With more and more installers using our Thin Floor Warming Cables with the revolutionary Ditra-Heat Membrane I thought it would be a good idea to provide some tips on how to choose the right cable wattage.  

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.