The science of windows

by Helena Morgan
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Ahead of a predictably sweltering summer, senior lecturer at RMIT and HVAC, solar thermal energy and heat transfer expert Tom Simko tells Facility Management why windows are the unsung heroes to lean on when sustainably cooling and heating a household or facility. 

Senior Lecturer at RMIT School of Property, Construction and Project Management Dr Tom Simko says the coldest winter he ever experienced was in Canberra, and he attributes an outdated and frustrating Australian construction penchant as the guilty culprit.

“Single-glazed windows – I couldn’t fathom it! But now things are changing,” he says.

Virtually ubiquitous in North America and Europe, double-glazed windows were initially slow to catch on in the sunburnt country, which baffles Simko.

“There were no nationally consistent minimum thermal efficiency standards until 2004 and at the early stages they were pretty low, so builders and designers could get away with it – there was no incentive for double-glazing,” he says.

With a PhD in mechanical engineering from the University of Sydney, Simko essentially researches how heat travels from the warm and toasty indoors to the frigid outdoors, and vice versa in cooling dominated environments. He fiercely advocates for an uptake in double-glazed windows – two sheets of glass separated by an air gap, or an argon-filled gap, because argon is less heat conductive. 

Simko also promotes the use in such windows of low-emittance (low-e) coatings because they are the key to eliminating a process called radiative heat transfer across the internal gap between the two glass sheets.

The physics of UV radiation

Most of us may not allocate too much thought as to what exact part of the solar spectrum is responsible for searing our skin on high UV days. In the window science community, however, it’s a primary concern.

According to the World Health Organisation, ultraviolet rays are separated into three regions: ultraviolet A, ultraviolet B and ultraviolet C. Simko likens a plot of the solar spectrum to an ‘upside down U, stretched out to the right’ with the intensity of radiation on the y-axis and the wavelength on the x-axis.

Solar irradiance spectral distribution vs. black-body spectral distribution at 5800 K.

“Around the peak of the U is the visible portion of the spectrum – the wavelengths or colours we see. To the right of the visible region is the infrared, the long wavelengths tailing way out, and to the left is the ultraviolet wavelength region, which falls off more sharply.”

Simko says ultraviolet C does not hold any interest or usefulness to us. “The ozone layer filters it out, so it doesn’t get down to the surface of the Earth.”

The ozone layer also absorbs most of the ultraviolet B radiation – but not all. Ultraviolet B represents about five percent of the ultraviolet radiation at the earth’s surface. Ultraviolet A covers the other 95 percent. Not as energetic and possessing longer wavelengths, ultraviolet A covers the remainder of the spectrum and penetrates deeper into human skin than ultraviolet B.

Simko says that regular window glass (soda lime glass) essentially filters out ultraviolet B, leaving ultraviolet A to dominate.

In assessing how to create glass capable of filtering out ultraviolet A, Simko reveals that experts examine the percentage of transmission for each wavelength with a spectrometer, a light meter that can be used to measure the intensity of the ultraviolet radiation at certain wavelengths behind a sheet of glass. 

“One can look at the transmission of solar radiation through glass and the amount of ultraviolet radiation getting through the glass starts to drop off as you move leftwards into the ultraviolet A region. By the end of the ultraviolet A region, it’s virtually gone and, by the point where you’re in the ultraviolet B region, it’s completely gone,” says Simko.

Radiative heat transfer

So what does this mean for window science? It means that ultraviolet B rays are completely screened out by regular glass, yet ultraviolet A rays slip through – almost 63 percent, according to a University of Florida paper published in June this year. 

In a double-glazed window (two sheets of glass), heat mostly enters and leaves the external glass surfaces through air flows (convection) and radiative exchange with the surroundings. It conducts through the solid glass panes, much as heat flows through the metal base of a pot on your stove. The sweet spot in window design is the narrow gap within the window between the two sheets of glass, where all three modes of heat transfer can occur.

Ultraviolet radiation transmission data of the most commonly used glazing types in the built environment

“Heat jumps across the gap between the internal glass surfaces in a similar way to how you feel the heat of sunshine from across the distance of space or the heat from a campfire from several metres away,” explains Simko, referring to this process as ‘radiative heat transfer’. Heat also crosses the gap by conduction through the thin layer of air.

“If the gap is wide enough, you can also have air currents circling around in there, but in practice the gaps aren’t that wide – so what we need to do is to knock back that radiative heat transfer and the conduction,” says Simko. 

The most common way to significantly reduce the radiative heat transfer is to cover one of the internal glass surfaces with a low-e coat – a thin transparent layer typically consisting of a semiconductor, like tin oxide, with added impurities.

The conduction across the gap is reduced by a gap of ideal width – about 12 millimetres – and by replacing the air with less thermally conductive argon gas.

