Based on the information you provided, I have made the following assumptions:
- "Electrically heated tile" means you intend on applying an electric-resistant radiant floor product that is either free-form cables or pre-formed cable mats.
- Your goal is to create a thermal barrier, with the XPS polystyrene, between the slab and the heating elements to minimize the chance of the heat sinking down into the slab rather than flowing up to the tile floor surface.
- The electric-resistant radiant floor product would go between the polystyrene and the tile.
- The application is for a dry area, not a wet area such as a bathroom.
I constructed an application similar to what I believe you are attempting to achieve -- a tile floor over resistant electric radiant heat over a slab-on-grade. (Disclosure: I had a professional licensed and bonded electrician and a tile mason perform this work based on my prescribed calculations.) The goal here is to get the heat to move up to the tile surface, not down into the slab where it naturally wants to migrate.
Assessing preliminary project requirements
The first step is to perform a heat loss calculation on the room to ensure that the heating product you have selected will perform to expectations and not send the heat down into the slab and the utility bills through the roof. Resources for performing a heat loss calc can be found at the following sites:
The next step will be to determine how much of a thermal barrier you need, with reference to R-value, to achieve your goals. There are a variety of options -- the best choice will depend on other variables in your application.
Designing your floor "sandwich"
Once you have determined your preliminary project requirements, you can start designing your floor "sandwich." Note: more likely than not, if you mix various manufacturers' products in the process of creating your floor "sandwich," you may effectively void the warranties for each layer. Generally, manufacturers will only warranty their products if they are installed with other "members of their product family."
- Typically the first layer will be some form of sealant (that may double as a crack isolation membrane) on the concrete slab, unless the slab is more than five years old and has never shown any signs of moisture (engaging a moisture meter on the slab will help you accurately determine the moisture level of the slab).
- This is followed by a thermal barrier that is adhered to the slab (make sure the adhesive is compatible with the sealant/isolation membrane). Again, this material should be selected based on desired R-value relative to threshold tolerance at different points of the room such as entry doors, closet doors, etc.; the need for addressing crack isolation; and the overall level of the floor. If there are irregularities in the floor, then you may want to use an underlayment such as cork to "absorb" some of those irregularities.
- The third layer will be thinset, followed by the electric-resistant radiant mat. The thinset is beneficial to the thermal mass, and thus allows the heating product to work more efficiently. For additional thermal barrier properties, you could consider laying your resistant electric wires in a radiant-positioning mat such as Master Heat's RPM.
- The wires are then covered by more thinset and then tile.
This is but one prescription for the application you described.
Some issues to consider with regard to the products you mentioned
- Extruded polystyrene foam is good at reducing heat transfer (acting as a thermal barrier) and possesses terrific compression strength, but the real question is -- can it stand up to the heat of the resistant radiant heat without being compromised?
- Is the slab so pristine and smooth that a rigid product like extruded polystyrene foam will lie perfectly flat, and without concern for flexing which could lead to cracked tiles?
- With respect to cost, performance, and environmental attributes, extruded polystyrene foam (XPS) can't hold a candle to cork. XPS is a petroleum-based product that will not biodegrade even in your grandchildren's lifetimes.
This is a very challenging application, and I encourage you to investigate all your options and address all the warranty restrictions that various product manufacturers may levy on you if you mix products in your prescription.