reinforcing bars on a minimum of 24" centers in each direction. If you're really concerned, #4's on 12" centers. Tie the tubes on the rebar grid, then place a 6" floor on top of that. If you're concerned 8" thick.

    You never mentioned what type of machinery you have. If it's tracked machines, remember the tracked machines are designed to distribute weight. A 50,000 pound wheeled machine would stress a floor more than a 110,000 pound tracked machine.

    Lastly, I'll say this. A heated floor is really nice. Open the wide shop doors to bring a machine in, the cold rushes in, close the doors, and the area is instantly warm again. It really is nice...VERY nice. If you looked at all the costs involved in a heated floor, and studied the energy savings compared to a couple of unit ceiling gas fired heaters, would a heated floor really ever pay for itself? But, if it's just something you want...and can afford it

    That's my opinions, for what it's worth.

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    Randy, in 2003 I built a new 70x120 ft shop with in floor heat. The heat I used was off-peak electric. I dug down 4ft all the way around the perimeter of the building and stood 2" thick styrofoam. the material inside the building was thourghly compacted sand with a 6inch base of 3/4inch minus road gravel, the cables had been plowed into the sand to a depth of about 12 inches. I then pour 4 inches of concrete in all bays except for one heavy duty bay where 6 inches was installed. I routinely place 120000 pd, loads on this bay with no problems what so ever in the bay itself. The only problem I've had is on the apron outside the building. The D8's coming and going in the bay has broken the apron on the outside end even tho they come in on conveyor belting. I hope this helps you in your decision makeing...good luck....oh one more thing, I buryed a 300 gal. barrel outside the building with a floor drain arrangement on the inside to dump used oil in...

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    I have both tracked and wheeled equipment, when we have the lowboys loaded we plan on backing them in so technically we would have up to and over 110,000 on my lowboys at times and want to unhook them so we can do repairs on the equipment dead while loaded, I've had a need for that quite a few times over the years, we can get them loaded wounded but have to fix them while on the lowboy and are planning for it to happen again. My shop will only measure 30x80 but the end 20 ft. will be for tooling and shop benches so only 60 will be used to back stuff in. We were hoping to pour the floor in sections 15 ft wide at a time so we can handle the flat cement work ourselves, we plan on having two layers of rebar every 24 inches each way on center at two different levels but stager the levels so we end up with a grid of rebar ever 12 inches when viewed from above. I originally wanted to tie the tubes to the bottom set of rebar but the cement crews I've talked to about pouring the service pit walls told me not to do that because it would weaken the floor by having hollow plastic tubes imbedded in the cement, thus making a hollow void and weaken my cement, don't know if it would or not but maybe it makes sense. The cement guys I've talked to were not as worried about the rubber tired machines and weight as the tracked stuff from the vibration aspect, even if I'd back them in on rubber belts, they all claimed the vibration was way worse than any constant pressure from more concentrated weight on smooth rolling tires. That surprised me but I don't have enough experience to argue with them either, any thoughts on that aspect from anyone else?

    Impact, the blue board your talking about is that the foam board most use for insulation? The guys around here are telling me not to put it in because that will be the weak point that can give and actually cushion and cause the cement to give when we drive heavy stuff in. Their concern was why put down the compacted layer of rock and have a soft spongy layer between the rock and the cement, they are telling me not to put anything down for insulation and just eliminate it, one told me to put the small thin layer of the flexable insulation thats only about 1/2 inches thick, his opinion was that way you put some down and I only have 1/2 inch to compress causing fewer problems later on down the road as far as crushing it from heavy weight. I honestly don't know or have any experience with any of it, we've put it in my folks basement and garage floor but maybe a car has driven on it and nothing heavy and thats a proven design and we knew it would work. None of the cement crews I've had look at would would put much of anything down first except for the one who recommended the 1/2 inch thick stuff, they all claimed it would crush and cause the cement to give up and down over time as much as the thickness we put in and cause the cement to fail over time and buckle. Any thoughts on this, are they right or wrong, if we didn't insulate it would all the heat go down into the ground like the heating guys are saying instead of up like common sense would predict. The heating guys don't have any idea of the crushing idea the cement guys are concerned with, they only know heating and the proven practice of insulating under the cement, as they say, nobody seems to agree on anything here, the cement guys think one way the heating guys the other. The heating guys are telling me to put the heat tubes attached to the rebar and cement them in and the cement guys say no way that'll weaken the cement. None of this needs to meet any code, we're in the country so thank god I don't need to deal with inspectors too yet who would have another opinion still.

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    Yes, typical foundation insulation is called Dow Board. I'm not engineer, but, I've put down numerous slabs that have been engineered by a registered engineer. The new energy codes require (at least here in ky) a 2' vertical board around the perimeter just inside the footings. I'd think the requirement would be 4' vertical in Iowa. then they require a 2' wide band 1" thick around the perimeter laying horizontal just under the floor. I've installed dozens or hundreds of slabs in this manner and have NEVEr once heard of any problems whatsoever. I know i'm just "Some guy on the internet", but honestly, it's simply should not be a concern. I wouldn't know where to send you to verify that information though. Same thing with the hollow tubes casts in the concrete. Not a concern.

    For loaded semi trailers grossing 110,000, I'd probably want 8" of concrete. The difference between a 6" slab and a 8" slab in a 30' x 60' area is only 11 yards. At $100 per yard, $1100 may be a good investment.

    As far as heat migrating through concrete, concrete is practically a 0 R-value. Yes, heat rises, but, if it was insulated beneath, it'd have no choice but to go up.

