Concrete Slab Insulation

I am putting a hot water heated concrete floor in my garage and I am wondering if it is a good idea to put insulation under it? I have been getting mixed opinions about this. I would also like to know if it is worth putting fiber in it if I am also using rods?

P.S. We live in northern WI.
I can understand why you are getting mixed opinions.  The right choice depends on additional information I need.

1.  Is that a slab-on-grade?  If so then it depends on the soil for support.  If you use some sort of insulation between the concrete and the soil it can weaken the system.
However, if when you said you are using "rods" (reinforcing bars?) you meant that it is a structural slab which can stand on its own then the support is not critical.  Was it designed by a structural engineer?

2.  If you can find insulation that is strong enough to support the weight of the concrete and your cars without crushing then I see no reason to worry.  But if that insulation is going to deform in any way it will damage the concrete.

3.  As for fibers - depends...
If you are adding fibers in order to gain some strength then forget it.  They don't.
If the purpose is to reduce the cracking in the slab then there are better ways to do it, especially when you have reinforcing bars (or is it wire mesh?)
Fibers add complications in the placing of the concrete and I see no benefit in your case.  You should however consider cutting joints that will split the slab into sections that are no bigger than 15x15 ft.  After the concrete matures (3-6 months) you can seal the joints and forget about them.

What were the concerns others had about the insulation?  Let me know if there is additional information about the slab and what you decided to do.

 

 The slab I am about to pour is on-grade. My main concern is the amount of heat loss going down into the  ground without insulation. I plan on insulating around the outside footings but not under the slab. Some say the loss is minimal others say it will be a great amount. I would like to hear your opinion on this matter.
 Thank you for your advice on fiber reinforcement. It seams everyone in this area is putting it in. Not having poured concrete with this type of reinforcement I wanted to get some input before I start.

 

 I am afraid I am not too familiar with the thermal properties of concrete in your area, or the temperature differences between inside and soil under the slab.
Another option you may want to consider is using Light Weight Aggregate Concrete.  This material can provide considerable insulation on its own but is not as strong as regular concrete.  Check with local Ready Mix plants to see if they can produce the strength you need (it is very possible if they know the material) at a price you can afford.  Since that is not standard material there may be a premium.
With appropriate reinforcement LWA concrete can be used for any structural purpose.  Personally I produced LWA concrete with strength of over 5000 psi without a problem.