Window Mullions and Transoms

6 min read Windows

What's the difference between a mullion and a transom?

The key difference between a transom and a mullion is the direction of their alignment on a window, but both mullions and transoms provide additional aesthetics to divide a window's pane of glass into smaller sections.

What is a window mullion?

Window mullion uPVC slimline windowA window mullion is a vertical beam that divides a window into smaller glass units. The purpose of a window on a mullion is to provide support to the glazing of a window, but they may also be required to provide structural support to a window arch. When used to support glazing, window mullions are often paired with transoms for extra support.

What is a window transom?

Window transom on flush timber casements

A transom is a horizontal beam that is paired with a mullion that divides a window’s upper part into additional lights. When transoms are positioned, generally they will be placed above head height to avoid obstructing the view from a window.

What is a mullion window?

Mullioned windows are common across Western and Gothic architecture as well as Islamic architecture. Mullion windows divide the glazing pane into adjacent smaller panes, providing better structural support to the window specification as well as the building around them. You can easily find mullioned windows on Gothic cathedrals, heritage properties and church architecture.

Historically, they were installed to support the glazing, however, nowadays modern mullion windows are added to a property for a decorative purpose.

What are transom windows?slimline uPVC window white internal view

A transom window is a bar which is found over the top of a window or door, segregating the main glazing panel and decorative ones which are commonly installed above a window. Transom windows originated in 14th Century Europe, and were commonly installed a doorframe to let in fresh air and natural light. They have emerged in popularity with homeowners nowadays looking to add additional style to new windows and doors, as well as to create a brighter atmosphere in their homes.

Looking for transom windows that open?

Transom windows used to have a purpose of adding additional ventilation to a building as they are fitted higher than a typical window or door, meaning air currents can easily enter the rooms when the sashes are opened.  When the transom is closed, the temperature can be regulated and kept consistent.

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Nowadays, it is uncommon to find transom windows which open at all as they only serve a decorative purpose, rather than a functional one. They'll assist in letting in more natural light however, for a brighter atmosphere within the home. However, if you are looking for transom windows that open, we can easily install this as part of your new windows. You can choose the appropriate opening mechanism to suit your rooms, maximising your home privacy, air ventilation and natural light.

Why do my windows need mullions and transoms?Tilt and turn windows

Mullions and transoms can be found on a variety of window materials, including uPVCtimber and aluminium windows. Often building regulations, structural implications or simply budget constraints mean that it would be impractical to fill a window space with one large pane of glass. Mullions and transoms make it possible to divide windows into adjacent units or panes. With the use of mullions and transoms, windows can be better tailored to suit a homeowner’s needs, for example, meeting the requirements of their interior space or their preference for external access.

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The use of angles, inserts, support metal and drilled anchors fix the mullion structure to the building securely. This transfers weather force away from the window glazing and onto the structure of the building. It is crucial that new or replacement windows are supported by strong mullions and transoms, and that the mullion system is installed by accredited industry experts.

Friction stay hinges are another crucial element of a window specification that assists the opening of the window. Take a read of this article for more information on friction stay hinges.

Do all window styles need mullions and transoms?

flush windowsOn most window styles, mullions and transforms are incorporated into the design for decorative reasons. They are most commonly used when the homeowner is looking for a heritage aesthetic as it separates the glazing into smaller window openings which were a popular trend on older properties. Still to this day, many double glazed window styles are split into smaller sections for added decor or to adhere to building regulations.

Many of our replacement window styles can be fitted with mullions and transoms, including our unique slimline windows, as well as popular styles like casement, flush casement, sash, mock sash and tilt and turn windows.

We're leading replacement window designers and installers, so can create your dream replacement window solution to meet your exact requirements with transoms and mullions to achieve both modern and traditional designs. So now we've answered all the commonly asked questions, including 'what is a transom?' and 'what is a mullion meaning', you're more than equipped to get inspired on designing your new double glazed windows, and here at Sheerwater Glass, we're always happy to help!

uPVC window mullions and transoms

PVC windows are the most favoured material choice with homeowners and commercial projects in the UK for a number of reasons, including being more affordable, low-maintenance, energy-efficient and of course, it's a versatile array of design choices.

uPVC window styles can easily be installed with decorative mullions and transoms to achieve your dream design and add the decorative touch that you are after for your replacement uPVC windows. We've installed plenty of PVC window mullions and transoms over our 50 years as a leading double glazing installer, so we can advise on the best style to suit your home.

Sheerwater Glass - 50 years of double glazing experience

If you're looking for replacement windows or more information on window mullions and transoms, get in contact with a member of the team who will be more than happy to help. We have nearly 50 years of experience in the double glazing industry, so our experts will be able to answer any questions you may have on all things window mullions and transoms.

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Related to this post:

What are the standard UK window sizes?

Our guide to the different types of windows (and which are the best!)

What are dummy sashes?

Should I repair or replace my broken window?

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