In the early days of modern construction, buildings were often draughty and inefficient. Cold air would pour through gaps in the walls around windows, making heating homes more challenging.
Over time (as technology improved), manufacturers began to develop various technologies to deal with these issues. Retaining heat in the home became a priority, particularly for working-class people wanting to stay warm during the winter.
Initially, no techniques existed except to fill holes around windows and the interface between window frames and masonry. However, secondary and double glazing continued development as the need for solutions to keep people warm, especially in cold climates like Britain, became paramount.
The Rise Of Secondary Glazing And Double Glazing: When Did They Become Popular?
Secondary glazing began emerging in the 19th century as homeowners tried to improve the thermal comfort of their houses. Some properties had a second sash window beyond their primary window to retain more heat. Wealthy homeowners saw that adding these could warm their properties in December, January and February, allowing them to keep more energy from wood burning.
Things stepped up a notch during the 1930s when American refrigeration engineer Charles D. Haven invented the Thermoplane system. This installation featured two window panes separated by an air gap considered by many to be the first precursor to modern double-glazing.
Unfortunately, Haven didn’t see much success from his ideas due to the interruption of World War II. The war effort required retooling most industrial facilities, leaving less room for his domestic production ideas. However, engineers soon picked up on the concept in the 1960s, creating secondary aluminium glazing leveraging industrial facilities already built for the extrusion industry.
Double glazing’s history is related to these developments, but different. Haven’s concept of insulating air is arguably where things started but it wasn’t until the 1940s that the first example of real double-glazing emerged with the Old John Hancock building in Boston. Engineers installed the new concept using a double layer of 12-metre-squared glass across over 10,000 windows. The idea was to keep more heat in the building while protecting occupants from sun damage.
After World War II, numerous manufacturers began building smaller double-glazing units for household installation in factories. These were made of two panels of glass embedded in an aluminium frame with argon or krypton gas pumped in between.
Later, prices reduced further when manufacturers switched to cheaper and lower-maintenance UPVC frames, allowing more people to insulate their homes affordably. By the end of the 1990s, over 60% of homes had double glazing, up from just 16% twenty years earlier.
But why did these installations become so popular, so quickly?
Secondary Glazing Was An Easy Retrofit
One reason was that secondary glazing was becoming an easy retrofit on older buildings. Double-glazing units in the right shapes weren’t always available, so adding a secondary glazing window could keep more heat in the home, improving the thermal experience.
Planning also played a role. Many building owners weren’t allowed to remove and replace their old windows with new ones, so secondary glazing was an obvious fix.
Double Glazing’s Mass Production
Meanwhile, double glazing’s popularity soared due to cheap mass production. Once manufacturers figured out how to make durable UPVC frames, costs came down and more people could afford to install double-glazed units on their homes.
Fundamentally, new double glazing changed the cost-benefit analysis. Suddenly, people could expect to be better off long-term if they installed double-glazing to reduce heating bills.
The 1970s Energy Crisis
The 1970s energy crisis also played a critical role in encouraging the British and others living in Northern Hemisphere countries to switch to double and secondary glazing. Higher and uncertain energy prices meant that every extra degree of heat in the home was precious.
Why Are There Two Types Of Solutions? Secondary Glazing And Double Glazing?
Many people (including those in the glazing industry) often use the terms “secondary glazing” and “double glazing” interchangeably. However, these solutions are different. Why?
Secondary glazing was the first to emerge because it used existing technology (e.g. sash windows) to create an air gap between the inner and outer panes. Early solutions had several inches between the two pieces of glass, creating a substantial barrier that heat found harder to travel through.
Engineers recognised that air was insulating in the 19th century because of advances in science. It was clear that leaving a gap could retain more warmth in homes for longer and amplify the effectiveness of fires. Therefore, it made sense to trap it between window panes and use it to prevent heat transfer.
Secondary glazing remains a popular solution to this day because of its versatility. Installing an additional glass pane over an existing window is often a superior retrofit method on heritage buildings requiring construction changes compared to conventional double glazing. Installers can fit it over existing window frames without making any modifications.
Double-glazing is the more advanced younger brother of secondary glazing. It is less versatile, but also effective at keeping heat inside.
Double-glazing consists of two panes of glass sharing a single sealed frame, usually with an insulating gas pumped between them. Due to superior energy-saving performance, these are now standard for new constructions.
So, why are there two approaches? The answer primarily lies around issues of cost and practicality. Secondary glazing is an efficient retrofit option for existing buildings, offering a quick and effective way to improve thermal performance without making structural changes. Meanwhile, double-glazing is ideal for buildings purpose-built to accept it, like new homes. These structures can benefit long-term from higher heating efficiency and lower bills.
The Pioneers Of The Glazing Industry: Who Led The Way?
The glazing industry’s invention wasn’t due to a single, lone genius. Instead, its emergence is due to numerous people working together who saw the concept’s potential. Visionaries combined with entrepreneurs and industry leaders to deliver products to market that people could use in their homes and businesses.
Interestingly, local craftspeople were likely the first to play around with secondary glazing. During the 19th century, they would recommend it to wealthy homeowners and estates wanting to keep their homes warm, offering them a removable glass pane or a wooden frame to affix to existing windows. You can sometimes see these on older Edwardian buildings in historic UK towns and cities as an inexpensive alternative to full-blown window replacement.
Pilkington, a firm founded in 1826, was one of the double-glazing leaders in the UK. The company began by making conventional glass products but popularised double glazing nationally and internationally. By the 1950s, it had one of the most advanced double-glazing production facilities globally, offering high-quality off-the-shelf glass units builders could install on residential and commercial projects.
Saint-Gobain, a leader in French glass manufacturing founded in 1665, was similarly precocious. During the 20th century, it researched and developed various double-glazing products and emphasised sustainability in glazing technologies, including its low-emissivity (Low-E) coatings for enhanced thermal performance.
Andersen Windows founded in the U.S. in 1903 also played a role in popularising windows for homes in North America. It started adding double glazing to its various product lines, making aesthetic options for the American market as early as the 1960s.
Finally, you have Velux, one of today’s most successful double-glazing companies. The Danish brand, primarily known for its skylights, contributed to the glazing industry by adapting double-glazing technology for sloped and vertical installations. These made it easier for manufacturers to deal with issues like condensation control and fitting insulation in hard-to-reach areas.
As these companies and various pioneers, like Haven, continued their work, the industry began reaching milestones. For example, during the mid-twentieth century, manufacturers began creating hermetically sealed double-glazed units that dealt with annoying problems like interior condensation. Water couldn’t get inside the cavity because there were no cracks, putting an end to the annoying steamed-up window issues many early adopters faced.
Later in the 1980s came low-emissivity coatings. Units with this technology reduced thermal heat losses by reflecting infrared radiation from radiators and fireplaces back into the room.
Gas-filled cavities began emerging at around the same time. These replaced air gaps with inert gases like argon or krypton that enhanced units’ insulating properties and made them even more efficient for homeowners wanting to avoid heat loss.
Of course, none of these technological milestones were easy to achieve. Innovators often faced pushback from within their own firms, telling them to stick with what’s known. But thanks to government subsidies and the energy crisis, the cultural context almost forced advanced glazing technologies into existence. Huge resources began pouring into their creation as soon as people realised they were a way to save money overall.
Today, the legacy of these early pioneers is obvious. Modern double and secondary glazing options are highly advanced and can improve the thermal efficiency of virtually any building.