How to Insulate Window Weight Pockets, Sash Weights

Key considerations:

In aggregate, these are a large leak demanding attention in buildings with weighted sash. Sash weight pockets, two per window, are very direct, in-to-out airways, not to mention gaps in insulation. In a typical home, these might add to the equivalent of 10 feet of uninsulated wall one story high.

The challenge is to air seal, and best, to insulate without loss of function, and in some cases existing character. Window replacement is always a (costly) option.

Possible downsides:

Some approaches to insulating these pockets leave windows hard to open. Large, heavy sash can be hard and dangerous to lift and to leave propped. One shudders at the though of anyone guillotined by one of these "fenester monsters."

Some of the better approaches that preserve function require removal of interior or exterior casings, a "bullet" to be bitten in many weatherization projects. This is not a small undertaking, but it is less costly than any decent, new windows. This will be of interest to those concerned to preserve the existing character of their buildings.

This image shows an open window weight pocket. This cavity has no insulation or air seal.


A short-term-quick fix for pulleys is to cover the window weight access ports with painters' blue mask in the coldest weather, form early fall roughly through what we call mud season. Blue tape pulls cleanly after many months. Longer term, the economical choice is plastic, Anderson Pulley Seals at two-and-change per pair. The premium choice is spring-loaded, Pullman Sash Balances at $35+ per pair that replace weight, pulley and cord. These are very rugged devices. See both Anderson and Pullman on-line. Inside and out, transitions from walls finish to casings should be tight, with caulking as required.

If Pullman balances are chosen sash will continue to operate and pockets can be drilled and "shot" with injection foam to fill and insulate them. Alternately, some have run the weights in conduit, carefully foaming the remaining space and substituting sash chain for rope to delay difficult sash cord replacements. Depending of the size of weight pockets, fitting foam board on the exterior side of the weight pocket might also be feasible. Retrofits that keep the weights and cords involve removal of either exterior or interior casings. Each building is little different so you may have to do some exploratory work on one window to properly plan your strategy.

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