How do you determine amount of rebar needed?

[dstig1]

2" of foam for a heated shed would be wise. 4" would be even better, but the expense does start to add up. Make sure to get a vapor barrier down and sealed. In the foam section you can tape the seams of the foam if you use urethane or XPS (the white bead board is vapor permeable so it will not do te same job as the other two in moisture control below a slab.)

 

[Piperflyer]

Go with fibermesh and you can forget using rebar

 

[Tractor Seabee]

Wow, I had heard that a minimum of 1 inch was needed, with 2 inches being better, but 4 to 6 inches seems like a lot. I realize that by adding more, we are certainly increasing the resistance to conduct the heat to the ground. I haven't checked locally, but a 4' x 8' x 2" sheet of foam board insulation was about $35. This could get expensive.

Home Depot has EPS 4' X 8' X 2" for $20/sheet, large qty ask for contractor price, retired military you get another 10% off. May find a better price if you shop around. EPS is R-5/inch, 6" is R-30. Do you want the heat in the ground or in the space, there is a choice. For cold room or heated space that is what you want, payback is quick. Urethane has better R value hence my 4-6". 6 for EPS, 4 for Urethane. Lay each layer at right angles to the other to minimize voids and fit in tight. These kinds of floor end usage you do not want to walk on the insulation or reinforcement while placing concrete. Place concrete with a pump and boom and get on top of placed concrete. That is the professional way for best results. Yes, it costs more to do it right. All depends on your end goals and tolerence for screw-ups. I am serious about the saw cutting, or use zip strip. Zip strip is labor intensive if you want it straight and is not my choice.

Ron

 

[Johnp33]

I finished building a 30x80 ft building we had to get an engineer to draw up the plans and the grid for the re bar. Without the engineer we could not get the building permit, we needed the exact specifications for snow load and earthquake resistance.
1/2 the building has a 4 ft wall back-filled, and needed counter-forts, everything had to be tied in to give us the proper stability. There is the footings with Re-bar the short walls the tall walls and the grid needed for the main pillars, the door opening 14x14 everything needs extra re-bar all tied in and secured to the floor witch had a 2x2 grid but then added pieces in each corner and pillar footing. There was about a ton of shortened pieces and bent stuff.
It was not cheap, but it is done and done right, I see other peoples buildings without certifications, and they are already having problems.
Just a thought!

 

[Raspy]

Wow, I had heard that a minimum of 1 inch was needed, with 2 inches being better, but 4 to 6 inches seems like a lot. I realize that by adding more, we are certainly increasing the resistance to conduct the heat to the ground. I haven't checked locally, but a 4' x 8' x 2" sheet of foam board insulation was about $35. This could get expensive.

Insulation "under" the lab is controversial. Slab edge insulation is important and keeping the radiant tubing back from the edges of the slab helps. Under slab insulation is not so important because, unless there is moving air or water under there, there is no place for the energy to go. The ground affectively increases the mass. In other words it's the same as making a thicker slab, and just slows the response time, maybe. Eventually the ground gets warm anyway, with insulation, as the insulation only slows the process.

But one thing about insulation is true, the insulation must be capable of supporting the concrete slab for the life of he building without breaking down. That's where it gets tricky.

I left it out on my house and I leave it out on all my radiant projects. They are all very efficient without it. If you do want more than just perimeter insulation, run it flat under the slab and just carry it from the outside edge to either two feet or four feet in, and leave the rest of the slab supported on the solid, compacted ground with a moisture barrier.

 

[dstig1]

Bzzt. Sorry but insulation under the slab is not controversial at all. Dumping heat into the ground is no different than dumping it into the air. Both the air and the ground are heat sinks that will take all the heat you will dump into them from a slab. You insulate to keep the heat where it does you good. This is well-understood building science.

Foam under a normal slab is completely capable of supporting it, and lasting a long, long time. Plastics do not break down in an environment like that in a timescale you will ever notice. I'm not sure where your concern is here, John.

 

[Tractor Seabee]

Bzzt. Sorry but insulation under the slab is not controversial at all. Dumping heat into the ground is no different than dumping it into the air. Both the air and the ground are heat sinks that will take all the heat you will dump into them from a slab. You insulate to keep the heat where it does you good. This is well-understood building science.

Foam under a normal slab is completely capable of supporting it, and lasting a long, long time. Plastics do not break down in an environment like that in a timescale you will ever notice. I'm not sure where your concern is here, John.

Amen, I was noting the original posters location, they have some **** cold winters, milder climates not as important, Florida, Hawaii, I would not insulate except under the coolder. Energy is not getting any cheaper.

Ron