We're renovating an old farm house and have commissioned tradition looking but double glazed wooden windows to be made. the windows are now complete and have been fitted, however during a recent spell of wet weather we discover that the sashes were leaking. The joiner took one to the windows away to replace the sash and has now returned it. we are not convinced that the the coverage of about 4mm (2mm greater than the origninal) he had given over the seal will keep the rain out this time either. He says that he is following a formula and that all his windows are made this way. Has anyone ever hear of this formula?