Back in 2001 we pioneered what I believe it the best way to install electric floor heating over wooden subfloors - especially in new construction.  It uses Diamond Lath Metal Mesh Reinforcement over our contractor toughThick Heating Cables as a means of reinforcing a subfloor under tile and heating the floors in a substrate only 3/8" thick.  Here's what's involved:

• Sweep & vacuum the floor, take measurements and plan layout.
• Put down 3/8" plywood under cabinetry and other un-heated areas.  You may also put down narrow strips around the room perimeter to act as a screed guide and to prevent leaks if any exist under the walls.  Be sure to caulk any cracks in the floor and around any ductwork which may penetrate the floor.
• Staple down our Thick Heating Cables using our T-37A Staplers and 9/16" adhesive coated staples.
• Spray the floor with a 50:50 diluted mixture of general purpose latex concrete admixture and water to seal the floor and allow to dry.  Alternately it can be rolled down with a paint roller either before or after cables are down.
• Cover the entire floor area with sheets of 2.5 lb diamond lath metal mesh.  It usually comes in 27" x 96" sheets which can easily be cut to fit using scissors or sheers.  Do not overlap the mesh unless you want to build the floor up more than 3/8" in height.
• Nail the mesh down over the heating cables approx every 6" using a pneumatic roofing nailer and 1-1.25" roofing nails.  Stay clear of the heating cables and try to keep the mesh tight by nailing between cable runs.
• Pour 3/8" - 7/16" of self-leveling cement over the mesh and cables.  A 50 lb bag of self-leveler will cover approx 14 SF at this depth.  Note that it is often OK to lower cost by adding up to 10 lbs of fine sand for each bag of self-leveler which stretches coverage to 17 SF per mix.

Here are some of the reasons this proven method is recommended:

• Most new homes will use 3/4" hardwood and tile.  Normally the subfloor under tile needs to be reinforced using 3/8" plywood to prevent deflection and raise the tile to the same height as the matching hardwood.  When typical Thin Heating Cables are used over the plywood reinforcement it results in a heated tile floor which is around 3/16" higher than the hardwood.  If the plywood is eliminated then you risk the chance of deflection cracking the tiles over time.  Therefore using the method described in this article solves both problems occupying the same amount of height as the plywood which would normally be used just to reinforce the floor.
• Something most people don't think about is that most typical heating cable or mat installations only last as long as the flooring which covers them.  This is because normal installation practices result in the removal and destruction of the thin heating elements when old tiles are removed since all the mortar and leveling compound comes up with the tiles.  When mesh is nailed down over the heating cables it mechanically bonds the cables and the bottom layer of mortar which encapsulates them to the subfloor, thereby making it possible to remove the top layer of flooring and mortar without inflicting any damage to the heating cables below or the reinforcing substrate.  
• Our Thick Heating Cables are very tough and are virtually impervious to damage when protected by the mesh that covers them.  This makes this installation method a lot safer to use on busy construction sites where the cables often have to be left exposed until an electrical inspection has taken place.
• Our Thick Heating Cables have 2-3 times more heat output per linear foot than most Thin heating cables.  This means that the cables can be spaced farther apart - usually 5-6" apart instead of 2-3 inches apart.  This means faster installations, less chance of damage and lower cost per Watt since less cable translates into lower material costs.  The savings in heating cable cost can then be used to offset the slightly higher cost of using 3/8" of self-leveling compound instead of 3/16"-1/4" required over Thinner products.
• Thick Heating Cables come in much larger sizes than most Thin Heating Cables which means that tackling large rooms can often be done using one cable and not two.  This lowers cost and layout complications greatly.

It should be noted that this method is not for most DIY installers and recommended for experienced tradespeople and contractors.   However, if you are planning on doing very large jobs comprising of several rooms or a whole house then it is worth investing in the tools and skills required since the finished product will be a heated reinforced substrate designed to be used almost under any flooring except staple-down hardwood.  This system should last as long as your home and survive several flooring changes over the years as well.