Double Sash and French Balcony: The Technique of Classic Elegance
Guide · 2026-05-23 · PenSoft Team · 7 min read
A double sash (French type) opening is one of the most elegant solutions in fenestration. The ability for two sashes to open without a fixed central mullion provides a spacious and airy transition; it is often preferred for balcony doors and large windows. However, behind this elegance lies a technique that must be carefully constructed. In this article, we will discuss double sash systems.
What is a Double Sash?
A double sash consists of two sashes side by side, both of which can open. The key feature is that there is no fixed mullion in the center; instead, one of the sashes (typically the "passive" sash) forms the closing surface for the other (active sash) when it is closed. Thus, when both sashes are opened, the entire opening is unobstructed.
This differs from a classic window where two sashes are separated by a fixed mullion; in that case, the mullion is always visible and divides the transition. In a double sash, however, the opening is unobstructed and seamless.
Active and Passive Sashes
The roles of the two sashes in a double sash are different:
- Active sash: The sash that is opened for daily use. It has a handle and opens normally (or with a tilt & turn mechanism).
- Passive sash: Normally remains closed; it forms the closing surface for the active sash. It usually does not have a handle and is secured with upper and lower bolts (espagnolette). If necessary (for large item passage, for example), the bolts can be released to open the passive sash as well.
This active-passive configuration directly influences the choice of hardware: the passive sash requires a specialized hardware set (bolt/lever) independent of the opening.
Movable Mullion
In a double sash, there may be a movable mullion where the two sashes meet. Unlike a fixed mullion, this profile moves with the sash and ensures airtightness when closed. The movable mullion forms the closing surface and locking points between the active and passive sashes. If not correctly configured, the double sash may not close properly or may become difficult to operate.
Hinge Direction and Handle Position
In a double sash, the hinges face outward: the left sash is hinged on the left, and the right sash is hinged on the right; the handles meet in the center. This symmetry must be established correctly from both aesthetic and functional perspectives. The hinge direction and handle side should be clarified during the measurement and quotation phase; otherwise, a reversed sash may occur during installation.
Where is it Used?
- Balcony and terrace doors: Ideal for wide, mullion-free transitions.
- Large living room windows: Offers a spacious appearance and easy cleaning.
- Classic/elegant architecture: French type openings add an aesthetic character.
The limitation is the weight of the sashes and wind load in very large openings. For very wide transitions, sliding or lift & slide systems may be more practical.
Static and Weight
Two opening sashes, especially if they are large and heavy, impose significant loads on the hinges and hardware. The sliding mechanism of the passive sash and the hinges of the active sash must be selected to bear this load. The quality of the hardware in heavy double sashes is key to longevity.
For the Fabricator: Correctly Configuring and Pricing the Double Sash
A double sash involves more parameters than a classic window due to the active/passive sash roles, movable mullion, and specialized hardware sets. Manually configuring these each time is both slow and prone to errors.
When you assign a double sash in PenSoft; the appropriate opening hardware for the active sash automatically comes for the passive sash, along with the company's passive (bolt/lever) accessories; the movable mullion is correctly configured; hinge directions are adjusted according to position (each sash outward). In the drawing, the opening lines are visible in opposition, and the quotation generates the correct accessory lines for both active and passive sashes. Thus, the double sash is both accurately drawn and fully priced.
Summary
- A double sash consists of two sashes that open without a central mullion; it offers a wide and elegant transition.
- The active sash opens daily; the passive sash is secured with bolts and can be opened when needed.
- The movable mullion ensures closure and airtightness.
- The hinge direction and handle position must be symmetrical and correctly established.
- In heavy sashes, the hardware quality is decisive.
- Correct configuration and complete pricing are facilitated by a digital tool.
Double Sash or Fixed Mullion?
A common question when deciding on a double sash is: is having a central mullion really a problem? The answer depends on usage.
- Fixed mullion (mullioned) two sashes: More economical and statically stronger; because the fixed central mullion provides structural support. However, it divides the view and leaves an obstacle in the transition.
- Mullion-free double sash (French): Opens the entire opening without a mullion; offers an elegant and spacious transition. However, it requires a movable mullion and specialized hardware, making it slightly more expensive.
In transitions between living rooms and terraces, where large items are expected to be moved or aesthetics are prioritized, the mullion-free double sash stands out. In standard windows, the fixed mullion solution is often sufficient and economical. Asking the customer this difference — "Would you like a visible central mullion, or should it be completely open?" — makes it easier to arrive at the right decision.
