On IRC some friends and I wondered if you could determine the rough square footage of an apartment or house by sealing off all the windows and doors, determine the height of the room, measure the PSI of the air, and use the resultant volume measurement divided by the height of the room to determine the square footage of the floorplan to an accurate degree. Obviously this would require a constant wall ceiling height, but this is pretty common.

Does it seem pretty silly or reasonable?

Oh, and another detail: Measure the PSI once, pump in a measured amount of air, then measure PSI again, and determine volume from that.

To determine square footage, it would work. However, just measuring the walls is much easier. It might take an hour with a tape measure for an average house while the sealing alone would take several times that.

Almost all houses leak like sieves, even with the windows and doors sealed, and the leak rate differs from house to house. You would have to determine the leakage before you could determine the pressure differential.

Furthermore, different groups use different metrics for square footage. Real-estate around here requires the outside dimensions, but the pressure method would give you the inside dimensions (maybe with some internal wall volume, depending on which side the vapour barrier is on). Some require closets, basements, stairways, and other areas to be excluded. It would generally be more trouble and less accurate than just hand measuring.

Energy efficiency firms use blower doors. A large fan sealed to a door. They check the air leaks, (which they reduce air leaks), but by no means make a house air tight. RJF

Your method would require an empty dwelling, no furniture, no cabinets, no appliances, totally nothing but the walls, not even a doorknob or light fixture.

For example a cluttered house vs a vacant house.