A double casement window, also known as a twin casement window, is a window configuration featuring two operable sashes hinged on opposite sides of a single frame, opening from the center outward or inward. This design creates a wider, more balanced opening than a single casement window, making it one of the most popular choices for medium-to-large window openings in both residential and commercial buildings.
Unlike sliding windows that only open halfway, double casement windows allow both sashes to swing fully open, delivering maximum ventilation and unobstructed views when needed. The symmetrical design also adds a classic, elegant touch to any building facade.
Double casement windows consist of two sash panels within a single frame. Each sash is hinged along its outer edge — the left sash hinges on the left side, and the right sash hinges on the right side. When opened, both sashes swing toward the center, meeting in the middle when closed.
Most models are operated using a crank handle or lever mechanism, making them easy to open and close even in hard-to-reach areas. Each sash can be operated independently, giving you precise control over how much fresh air enters your space.
Key Fact: Double casement windows are available with or without a center mullion. Mullion-free designs — often called French casements — provide a completely unobstructed view when both sashes are open.
With two fully operable panels, double casement windows deliver more natural airflow than single casement or sliding windows. You can open one side for gentle ventilation or both sides for maximum air circulation, making them ideal for living rooms, kitchens, and bedrooms.
When closed, double casement windows seal tightly with multiple locking points around the perimeter, creating an airtight seal that minimizes drafts and heat transfer. Aluminum casement window models with thermal break technology can achieve K-values as low as 1.3, significantly reducing heating and cooling costs.
The balanced, mirror-image design of double casement windows adds visual harmony to building facades. Whether your project is modern, traditional, or classical, these windows complement virtually any architectural style with their clean, proportioned appearance.
Quality double casement windows feature multi-point locking systems and anti-pry hardware, making them more secure than many sliding window alternatives. When properly locked, the sashes meet tightly in the center, creating a strong, tamper-resistant barrier.
Crank handles make double casement windows effortless to operate, even in hard-to-reach locations like above kitchen sinks. Most designs also allow for easy cleaning from the inside, especially with inward-opening configurations.
One important design decision when choosing double casement windows is whether to include a center mullion — the vertical post that separates the two sashes when closed.
| Feature | With Mullion | Mullion-Free (French Casement) |
|---|---|---|
| View When Closed | Center post visible | Sashes meet flush, minimal center line |
| View When Open | Mullion remains in center | Completely unobstructed opening |
| Structural Strength | Higher — mullion adds rigidity | Requires reinforced frame design |
| Cost | Generally lower | Higher due to engineering |
While double casement windows are available in various materials including wood, vinyl, and uPVC, aluminum frames offer distinct advantages that make them the preferred choice for many architects, builders, and homeowners.
Aluminum is inherently strong and resistant to warping, rotting, and corrosion — ideal for double casement designs that need to support two sashes.
Modern thermal break aluminum profiles with PA66GF25 insulation strips deliver excellent energy efficiency without sacrificing strength.
Aluminum frames require minimal upkeep — no painting or staining needed. A simple wipe-down keeps them looking new for decades.
Aluminum can be powder-coated in virtually any color and finish, from matte black to wood grain textures.
As a professional casement window manufacturer based in China, ALPES offers a range of high-quality aluminum double casement windows designed for residential and commercial projects worldwide.
ALPES also offers the C116 Outward-Open Window with an ultra-slim 22mm medium mullion for maximized field of vision, and the C113 series with full glue-injection corner processing for enhanced structural integrity. All products are manufactured in ALPES's 100,000 m² production facilities in Foshan and Zhaoqing, meeting international building standards.
Consider these factors when selecting double casement windows for your project:
Double casement windows work best for medium-to-large openings. For very wide openings (over 2 meters), consider triple casement or sliding alternatives.
Outward-opening windows save interior space but require clearance outside. Inward-opening designs allow for easy interior cleaning but take up indoor space when open.
Look for thermal break frames, multi-chamber design, and low-e glass if energy efficiency is a priority. For cold climates, seek K-values of 1.6 or lower.
Invest in reputable hardware brands like ROTO, Winkhaus, or CMECH. The hardware handles repeated daily use, so quality matters for long-term reliability.
Ensure the windows meet relevant standards such as AAMA, EN 14351-1, or local building codes for wind pressure, water tightness, and air infiltration.
Double casement windows combine timeless elegance with practical functionality, offering superior ventilation, energy efficiency, and visual appeal compared to single-panel alternatives. Whether you're building a new home or upgrading existing windows, this configuration provides an excellent balance of performance and aesthetics.
For projects requiring premium quality and custom solutions, working with an experienced casement window manufacturer like ALPES ensures you get windows engineered to meet your specific requirements — from thermal performance to hardware selection and custom finishes.
