Electric radiant heat in the garage
When installing electric (or any) radiant heat on a slab, insulation is the key to a successful and economical outcome. There are several types of insulating materials available with which this may be accomplished, depending on what is available locally, and cost per sq. ft. you are willing to spend.
1. Cork acoustic membrane. This is a decent insulator for the the thickness (1/8" to 1/4" is all that is necessary)
This has been used as an isolation membrane for tile floor installations for many years, and has a good track record and is TCA (Tile Council of America) approved.
2. Polystyrene crack/ Acoustic Isolation Membranes.
These are basically synthetic versions of cork. While the actual chemical compositions may vary ( I am being purposely generic) these usually have about the same R value and K value as cork, and are not prone to moisture absorbtion or insect infestation.
3. "Blue board" type sandwich boards.
Look up "Wedi" in your browser if you are not familiar with this type. It has by far the best R and K value, but is also the thickest and most expensive, and is also tested and approved by the TCA.
Also, it is worth mentioning that with that old of a slab, some specific crack isolation membrane may be adviseable, possibly in the form of a "roll-on" product.
I hope this helps you!