The code would say that you have to take the same amount
of air out of a room that you are putting into the room.
If you do not have a return in the bed room and you have one 6"rd. heat run than you might be moving at least 100 cfm of air into that room. and if so the inspector would probable tell you that you need 1", normally they will not let you get by with 1/2". later paul

With all due respect Paul, 1" is ridiculous for a gap on a door.

perhaps you believe that it is ridiculous..but
without a transfer or jump duct the undercut of doors
is the only pathway for air from rooms to return.

this is a common way to do this..however if room has more than one supply grill even one inch is not enough return.

I've seen a lot of master bedroom suites with 2 supply grills in bedroom, one in each closet and one in bath..return path..1" undercut. put carpet on the floor in bedroom and you see dust trails where air is squeezing under door trying to make it to return air.

It isn't smart, or right..but it isn't uncommon.

OP ask about transfer grills..
these can be offset in
wall cavity one high and one low. this will return air and not transfer noise or light. this would be the less costly option.
jump ducts would require a duct run from in the attic from bedroom (s) to hallway..
more costly and creates more air leakage sites from attic to conditioned space.

personally I would fit the doors tighter and install transfer grills...just remeber to offset on oppsite sides of wall or you will also transfer sound and light.

best of luck.

energy_rater_la,

I understand the issue - I'm just saying that if you are looking at a 1" gap you might as well just install a screen door.

Really, if the OP is only willing to leave the door open "once in a while" then he needs to install transfer grills or returns.

yeah but just think...if you were lock the teenager
in their room..you can still slide a sandwich on a paper
towel in to feed them...just a thought.

"The contractor has hung them all with rather large clearances between door's edge and floor  partially defeating the sound-insulation... When confronted, he pointed out, that Code mandates clearance between 5/8" and 7/8" in our case, where the central air system pushes cold air into every room, but has only one return per floor (in the middle). "
There is absolutely no such mechanical code "requirement" that there needs to be a minimum door height above the floor for AC or forced air heating purposes.

NONE.

What you have been told is complete and utter NONSENSE.

I am a nationally certified Residential and Commercial Mechanical Code Inspector and challenge anyone to provide a Code section from any Code that has ANY such a requirement.

While a design to provide a balanced air output and return air within a forced air system IS required, the 'means' by which this should occur is written into NO CODE and depends upon the design and the circumstances.

Particularly on older existing homes with centralized (not individual room) air returns like the original poster's...the house should simply not ever be 'tight' enough to restrict air changes per hour or to restrict air flow when interior doors are closed.

Those who think doors need to be cut extra high above floors to maintain air flow are simply ill informed.

There is nothing in any Mechanical Code that prevents even sealed door units and sealed thresholds from being installed.

Nothing.

In some situations, Mechanical Codes actually require sealed doors....(like between bedrooms and closets containing boilers, furnances and water heaters).

Likewise...some rooms, like bathrooms and kitchens, garages and some types of basements and crawl spaces are prevented from having return air vents and ducts installed within them.
----------------

I don't believe your contractor is lying to you. He does not stand to benefit enough for his false claims.

I DO beleive he just doesn't know what he is talking about, and thought he 'learned' something along the way that simply is not true.

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Tell him to repair the doors he damaged at his own expense....and do not pay him until he does.

The code says to install a proper air distribution system.
If you don't let air back out of a room than the room will
pressurize and you will not get enough air into that room to heat or cool that room. So we give the customer the option
of, ducting return air to that room, a jumper duct, relief air
vents through the wall, or 1" space under the door. By doing this we know that we can heat the room. It is the job
of the inspector to make sure things like this are done.
You are right that we don't do this to bathrooms, but most
of them have doors open most or the time. Bedroom doors might be closed most of the time. later paulbm