I am doing some work for some friends who recently bought a house. The house is 18 years old and has Hurd brand factory made wood windows. They want to replace the glass in several because the seal has broken and leaked the insulating gas. It seems like it would be easiest to replace the entire sash, but am not sure this is available from manufacturer. If I have to replace just the glass pane, is this extremely difficult/impossible to do on a factory made wood window? Any pointers?