What should be placed over a sub-slab to locate heating ducts while the heating system is on?

Damp newspaper is an effective choice for locating heating ducts when the heating system is operational because it can absorb heat and will exhibit a temperature change more quickly than dry materials. When placed over the sub-slab, the damp newspaper will warm up as the heated air from the ducts rises, allowing the technician to identify the location of the ducts based on the areas of the newspaper that dry out and warm up faster. This method is straightforward and provides a clear visual cue for the presence of heating ducts.

Other materials, like plastic sheeting, could create a barrier without the same thermal conductivity, making it more difficult to detect temperature changes. Cardboard boxes may not react in a thermally responsive way suitable for this application. Foil wrap could reflect heat but might not effectively signal the heat patterns as well as damp newspaper. Thus, using damp newspaper maximizes the ability to identify the location of heating ducts accurately and quickly.