Acelab Insights

Window Selection, Pt. 1: Fixed vs. Operable


Fixed vs. Operable Windows

You may not realize it, but windows get stereotyped. There are clearly times when a curtain wall is the right choice - when you have a wall made up of mullions, holding glass, covering large portions of the facade. There are clearly times when a window wants to be punched opening - when a window (or several) are used in a surface that's mostly solid wall. But it could be argued that, most of the time, the decision is somewhere in the more nuanced middle ground.

Based on the project type, there are default options that fit better than others. We’re here to help when you want to navigate past the default options.

Here are three window systems which solve a particular problem, and why you might choose each. Fixed windows are a go-to for higher performance, sliding doors are for when you need a door to be more like a window, and field-mulled windows are for when you need a large expanse of glass.

High Performance Punched Openings

Picture windows, also known as “fixed windows” solve a lot of performance problems. The reason we often review both the “center of glass” U-value and the “unit” U-value is that the frame is often the weak link. In a fixed window, the frame is a little bit less of a concern. They have less complicated frames, less frame to worry about, and no openings or moving parts. Within any manufacturer's lineup, fixed windows generally cost less and perform better than operable windows. Of course, opening up a window for fresh air and a connection to the outdoors is great, but consider that most operable windows don’t get opened and opt for fixed picture windows wherever it makes sense.

High-performance fixed windows

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Sliding Doors

Balconies and decks are the most common times you want a door that mostly serves as a window, but there are often openings in a building that meet this condition. Sliding doors can be a great choice. They’re continuously supported so the frames can be lighter and less robust, plus there’s no need to transfer the weight of the glass sideways to a hinge. Lighter frames translate to more glass, more view, and better performance. Of course, a sliding door is still a door, even if it makes a great window. A big bonus is that screen doors get out of the way and stay out of the way when people are going in and out a lot, which is why they are so common on balconies and decks.

Our top sliding doors

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Mulled Windows

Mulling windows comes from the same root as mullion, and it’s really the same thing. Historically, a curtain wall mullion was a two-to-three inch slender frame of metal between two pieces of glass, and a window mullion was an eight-to-ten inch column between similar pieces of glass. The structural element between the windows was functioning as a wall would, complete with all the flashing, trim, and insulation that would be found in a wall. Many modern windows can be factory mulled together with a profile closer in width to a storefront window, or mulled to a single stud framing member, or even field mulled with a custom connector which gives you best of both. Mulling windows at large openings can often look better, perform better, and match the other windows on a building where the default storefront option is only needed for a few locations.

Options for mulled windows

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What to look out for:

Each of these suggestions are generalizations which may not work in your particular situation. For example, sometimes the most effective way to create a punch window is actually to use a storefront system. Sometimes sliding doors are not an option because the project doesn’t have enough space to recess the sill properly for water proofing and accessibility purposes.

Remember that most sliding doors do not have ADA accessible thresholds when installed at the same level as the floor (but there are swing doors which can meet this requirement).

Finally, windows come in many, many shapes and sizes which can be mulled together to look great and perform very well, but there remain situations where storefront is required to meet the size, durability, or accessibility requirements. Specify what you need to specify, but remember that the defaults aren’t always the best fit.

Consider ventilation

Windows are very good at providing views and daylight into buildings.  They can also be good at providing ventilation.  Sometimes, it’s an effective solution to combine those functions, but there are also many times that we default to combining them when perhaps it would be better to separate them. 

For example, have you ever found yourself making a whole row of windows operable double-hung just so that they all match, without considering which are likely to get opened and which will never get opened?

This is where the fixed vs. operable conversation gets more nuanced. Sliding doors are an interesting consideration for ventilation. They can be left a little bit open, or a lot open, with no fear of the wind blowing the door shut. A swing door propped open a few inches looks like a mistake, but a sliding door open a few inches looks like a spring breeze.

In their award-winning Empire House in Canberra, Australia, Austin Maynard Architects put together an elegantly crafted example.  By separating the vision from the ventilation, they produced a more beautiful and functional design for a lower cost.  That’s something we’re all looking for.

Empire House, from Austin Maynard Architects

In a different case study of a modern classic, the Allston Branch of the Boston Library designed by Machado Silvetti demonstrates a beautiful example of using mulled windows to create a more warm, inviting, and human-centric ribbon of glass in a situation where aluminum storefront is often the default. The materiality which was made possible by using wood windows instead of aluminum storefront results in a better experience for both the community passing on the outside and occupants on the inside.

Boston Public Library by Machado Silvetti

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