Spandrel glass
Spandrel is the trick that lets a tower look like glass from sidewalk to roofline. It is the opaque glazing that hides the floor slabs, columns, insulation, and ductwork between the windows, dressed up to blend in with the vision glass around it.
What is spandrel glass?
In a multi-story curtainwall, not every panel is a window you see through. The horizontal bands between floors have to hide a lot of unglamorous stuff, structural slabs, insulation, fire-stops, the tops and bottoms of the floor plates, while still reading as part of the glass skin. The panel that pulls that off is spandrel glass.
The job is to look opaque from outside while matching the color and shine of the vision glass next to it as closely as possible. Get it right and you get that seamless all-glass tower everyone pictures. Get it wrong and the mismatched panels jump out from across the street, permanently.
Types of spandrel glass
Ceramic frit
Ceramic frit is a glass-based paint fired right into the surface during tempering or heat strengthening. The particles fuse into the glass for good, so it is tough as nails and does not care about UV. On a spandrel panel it goes on surface 2 (the inside face of the outer lite), tucked away from the weather while still facing out.
Ceramic frit is available in a very wide range of colors and patterns, including dot-screen patterns (called fritting) that can create gradients from opaque to translucent. It is the most durable and weather-resistant spandrel option and is specified for the majority of commercial curtainwall projects.
Opaci-Coat
Opaci-Coat is a proprietary silicone-based coating rolled or sprayed onto the back of the glass at the fab shop (not in the furnace) and then cured. It gives you a dense, even, matte-to-satin finish with excellent color consistency and very high opacity.
Because it is applied after fabrication, Opaci-Coat is flexible for custom colors and can match almost any RAL or Pantone color exactly. It is resistant to UV, moisture, and alkali from concrete backing. However, it requires careful application, uneven coverage or contamination can cause adhesion failure over time.
Back-painted glass (painted back)
Back-painted glass is just ordinary paint on the back of the lite. It is cheaper than frit but nowhere near as tough, UV, moisture, and temperature swings can make it peel, bubble, or fade, especially on a panel that bakes in the sun. Keep it off exterior spandrel. It is fine indoors, kitchen splashbacks and decorative wall panels, where UV is not an issue.
| Method | Durability | UV resistance | Color range | Best for |
|---|---|---|---|---|
| Ceramic frit | Excellent, permanent | Excellent | Wide; custom screens | Commercial curtainwall, exterior |
| Opaci-Coat | Very good | Very good | Any RAL/Pantone | Exterior where exact color match needed |
| Back-painted | Fair, degrades in UV | Poor | Wide | Interior only |
Heat treatment requirements
Spandrel panels are almost always heat strengthened or tempered, and the reason is thermal stress. Unlike a vision pane, a spandrel panel usually has insulation or a dark barrier packed right behind it, trapping heat. That drives the center of the panel much hotter than the shaded edges clamped in the frame, and a steep temperature gradient across a pane is exactly what cracks glass.
Annealed glass just cannot be trusted with that kind of stress; it will crack on its own. Heat strengthened is the usual answer: enough thermal-stress resistance to be safe, without the spontaneous-breakage risk and roller wave that come with fully tempered.
Color matching vision glass
The hardest part of speccing spandrel is getting it to visually match the vision glass beside it. Vision glass both reflects and transmits light; spandrel only reflects. So the two will never be truly identical, but you can shrink the gap with careful choices.
Key factors that affect the match:
- Frit or coating color: Typically specified to match the reflective appearance of the vision glass at a set viewing angle and lighting condition.
- Glass substrate: The base glass (clear, tinted, or coated) should match or be coordinated with the vision glass substrate.
- Back pan color: An opaque back pan behind the spandrel IGU eliminates variable background colors that shift the apparent spandrel color in different conditions.
- Viewing conditions: Spandrel panels look different at dawn, midday, cloudy, and direct sun. Mockups are essential for verifying appearance across conditions.
Spandrel in insulated glass units
Most commercial spandrel panels are built as sealed IGUs: an outer lite carrying the spandrel treatment on surface 2, a sealed air or argon gap, and an inner lite (usually clear or lightly tinted).
The sealed IGU approach has several advantages over single-lite spandrel:
- Eliminates condensation on the interior face of the spandrel panel
- Provides improved thermal performance (reduces heat loss through the spandrel zone)
- The sealed air space provides a consistent dark background that improves color uniformity
- Protects the coating from damage during installation and building operation
Edge deletion
When frit or a coating sits on the back of a lite that is going into an IGU, you have to strip it off a band around the perimeter, usually 10–15mm, before sealing. That step is called edge deletion.
It matters because most IGU sealants will not grip a coated surface reliably. Let the seal bond to coating instead of bare glass and the unit fails early. Fabricators strip that band with automated abrasive or laser tools, and you should always match the deletion width to what the sealant maker specifies for that exact coating.