Historic steel: fix alignment before new gasket?

Short answer. Weatherstripping should not be used to disguise a misaligned historic steel casement. The NPS calls for checking bowing, bent sections, sash alignment, corrosion, paint, glazing, hinges and other hardware before deciding the repair scope. source

Separate window condition from perimeter sealing

Look at the closed assembly as a whole rather than beginning with a catalogue photo of a gasket. Make a four-edge condition record: (1) whether the sash appears square in its opening; (2) whether the contact line changes substantially from bottom to top; (3) whether hardware is operating without force; (4) where rust or paint loss is visible. Treat the notes as questions for diagnosis, not proof of any specific defect.

A 1984 NPS steel-window example describes tall sash bowing at the top and says that, when the referenced gap exceeds roughly one-quarter inch, realignment may be needed rather than using foam to bridge it. The number is not a published tolerance for a different window. source

Three decisions, in order

Window is visibly bent, binding or mechanically uncertain: stop at condition assessment. Do not use more seal thickness to force a poor fit; the NPS warns that this can spring hinges and produce further gaps. source

Window shape seems stable but a defined operable joint leaks: ask which weatherstrip system belongs at the meeting line of movable sash and fixed frame; NPS places historic steel weatherstripping at that interface. source

Air movement seems to come from stationary masonry/frame surrounds: record that location separately. NPS distinguishes perimeter weatherstripping from caulking around masonry openings. source

Hand-off sheet

Provide labeled photos from the interior and exterior where safe, the edge at which contact changes, notes on operating difficulty, and details of past repairs if known. Do not undertake invasive paint stripping, sash bending, hinge reworking or hardware alteration from this page. A preservation professional should resolve structure and historic significance before recommending an exact seal. This article is the alignment-first decision, not a guide to foam product sizing or rust remediation.

Scope and references

A strip cannot compensate for a sash that is out of alignment or fails to close properly. Note where the steel sash rubs, the location of a visible gap and any evidence of hinge or frame movement before deciding on a seal. Preserve existing hardware and finishes; a preservation-aware repairer should assess alignment and corrosion first rather than forcing a thicker strip into the opening.

• — NPS, NPS Preservation Brief 13, Sharon C. Park, historical publication 1984; hosted original PDF, no revised edition indicated; Printed p.3, Physical Evaluation. Primary source: https://home.nps.gov/orgs/1739/upload/preservation-brief-13-steel-windows.pdf
• — NPS, NPS Preservation Brief 13, Sharon C. Park, historical publication 1984; hosted original PDF, no revised edition indicated; Printed p.10, Weatherstripping / compressible foam. Primary source: https://home.nps.gov/orgs/1739/upload/preservation-brief-13-steel-windows.pdf
• — NPS, NPS Preservation Brief 13, Sharon C. Park, historical publication 1984; hosted original PDF, no revised edition indicated; Printed p.10 end of Weatherstripping, below Fig.8. Primary source: https://home.nps.gov/orgs/1739/upload/preservation-brief-13-steel-windows.pdf
• — NPS, NPS Preservation Brief 13, Sharon C. Park, historical publication 1984; hosted original PDF, no revised edition indicated; Printed p.10, Weatherization and Weatherstripping. Primary source: https://home.nps.gov/orgs/1739/upload/preservation-brief-13-steel-windows.pdf