Over the years I've run into several contractors and wholesalers who recommend the use of rolled-up reflective foil insulation as a means of insulating below or above a concrete slab.  These products are very easy to install and are typically 1/4 to 1/2" thick foam or bubble-wrap covered on one or both sides with aluminum foil.  They also claim to have an R-10 insulation rating equivalent to 2" of rigid foam insulation.  

Unfortunately these claims are completely unfounded and the insulation when used under a slab will only have an R-Value of around 1 not 10.  It might make a great thermal break and vapour barrier but that's it!  The reason these products claim to have a high R-Value is that they were designed and tested in cooling applications where the reflective surface bounces unwanted infra-red energy away from occupied space like up in an attic.  Furthermore, these products require at least 2" of air space in a dust free environment to perform in a way that is equivalent to 2" of rigid foam insulation.  In fact if you check the Reflective Insulation Association website you will see that they warn consumers about false claims when these products are used in slab heating applications.

When you're pouring concrete over a reflective insulation product its R-Value drops down to that of the foam or bubble wrap which is going to be only 10% of the advertised value.  Considering that many of these products are very expensive it is a shame that so many people are misled into using them in slab insulation applications.

Rigid foam is the best and most economical way to insulate under a slab.  2" thick insulation has been the standard for years but 3-4" is now becoming more common.  When insulating over an existing un-insulated slab you may have to compromise and only use 1" EPS-II or XPS but this is far better than none and offers an excellent thermal break.  

If you're doing an overpour over an existing slab and over 1" or more rigid insulation you will need to consider the holding strength of the foam.  If you're pouring 1.5" or more concrete then you need not worry.  However, if you're trying to keep the overall height as low as possible then you will need to reinforce the concrete or mortar so that it won't crack.  Many people renovating a basement will glue and pin 5/8" plywood over the rigid foam and proceed with a normal subfloor installation.  You can also cover the foam with 2.5 lb diamond lath mesh and this will reinforce self-leveling cement or a latex modified thinset mortar 3/8" to 1/2" thick.  For hybrid installations like these we recommend you give us a call to discuss your project so that we can take all your circumstances into consideration before making a recommendation on how to insulate and heat the slab.