Security and Correct Installation of the Passive Sash
In a double sash, the critical point for security is the passive sash. The passive sash is secured to the frame with upper and lower bolts (espagnolette); if these bolts do not fit properly, the sash becomes vulnerable to external force. Therefore:
- The sliding mechanism of the passive sash must be sturdy and full-length; it should lock securely at the top and bottom.
- The active sash should lock at multiple points against the passive sash.
- In security-priority areas (ground floor), the bolts of the passive sash and the lock of the active sash should be selected with reinforced hardware.
A correctly installed passive sash provides security without compromising the elegance of the double sash. If installed incorrectly, it becomes the weakest point of the opening. Designing this detail correctly from the start makes the double sash both stylish and secure.
Frequently Asked Questions
How is the passive sash opened in a double sash? The passive sash normally remains closed and is secured with upper and lower bolts (espagnolette); it usually does not have a handle. In situations like moving large items, these bolts can be released to open the passive sash as well. For daily use, only the active sash operates; the passive sash forms the closing surface for the active sash.
Does the absence of a central mullion create static issues? No, when installed correctly. In a double sash, the movable mullion and strong hardware provide closure and airtightness instead of a fixed mullion. However, in very large and heavy sashes, the capacity of the hinges and hardware becomes critical; therefore, the quality of the hardware should not be compromised in wide double sashes. Sliding systems may be more practical in very wide openings.
Should I choose a double sash or two sashes with a fixed mullion? If an aesthetic and spacious mullion-free transition is desired, choose a double sash (French); if economy and static priority are needed, choose two sashes with a fixed mullion. In transitions between living rooms and terraces, and where large items are expected to be moved, the double sash stands out. Asking the customer, "Would you like a visible central mullion?" simplifies the decision.
Is a double sash secure? Yes, when installed correctly. The critical point is the passive sash: the upper and lower bolts must fit securely, and the active sash must lock at multiple points. In security-priority areas (ground floor), reinforced bolts and lock hardware should be used. An incorrectly installed passive sash can become the weakest point of the opening.
Insulation and Airtightness in Double Sashes
The elegance of a double sash is undeniable; however, the aspect of this opening type that requires the most attention technically is insulation and airtightness. This is because instead of a fixed central mullion, there is a movable line where the sashes meet, and if this line is not properly constructed, it can become the weakest point of the window.
The source of the problem is this: in a classic window, the sash closes by pressing against a fixed frame or mullion. In a double sash, however, the active sash presses against the passive sash (and the movable mullion). The quality seals and properly pressuring hardware are what ensure airtightness at this junction. A well-constructed double sash offers airtightness similar to that of turnable sashes; a poorly constructed double sash, however, allows air, water, and sound to leak through this junction.
Therefore, several points become critical in a double sash. Seal quality and continuity: a continuous, flexible gasket is required along the junction line. Hardware pressure: the active sash must lock against the passive sash at multiple points with sufficient pressure; the upper and lower bolts of the passive sash must fit perfectly. Movable mullion configuration: this profile must be designed to correctly form the closing surface and locking points.
Additionally, when a double sash is wide and has large glazing units, thermal expansion and weight also affect this delicate balance. The hardware and profile must be selected to bear these loads; otherwise, over time, the sashes may sag, and the junction line may deteriorate.
All of this demonstrates that a double sash is not just a "beautiful-looking opening" but a delicate system that requires correct configuration. When well installed, it is both elegant and insulated; when details are neglected, it becomes a beautiful yet problematic window. Therefore, a double sash must be approached with both aesthetic intent and technical care.
Daily User Experience
When selecting a double sash, aesthetics take precedence; however, the true test of this window occurs in daily use. The customer will open and close the double sash every day; thus, having a smooth and trouble-free daily experience is key to long-term satisfaction. In a well-installed double sash, the active sash opens and closes easily with a single motion; the bolts of the passive sash engage easily when needed. In a poorly installed double sash, the active sash may rub against the passive sash, handles may turn stiffly, or the closure may not fit properly. These small difficulties may seem insignificant at first, but when repeated daily, they can become a serious source of discomfort. Therefore, fine-tuning after installation — aligning the sashes, adjusting pressure, and ensuring the bolts fit properly — should be done meticulously. Additionally, showing the customer how to open the passive sash prevents confusion in daily use. Most users are unaware that the passive sash can also be opened and wonder, "Why won't this sash open?" A well-installed and well-explained double sash becomes a window that is both enjoyable to use daily and functions flawlessly for years; the experience complements the aesthetics.
A double sash connects the classic elegance of joinery with modern technique. When you configure the details correctly, you achieve a beautiful transition that works flawlessly for years.