The short answer is, if you need this device, you’ve likely got bigger problems than this gizmo will handle. Plus, there are simpler, more elegant ways of replicating the same functionality without all of the negatives.
What is a Pulley Seal? A Pulley Seal is a plastic flap that fits over the pulley near the top of the jamb on a double hung window. The rope moves in and out of an opening that has a smaller gap than the space around the pulley. It seals around the pulley with foam covered with adhesive. Additionally it is screwed into the jamb with two wood screws. The purpose of this device is to block outside air from entering your house through the gap around the pulley.
Is it necessary? First, why would there be outside air entering your weight chamber? If you have this problem, then something on the exterior of your house is not sealed properly – like where your window casings meet your siding or blind stops. A simple solution to this problem can be found in a tube of caulk – just caulk between your casings and your siding on the outside of your house. It’s also possible that air is entering the weight chamber through the top or bottom of the window, so if this space leads to an uninsulated attic or basement, a pulley seal may be effective in blocking this intrusion. However, framing members should seal the window from the top and the sill should seal it from the bottom.
What are the problems with the Pulley Seal? The biggest problem is the lower sash can’t extend all the way up. Sashes using the rope and pulley system have stiles that are bored and plowed so the plowed channel ingeniously clears the protruding pulley when the sash is raised. The Pulley Seal prohibits this movement by about 6” – a huge obstacle on a nice summer day. In addition, the plastic used to make Pulley Seals becomes brittle and cracks over time.
If you feel you need this item, a better solution is to take a small piece of carpet padding (1 1/2” X 4”), pull your rope out of the opening by about an inch, insert the carpet padding piece, release the rope, and let the rope hold the padding tight against the pulley covering the gap. When winter is over and the windows are ready to be opened, pull the rope out and remove the padding.