The Science Behind Double-Glazed Windows: How They Improve Energy Efficiency

With rising energy costs and environmental awareness, homeowners are seeking ways to reduce heating and cooling expenses. Upgrading to double-glazed windows is one of the most effective options, providing year-round efficiency. But how exactly do these windows save energy? The science behind double glazing reveals the clever engineering that slashes energy loss.

What is Double Glazing?

Double-glazed windows contain two parallel panes of glass enclosing a sealed airspace. The twin layers create an insulated barrier limiting heat transfer in either direction. Compared to single pane windows with higher conduction, double glazing reduces energy loss by around 50% in many climates. Additional benefits like noise reduction make them a popular green upgrade.

Insulation is further enhanced by filling the gap between glass with argon or krypton gas. These inert gases slow conduction and convection versus regular air. Hermetic edge seals prevent gas leakage while maintaining the insulating gas concentration. Advanced glazing options like low-emissivity coatings or electrochromic tints offer even greater efficiency.

How Heat Moves Through Windows

To understand double glazing’s advantage, it helps to know how windows lose heat. The main mechanisms are:

  • Conduction – Direct transfer of heat energy through materials, like from warm indoor air through glass to cold outdoor temperatures. The rate depends on the material’s thermal conductivity. Still air has lower conductivity than many building materials, so trapping it within double glazing slows conduction.
  • Convection – Heat transfer via air circulation, like drafts along cold window glass. Double glazing separates and slows air flow to reduce convection.
  • Radiation – Infrared heat radiating outward from warm objects towards colder surfaces. Low-emissivity coatings minimize radiative heat loss through the glass.
  • Infiltration – Outside air penetrating indoors through cracks and gaps. Tight double-glazing construction eliminates infiltration drafts.
  • Minimizing these modes of heat transfer is key to energy efficient windows. Double glazing strategically targets each one through clever engineering and material selection.

Conduction Through the Airspace

Adding a second pane and enclosed insulating airspace cuts conductive heat flow significantly. While glass has high conductivity, still air does not. The gap width is optimized between 20-16mm to balance insulation versus glass clarity and spacing. Wider gaps above 20mm increase convection while thinner gaps sacrifice insulation value. The double airspace acts like a thermal chokepoint.

LBS mentioned that filling the gap with argon or krypton further reduces conduction. These monatomic noble gases have lower thermal conductivity than the diatomic nitrogen and oxygen molecules in air. Krypton provides the best insulation value with 60% less conductivity than air. However, its higher cost limits use to very cold climates or specialized glazing. Affordable argon still lowers conductivity around 10% versus air. Sealed gas fills maintain this improved insulation for decades if edge seals remain intact.

Convection Layers and Air Vents

What little convection occurs in the airspace can be further limited by adding convection barriers. Horizontal or vertical capillary tubes within the gap impede small convection currents between hot and cold glass. Staggered vertical pillar spacers also divide the gap into discreet convective layers with less active air motion.

Vents on the window’s perimeter allow any pressure changes within the unit to equalize to outside air. This prevents expansions from forcing gas out of the gap. But the vents are too narrow to enable convection drafts. Carefully engineered ventilation maintains the airspace’s insulating gas concentration over years of thermal cycling.

Low-E and Reflective Coatings

Another double-glazing advantage is specialized glass coatings limiting radiation and conduction simultaneously. Low-emissivity or Low-E coatings reduce the ability of infrared wavelengths to radiatively exit the glass. This “invisible insulation” blocks heat loss through radiation without sacrificing clarity like tinted glass. Reflective metallic coatings also minimize solar heat gain during summer.

Low-E coatings work best when facing the air gap rather than outward. This allows the glass to absorb radiant heat from indoors and reflect it back rather than losing it to the cold outer pane. Proper orientation and optimal coatings application significantly improve double glazing performance.

Optimizing Double Glazing Performance

Beyond the core concept, several design choices impact double glazing’s real-world energy efficiency:

Frame Material

Double glazing units can be installed in frames of wood, vinyl, fiberglass, metal, or composite materials. Wood offers the best insulation but requires more maintenance. Vinyl frames insulate well but can become brittle and warp over time. Composite frames blend durability, performance, and low maintenance. The frame choice affects the window’s overall U-value rating.

Metal Spacer Bars

Aluminium spacer bars between the glass panes cause thermal bridging that reduces insulation. Using less conductive materials for spacers improves performance. Options include fiberglass, acrylic foam, silicone foam or stainless steel with thermal breaks. Warmer edge spacers also minimize interior condensation.

Edge Sealants

Most units use polyisobutylene as the adhesive sealant. Advanced sealants like silicone, polysulfide or hot melt butyl may prolong edge seal integrity and gas containment. This maintains the airspace’s insulating properties over decades. However, advanced seals cost significantly more.

Number of Gases

Triple glazing with two enclosed gas spaces provides even greater insulation. Krypton is commonly used for the inner gap while cheaper argon fills the outer space. The additional gas barrier improves the U-value up to 30% over double glazing. However, triple glazing costs much more and weighs considerably more requiring structural support.

IGU Glass Thickness

Thicker glass panes improve insulation further, but also increase weight and cost. Most residential windows utilize 4 mm – 6 mm annealed float glass. annealing removes internal stresses for durability. Tempered safety glass can be used where needed to resist impact. In cold climates, thicker 8 mm – 10 mm glass enhances energy savings.

Gap Width

The ideal gap width is between 16 mm and 20 mm. Narrower spaces risk convection while wider gaps reduce support and increase longwave radiation heat transfer. Specialized pressurized and corrugated gap technologies can allow wider gaps with minimized convection. This improves conduction insulation further.

Final words

In summary, the combination of twin insulation layers, conductively inert gases, convection barriers and advanced coatings gives double glazing outstanding thermal advantages over single panes. The clever engineering science behind the concept makes passive energy savings a transparent reality.