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    Several posts have mentioned that "heat rises" which is correct only respecting the air in your shop so install ceiling fans. Hot air rises because it is lighter (less dense), not because it is hotter than cold air.

    As far as heat flow in a solid, like your floor, heat will only flow from hot to cold. If the ground under your slab is as hot or hotter than the slab then the heat will rise to heat the air in the shop. If the slab is hotter than the ground beneath the slab, heat will flow into the ground. If your slab is hotter than either the air above or the ground below, you will have heat going up and down. That is why you should have some insulation to keep the heat in the shop where you want it.

    I don't know what the compressibility of insulation is (consult the manufacturer or an engineer) but if it was buried deep enough, any load above would be spread over a large enough area to not compress the insulation too much, same principle as burying a culvert under a road. I know the highway dept. here has installed styrofoam insulation under major highways where they have serious frost heaving problems.

    Perimeter insulation as mentioned by others around the slab would also be beneficial.
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    Senior Member bill onthehill's Avatar
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    I have worked in shops heated by the slab heat and it is very comfortable. The concrete stays warm and dry and it recovers from doors opening quickly. Since you plan to do 15ft. slabs in a 30 ft. area consider zoning it into 4 zones. Each zone would be 15X 40 and allow for good heat transfer. I zip tie my tubing to wire but rebar will do just as well to hold it in place while pouring. I am in the process of building a second greenhouse for the wife and it will have pex tube in the floor the same as the one I did last year. I have it in my porches as well as my basement floors. I used the same soft foam insulation under the tube in all of them. Once you have a heated floor you will never want to go back to overhead heaters.
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    Impact and norite, thanks for the info, around here they are saying to keep the tubes below the cement and not in it at all. I originally wanted two layers of rebar one at about 3 1/2 inches up from the bottom and the other 3 1/2 inches down from the top and tie the heat tubes to the bottom rebar but all the cement guys are saying no, but get this, they want me to use wire mesh and tie the tubes to that and lay it in the bottom and cover it up and put no rebar in above, that would be the reinforcing, I've spoke to six differnt cement guys and this is the same story I've gotten use the mesh on the bottom for the rebar and foreget anything else? My thoughts are what good is it doing under the cement to put the mesh or rebar and nothing above, that was somewhat idiotic I thought but can all of them be wrong, their reasons were the tubes in the cement would create a hollow space and weaken it. My thoughts were two layers of rebar would be better than one under the cement and tie it to the bottom layer and it would have to be stronger than even with the tube in it to ever weaken it.

    One guy told me he did his himself for his house and to staple the heat tube to the insulation board and put rebar above, thats what I did to the milk house and basement and garage but the cement was only 4 inches thick there and everyone seems to be in agreement that it would work fine on 6 inches thick but not thicker so my thoughts were to put it basically in the cement 6 inches from the top and incase it in the cement. The heat guys are telling me on 6 inches or less staple it to the insulation board and don't have an answer on thicker cement period but they insist it needs to be under the cement not in it.

    My initial concern with stapling it to the insulation board was if rodents got under the cement they could basically live in the insulation baord and chew holes in the lines causing them to leak but if it were incased in cement then nothing could destroy the lines but again I do a lot of demolition and it seems every buiding we destroy has critters living in or under the cement. Maybe its nothing to be concerned with but thats my concern, if they ever got under the cement and into the insulation it would heaven for living for them all winter, mice never had it so good if they found that place to live.

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    Senior Member FSERVICE's Avatar
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    Randy are you doing the install of the heat yourself or are you going to hire it out?? you should contact your supplier for the tubing/heat equipment they will be more than happy to help you with how deep in the concrete it needs to be or any other concerns that you may have! there are several makers of the tubing/furnaces they all have the engingers to serve you if you have any questions just message me i have put in several of these systems

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    Senior Member bill onthehill's Avatar
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    I attached my tube to the wire and just pull it up about 2 inches as we poured the concrete. This lets the wire do it's job and gets the tube up in the concrete where it belongs. I agree on the critters getting in under for a warm house and once they chew a hole in it that section is done for.
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    Senior Member EZ TRBO's Avatar
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    First off...cement will not hold worth a darn...really powdery and loose. Concrete on the other hand, that will do you good. lol, when you work for a company that sells concrete they have a big issue when you call it cement.

    Curious as to where abouts you are located Randy?

    Better to go thick than too thin. I know my uncles floor never had anything but some fines under it where the tube laid on and it works wonderful.

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    You could use welded black pipe for the zones in the floor- would solve most of the issues discussed sofar...

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    Senior Member Nac's Avatar
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    I am planning on building my 3 bay shop next year and I am a concrete contractor. I will have 2" rigid insulation along the foundation walls and 2' along the perimeter of the floor. Then I will have an 8" base of 3/4 clean stone compacted a layer of the bubble rap foil insulation for the base. Then a 6" 5,000psi slab with #5 rebar 12" OC with 2 1/2" steel chairs holding it in the center of the slab with the pex tubing tied to it. After it is poured and bull floated we will add a dry shake emery powder to it before floating to give it more strength and impact resistance.
    PS: You mentioned you will have your lowboy in there sitting on the floor unhooked from the tractor. If you know where it going to sit you can place some steel plate insterts before hand and pour around it to help spread the spot loads. You would have to pour a seperate spread footing and anchor some type of beam with a plate welded to top at the right elavation and then can pour around it when doing your slab.
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