This is a partial solution but it may prove worthwhile nonetheless;
and although it won't eliminate air leakage around the sash, it will
help reduce some of the heat loss through the glass itself.

First of all, go for the 3M brand of window kits. Nothing else, in my
experience, comes remotely close to the quality of this product and
for the few extra dollars, it's well worth it.

Instead of applying the sticky tape around the outer edge of the
window trim, apply it directly to the front of the lower sash. In
this case, the window film will cover just the lower half of the glass
but you can still open and close the window as you wish. You might
fully cover all of the windows in your home with the exception of the
one or two you wish to use for ventilation.

Three years ago, I installed 3M window kits on both the inside AND
outside of six large fixed windows in my home (a combined glass area
of about 135 sq. ft.). Applying the film directly to the window
itself and not the outer trim means these films virtually disappear
from view. [The outer film is protected by a wooden storm.]

See:
http://server3.pictiger.com/img/6221...indow-kits.php

Taken together, these kits reduce my home's heat loss by over 1,800
kWh/year and have saved more than 5,500 kWh to date (over $600.00 at
today's prices).

Cheers,
Paul

On 24 Oct 2006 09:29:43 -0700, "ap" wrote:

Hello,
Has anyone found a way to insulate/weatherstrip their windows,
but still be able to open it occasionally to let fresh air in?

I googled and found plastic sheeting, but this completely seals off
the windows.

I have the single hung type of window where the bottom sash moves
vertically up to let air in.

Thanks for any ideas.