Are you restoring your windows to last for the next 10 or 15 years or are you trying to restore and rehabilitate your windows to last another century?
Heritage consultants are knowledgeable in historical construction methods, products and quality. When you consider the joinery alone, always look for ‘through’ mortise and tenon joinery. The difference in something so small will mean adding 80 years to the life of your sash.
The joint has two components: a mortise which is essentially a hole cut into one board, and a tenon which is a tongue that is shaped onto the end of another board which will fit into the mortise.
Historically, windows were made to release water. When a bottom sash rail gets wet the moisture travels into the tenon and out the other side allowing that sash to breathe.
When manufacturers produce sash with stub tenons or dowels, the only way to keep them tight is with glue. As the bottom rail gets wet the moisture travels into the tenon area. The glue then forms a pocket around the stub tenon, preventing evaporation and rot incurs.
Do not compromise projects for short term cost savings. This causes you to restore those same windows again in 20 years. No matter the ‘tests results’ a company has provided on their product, there is no test like the test of time.