Why Thermal Board works so well:
Non-structural Thermalboard™ is designed specifically for subfloor applications. Thermalboard™ is constructed of a dense composite board covered with aluminum that spreads the heat evenly and quickly from the hydronic tubing. It heats rapidly and is easy to control with setback thermostats for maximum energy efficiency. It contains just enough thermal mass to be effective, but not so much that it’s difficult to control. No other product offers this combination of performance, ease of installation and cost-effectiveness. (Up to 40 BTU SF)

ESTIMATING THE REQUIRED NUMBER OF THERMALBOARDS:
The following calculations can be used for estimating the required number of boards. For experienced installers:
calculate the net square footage of each room and multiply sq ft by
the following factors: Straight – 0.133 Utility – 0.028 Combo End and Utility pieces

Example: For a 600 Sq. ft. room, multiplying 600 by 0.133 gives approximately
80 straight boards. Multiplying 600 by 0.028 gives 17 Utility pieces.
Multiplying 600 by 0.028 gives 17 Combo end pieces. It is always recommended
that additional 10% material excess is added to the estimation.