Windows Are Absurd

Main Ideas

Windows and Heat Transfer

Windows bring in daylight and provide views, but replacing part of a wall with glass affects a building’s structure, insulation, ventilation, sound, safety, and security.

Glass generally insulates much less effectively than a typical exterior wall. Heat moves through buildings by:

• Conduction: Energy passing through solid materials.
• Convection: Heat carried by moving air or liquid.
• Radiation: Energy transferred by electromagnetic waves.

A typical exterior wall may have an R-value of about 10–25, while a single pane of glass is around R-1. R-value measures resistance to heat flow.

In a demonstration, the host placed a 25-watt heat emitter inside a box and compared different lids. The interior-to-exterior temperature difference settled at about 43°F with an R-5 foam lid, 22°F with a single pane of glass, and 30°F with a double-pane window. The results illustrate how readily heat escapes through single-pane glass and how a double-pane unit improves performance.

Most modern windows have two panes, and some have three. The panes are separated by a sealed gap that may contain air, a heavier gas such as argon or krypton, or, in some units, a vacuum. Depending on construction, double-pane windows can provide roughly two to four times the insulating value of a single pane.

Sealed windows can also be affected by pressure differences. If a window is manufactured at one elevation and installed at a much higher elevation, the pressure difference may cause the panes to bow or potentially break. Manufacturers can adjust the pressure or use a narrow capillary tube to equalize it while limiting moisture entering the space between panes.

Coatings and Solar Heat

Glass lets visible light through but blocks much of the longwave infrared radiation emitted by warm objects. It absorbs that radiation, warms up, and can then transfer heat onward. Shortwave infrared from sunlight can pass through glass and warm indoor surfaces—the principle behind a greenhouse.

Low-emissivity (low-e) coatings act like mirrors for infrared radiation, reducing unwanted heat transfer. Coatings can be selected and positioned for a building’s climate: colder climates may favor letting solar heat in, while warmer climates may favor reflecting it. The coating’s position can also affect how heat is reflected between the panes and the room.

Bird-Safe Glass

Birds may mistake reflections of trees and sky for open space, or see through glass toward something they want to reach. Patterns on windows help make the glass visible as a barrier.

At Chicago’s McCormick Place Convention Center, a dot pattern reportedly reduced bird strikes by 95%. Some products use ultraviolet-visible patterns that are difficult for humans to see. Some cities are adding bird-safe-glass requirements, particularly in migration corridors and on large glass façades.

Curtain Walls and Glass Support

In many tall buildings, interior columns carry the structural loads while the exterior glass façade hangs from the building like a coat. These curtain walls are non-load-bearing façades. They are lightweight and relatively thin, can be assembled in large factory-made sections, and allow substantial daylight into the building.

Glass requires specialized support:

• Captured glazing holds glass within a frame, using softer materials to seal it and distribute forces along its edges.
• Structural silicone glazing bonds glass panels to the building with purpose-made structural sealant. The sealant may be applied on all four edges for a smooth exterior appearance.

The host contrasts structural silicone with ordinary bathroom caulk. In the demonstration, the caulk failed under a 45-pound load, while cured structural silicone held the load and an added small anvil. The video notes that real installations may also include mechanical restraints in case the sealant fails.

Glass Treatments and Breakage

Different types of glass address different breakage risks:

• Annealed glass is slowly cooled after manufacture. It can be cut or drilled, but tends to break into large, sharp shards.
• Tempered glass is rapidly cooled, leaving its outer surfaces in compression. It is stronger and breaks into smaller pieces, reducing—but not eliminating—the risk of injury. It is commonly required in locations such as doors, showers, and some low windows. It must be cut or drilled before tempering.
• Laminated glass sandwiches a plastic interlayer between panes. When cracked, the glass generally stays attached to the plastic, helping it remain a barrier. It is useful overhead, in ground-floor commercial windows, and in vehicle windshields.

Overall Lesson

Windows are not simple holes filled with glass. Their performance depends on layered design choices—including panes, gaps, coatings, frames, mounting systems, and glass treatments—that balance light and views against heat loss, bird collisions, structural demands, and human safety.

Demonstrations and Methodology

• Thermal-performance test: An insulated box was fitted with a 25-watt ceramic heat emitter, closed, and left until its temperature stabilized. The temperature difference was compared using an R-5 foam lid, a single glass pane, and a double-pane window.
• Sealant-strength test: A glass block was bonded to a metal plate with ordinary silicone caulk and tested with a 45-pound load. The test was repeated with cured structural silicone. The demonstration is illustrative and is not a substitute for testing an actual building installation.
• Infrared comparison: An infrared camera showed that ordinary glass blocks the view of warm objects beyond it and compared how a low-e-coated pane reflects infrared radiation.

Speakers and Sources Featured

• Grady Hillhouse — Host and narrator of Practical Engineering; presents the explanations and demonstrations.
• Send Cut Send — The video’s sponsor, discussed by Hillhouse in a sponsored segment about custom fabrication services.
• Other organizations and examples: McCormick Place Convention Center, glass manufacturers and bird-safe-glass products, and cities adopting bird-safe-glass requirements. No other interview speakers are featured.