For most applications Ditra recommends using three knob spacing which works out to 3.5" on-center spacing.  This spacing when used with our cables works out to 13.53 Watts per SF which is a comfortable output well suited for toasty floors and a primary heat source in many parts of the country.  Therefore, to choose the best cable for the job simply multiply your net square footage by 13.53 W/SF to calculate the target wattage required and choose the cable set which matches is most closely.  

3 knob or 3.5" spacing = 13.53 W/SF

For floor warming applications you may use four knob spacing which works out to 4.72" on-center spacing. This spacing when used with our cables works out to 10.16 Watts per SF which is a comfortable output well suited for floor warming and a secondary heat source. Therefore, to choose the best cable for the job simply multiply your net square footage by 10.16 W/SF to calculate the target wattage required and choose the cable set which matches is most closely.  

4 knob or 4.72" spacing = 10.16 W/SF

If your heated area falls in between the target area for three & four knob spacing then simply alternate spacing every other row to achieve an average of 11.85 W/SF which is an ideal output for use under tile for both primary or secondary heating applications.

Alternate 3 & 4 knob spacing = 11.85 W/SF

Bear in mind that selecting a lower wattage cable will mean some unheated area or the need to space some of the cable using four knob spacing to make it fit.  Choosing a cable with higher output will mean you will need to increase your heating area to make the cable fit.  

Target Cable Wattage = Net Heating Area x Desired W/SF above

With our cables you should not use two knob spacing since this would exceed the maximum output of 15W/SF allowed.  Also remember that you are expected to keep cables 1/2 on-center from walls and fixtures in order to make the above calculations work.  That means that your cables should be one and a half to two knobs away from perimeters.  Ditra also recommends that you introduce a single knob tightening loop at least every 10' if your runs are that long.  This is necessary to keep the cables from going slack and we recommend that you divide long runs under 20' in half using this technique with our cables.

One of the advantages of using Ditra-Heat membrane is that you don't need to cover the cables with self-leveling cement or thinset mortar before laying tiles.  However, this also means you need to be careful with your trowel when laying tile and that you do not use a knife or sharp trowel edge to clear thinset from grout lines.  Use a plastic trowel edge to remove the excess thinset so that you don't accidentally cut or damage the cables when doing so.  You can download the Ditra-Heat Installation Manual - English for installation instructions but use the sizing calculations mentioned in this article instead of those in the manual since they refer to Ditra's own expensive line of floor heating cables.

If you have any questions on which cable size to use for your project then simply give us a call and we'll figure it out together.  I welcome any input from you on other tips and tricks for laying our cables with Ditra-Heat membrane.

Schluter has an excellent reputation with tile setters with their decoupling & anti-fracture membranes.  However, until recently you had to install our cables, embed them in self-leveler before installing the membrane and then the tile.  This was a costly and thick installation approach and therefore was never that popular.  Now that Ditra-Heat has been released in Canada you now have what we believe is the best way to install our Thin Floor Warming Cables.  

This new approach eliminates the need to pour self-leveling cement over our cables.  Simply install the Ditra-Heat membrane to your un-reinforced sub-floor, clip our Thin Floor Warming Cables to the grid and then proceed directly to tile installation using thinset.  The membrane not only holds the cables in place and eliminates the need for reinforcement but it also provides a perfectly flat surface for easy and professional tile installation.

For those of you who have installed heating cables and mats of any kind before, you know that pouring the self-leveling cement is the most critical part of the installation and that it doesn't always work out perfectly the first time.  Schluter's Ditra-Heat allows novices and professionals alike to install our Thin Floor Warming Cables and tile with professional results every time.

Of course you may ask why you should use our ThermaWire Thin Floor Warming Cables and not the ones that are offered with the Ditra-Heat system.  The answer is price; our cables are priced at less than half the cost of the Ditra version.  Then there's our outstanding customer support, free air freight and excellent technical support available day or night should you need it.

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.

• First you'll have to sweep and mop the floor to remove any dust, grime and oil.
• If you are laying mats down, you'll want to dry fit them first.  That means unrolling them, cutting the netting to flip them and making sure that they fit the way you 'd like them to.  Once you are satisfied, roll them back up again.
• Purchase some general purpose latex concrete admixture or primer as well as a can of spray-on adhesive contact cement and a roll of aluminum tape from your local building supply or concrete products wholesaler.  
• Roll on the primer to seal the floor.  It shouldn't have to be diluted over concrete but read the instructions to make sure.  Allow it to dry completely before proceeding.
• Un-roll the pre-cut mats or start laying cable.  The mats have an adhesive backing which should stick nicely to the primed concrete.  For Thin cables, use the aluminum tape to surround the cable and it should also stick very nicely to the primed concrete.
• For areas where tape or adhesive backing don't stick well on their own, spray the desired area with the contact cement and allow to dry for a minute before taping cables or sticking mat backing down.
• Use the spray-on contact cement under areas where cold leads and sensors will be going, allow to dry and then use the aluminum tape to secure the wires tight to the floor.
• If you are using mats it is recommended that you try to lay all or most of them with mat fabric facing up to keep the wire loops down.  Once the mats are down you can use Tuck tape to tape the edges of adjacent runs together so they act as a cohesive surface.  This makes it much easier to cover them with self-leveling cement or mortar since they will be weighed-down - preventing them from floating up.
• Note that a cold chisel and hammer may be required to chisel out grooves for the cold leads if you are tying to use less than 1/4" of self-leveler or thinset over the cables.
• If your installation will need to be inspected by the local electrical authority before cement can be applied it is recommended that you not waste much time trying to get all the loops or corners down with tape since the tape tends to come up after a day or two.  You are better off doing this just prior to cement work.

Anyway, hope this advice helps!

Stapling cables down with these staplers is way faster than installing and using plastic clip strips, hot glue or taping.  Most competing cables can't be stapled because they are not tough enough to withstand slight impact or the tension associated with stapling.  Cables should be stapled approx every 8-12" with three staples to support each loop.  When laying cables it is recommended that only the end loops and maybe one or two points in-between be stapled in order to get the cable down to make sure it fits before going back and keeping it tight to the floor with more staples.  

Although it doesn't happen often, it is possible to staple through a cable by accident.  Using a cable fault alarm or testing the cable resistance and earth leakage periodically will help identify if a fault occurs.  However, it's kind of hard not to notice if you staple through a cable or not - just don't ignore this if it happens and make sure you take-up and replace the cable if this happens. 

• 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.