While this practice is widespread, and conserves floor space, it doesn't perform well. During construction, the drywall is rarely caulked to these studs or joists. Since most lumber is S-dry or green nowadays, there is considerable shrinkage. The resulting gaps cause shifts in the air balancing of the ducts. What worked at settlement may develop problems down the road. I come in investigating soot claims (Black Particulate Matter) and find air shunting under wall plates, esp. near returns. The BPM gets filtered by the carpeting along the baseboard on those walls. We also see BPM stains telegraphing the 4x8 plywood subfloors when there is panning underneath of light colored carpets.

I doubt anyone would argue panning performs better than properly sized and installed hard ducting but it just isn't economical for the whole house with the price of each square foot so high. It would take a colossal chase to house one hard duct for the entire home and that just ain't happening. Besides, the code wants a return is each room (except kitchens and baths) properly sized, sealed, and balanced in most cases in lieu of one central return.

Maybe some of our HVAC friends could comment on the effects on static pressures, Delta T, etc. on panning versus hard duct returns?
TIA,
Bob

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