What is a transom window?

A transom window, also called a transom light, is the window above the horizontal crosspiece over a door. Operable examples can move air as well as admit daylight. For historic-window work, Standard 6 favors repair over replacement, so request a condition assessment before specifying a new unit.

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By InstaBids editorial team with AI research agents. Updated 4 November 2026

What this means for your home project

For a historic transom, photograph its condition clearly enough to show the range of damage before seeking replacement. Source: National Park Service, read 2026-11-04.
Request a written repair scope identifying proposed finish renewal, component replacement and weather stripping, so each task is named in the reply. Source: National Park Service, read 2026-11-04.
If repair feasibility is uncertain, request a sample repair to help assess the result before deciding on replacement. Source: National Park Service, read 2026-11-04.

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What is the purpose of a transom window?

Its uses include ventilation, so a scope for an older interior can consider retaining the opening rather than removing it. “Transoms and screen doors are distinctive and practical features that provided cross ventilation in this historic hotel.” Source: National Park Service, read 2026-11-04.

Can transom windows be opened?

Some do open; for an existing historic interior unit, identify whether it operates before deciding what work to request. “The wall and door glazing ensures that the corridor receives natural daylight and the operable transom helps air to circulate in this historic office building.” Source: National Park Service, read 2026-11-04.

What is the difference between a transom and a mullion?

The distinction is horizontal versus vertical; use the member’s direction to identify the part in a drawing or repair request. “This contrasts with a mullion, a vertical structural member.” Source: University of Arizona Cooperative Extension, read 2026-11-04.

What is the difference between a sidelight and a transom?

A sidelight is beside the entrance; when describing the glass you want worked on, distinguish the side panels from the overhead unit. “One of a pair of narrow windows flanking a door.” Source: Delaware Department of Transportation, read 2026-11-04.

What’s the difference between a transom and a fanlight?

The fanlight name identifies a curved outline and fanlike divisions; include both features when describing a matching historic design. “A semicircular window over the opening of a door, with radiating bars in the form of an open fan.” Source: City of Ann Arbor, Michigan, read 2026-11-04.

Sources

Housing and Interior Design: Transom Window, page 34 — University of Arizona Cooperative Extension; read 2026-11-04.
Illustrated Guidelines on Sustainability for Rehabilitating Historic Buildings: Windows, page 7 — National Park Service; read 2026-11-04.
Historic District Design Guidelines: Historic Door Parts, page 4-2 — City of Ann Arbor, Michigan; read 2026-11-04.
Archaeology and Historic Preservation Glossary: Sidelight — Delaware Department of Transportation; read 2026-11-04.
Evaluating Historic Windows for Repair or Replacement (2007) — National Park Service; published 2007; read 2026-11-04.