Weatherstrip old wood windows for energy efficiency
If you have older wooden windows, you may have noticed drafts leaking around
the edges of the moveable sash. This air leakage will cool your house in the
winter, heat it up in the summer, and allow dust and debris into your home during
windy weather.
Traditional wooden double-hung windows, with an upper and lower sash that
slide past one another, are fairly easy to weatherstrip if you’re handy with tools.
Paint is the primary obstacle to weatherstripping windows. The upper sash has
often slipped down and is locked in place by layers of paint, making it impossible
to seal effectively. Unless the window slides freely and the paint is in good
shape, plan on stripping and repainting the window before weatherstripping.
To weatherstrip double-hung wood windows:

Cut the paint seal with a knife around both the upper and lower sash, both
inside and outside.

Pry off one of the lower sash’s wooden stops -- these strips hold the window
in place. Be sure to cut a line through the paint where the stop meets the
jamb so paint doesn’t pop off as you pry.

Remove the lower sash. If it was too tight in the opening, use a wood plane
or rasp to remove enough wood so it slides freely.

Use a strong board and block, or a small hydraulic jack, to boost the upper
sash back to its original position. Be careful to apply pressure only at sash
corners so you don’t break the glass.

Block, screw, or nail the upper sash in place. The upper sash will now be
immovable, but your weatherstrip will work better if only the lower window
needs to slide.

Scrape excess paint from the sashes and windowsill.

Apply vinyl V-strip to the side jambs, and bronze or vinyl V-strip to the
meeting rail on the top sash. You can get these materials at most hardware
stores.

Reinstall the lower sash and wooden stop.
When you repaint the window, don’t paint the weatherstrip or the channels where
the sash slides. If the window sticks, lubricate the channels with a bar of soap.