By the way, you may *think* the windows don't let any air through, but this is usually not the case, unless they have been sealed.
The common test for air passageways in a room is to depressurize the room and use smoke. By tracing its path you see where the faults are. A standard test like this can be ordered from "machon hatkanim" I think for about 300 NIS.

Regards,
Assaf.

Hi Mordechai,

The seal is automatic - it closes when you close the door, and opens when you open it.
Some seals are also made from vinyl.
Sealing doors is not just an acoustics problem - it has to do with heat insulation (air convection can cause major heat problems), with dust, dirt and so forth. Therefore it is a standard practice.
So, by going to a good hardware store and talking to the salesperson there you should be able to get detailed instructions on how to seal the bottom AND the sides. It will be much more effective than me writing it.
A bottom drop seal should cost about 100 NIS, maybe less.

Assaf

One extra point.
Although I have never worked with them myself, try talking to Tenegal Agencies. They are the sales representatives of zero international, I've heard good things about them. They deal with insulation problems related to doors.
Info:

TENEGAL AGENCIES
52a, Hamasger St. Tel-Aviv
P.O. Box 57355 Tel-Aviv 61573, Israel.
E-mail:
menahem@tenegal.com,
mati@tenegal.com
Tel : 03-5617411,5617412, 5617413
Fax : 972-3-5616717
Attn.: Mr. Menahem M. Tiano
Website: www.tenegal.com

If you do business with them, please share your experiences so we may all learn.

Regards,
Assaf.