The problem is the quality of the wood being used in wood replacement sashes. Compared to the fine grain hardwoods used up to the Second World War, today’s wood is little better than balsa wood (a very soft wood used for model airplanes).
The weak point in the design of the double hung sash window is in the joints. A sash, the part of the window that goes up and down, is composed of pieces of wood called “rails” surrounding panes of glass. The rails are held together by complex mortise and tenon joints, so tightly made that you usually can’t even see the seams. They are vulnerable because the bottom sashes sit on the sill with their end grains exposed so that in the summer when the storms are open and rain blows in, they soak moisture into the adjacent joint, and the bottom rail of the top sash has a horizontal surface where condensation can sit and seep into the joints.
Compounding these weaknesses is the tendency for people to not maintain wood sashes. When putty is allowed to fall out it exposes the joints to rot, and when the seams open up on the inside they are vulnerable to condensation throughout the winter.
Because the wood in old sashes is so hard, it is usually a better investment to restore them no matter how rotted they are. When the joints are rebuilt with epoxy, and the putty replaced with permanent caulking, the result is a sash that will last another hundred years. It is our opinion that such a sash will last much longer than any replacement sash, made of anything.
Let us present you with the information in more detail, talk to others for whom we have performed this service, compare our product with replacement alternatives, and make up your own mind.