Heating dominated environments 

The science of building windows can be a form of climate-responsive design – a trope that is definitely here to stay in the wider architecture and design community. 

As the majority of the utility bills for a Melburnian household or facility over the course of a year go into keeping interior space warm, the Victorian capital is referred to as a ‘heating dominated environment’. Simko says windows need to be designed to function well in the winter by keeping the heat inside and allowing for an appropriate amount of solar radiation to pass through – resulting in the perk of free solar heating. 

“In winter, we want the indoor pane of glass to stay as warm as possible, so we coat it with a low-e coat so it will give up radiative heat less easily to the cold pane on the other side of the gap,” says Simko.

A low-e coat also carries the added benefit of UV protection, reducing the UV transmission across a double-glazed window by about half, according to the aforementioned University of Florida paper. Simko stresses that low-e coats don’t affect the visual clarity of windows.

Cooling dominated environments

Comparatively, houses and facilities in much of Queensland and the Top End exist in a ‘cooling dominated environment’ and the outer sheet of glass in question is naturally going to get hot in the summer. 

“In these climates we don’t want that heat radiating across the gap and flowing inwards, so we put the low-e coat on the gap-side surface of the external pane of glass to reduce that radiative transfer,” says Simko.

The solar heating expert warns against choosing reflective or tinted film for the added benefit of UV protection in a heating dominated environment that requires supplementary wintertime heating.

“Reflective or tinted films stop free sunshine coming in during winter. If you don’t have them, you’ll pay for the unwanted incoming solar gain in the summertime with extra cooling, but if a window has been properly selected, this should be offset by the desirable wintertime solar gains.”

“You don’t need tinted windows in a heating dominated environment unless it’s for a particular room or there’s a compelling comfort issue. But in Darwin, tinting and reflective films are fine,” says Simko.

Double double gives no trouble 

Simko says that in years past the Australian architecture and construction industry suffered from a misconception that double-glazing was reserved for cold climates such as Northern Europe or Canada.

“What really matters is just the temperature difference between the inside and the outside of a property,” says Simko.

More rewarding from a sustainability standpoint, advanced double-glazed windows can be about three times more insulating than basic single-glazed windows.

Simko advocates for the case of double-glazed windows for many reasons, yet a standout one is the lack of maintenance or retrofits required – appealing to facility managers. 

“You don’t really have to do anything to the windows. You don’t have to put films on them years later. You just have to start with the right windows to begin with which are double-glazed, low-e coated and argon-filled,” says Simko.  

Although, he says one minor complication relating to maintaining double-glazed windows involves framing. 

“Maintenance will differ depending on whether the frames are timber or aluminium – but double-glazing is not rocket science – double-glazed is a double win,” concludes Simko.

In October a decision was made to make all new house builds in Australia have a 7 NatHERS energy star rating. Simko labels this as a step in the right direction, addressing the limitations inherent in the lingering inclination to build with single-glazed windows. 

“The old fashioned Australian single-glazed windows are just not going to cut it anymore,” says Simko.“And depending on the house design and other insulating measures, even basic double-glazing may not be enough. Low-e, argon-filled windows will become more commonplace because of the higher star rating.”

On the rewards of glass

Apart from protecting ourselves against sun damage, why do we need to block out UV radiation from reaching indoor spaces? Ultraviolet radiation will, over time, bleach or fade the colours in carpets and furnishings. 

Simko says selecting the correct window glass for UV protection also carries safety benefits.

The best UV protection comes from windows made with laminated glass: two sheets of glass bonded together with a thin sheet of transparent plastic between them. The inclusion of the plastic means that, if smashed or broken, the glass will hold together instead of producing large and dangerous shards.

The front and back windows of cars are generally made from laminated glass, so in the event of an accident or rock strike, the glass can hold together. The plastic sheet is also an excellent UV sunscreen – leading laminated glass blocks up to 99.9% of UV radiation.

“There are plenty of film products on the market for various purposes that can be applied to windows – if appropriate for the site-specific situation and climate –  that will also block UV radiation,” says Simko. “And green window glass tinted with ferric oxides also screen out more UV than regular glass.”

Light and dark green glass will reduce the UV radiation by almost a half and a third respectively, compared to a single sheet of clear float glass. 

The forgotten feature 

Amid a general societal tendency towards energy efficiency and building more sustainably, Simko reports a general unawareness of the power of windows. 

“People are clearly very receptive and sympathetic towards building sustainably – but we don’t think about our windows. In selecting windows for a new home or retrofits, it could just be a simple matter of asking for a low-e coat design, which some companies offer as a free upgrade or for a relatively small extra cost,” says Simko. 

In encouraging more people to prioritise incorporating sustainable practices into builds or retrofits, Simko applauds the recent Victorian State Government decision to ban gas connections in new homes from the start of next year.

“It’s a great example of the government mandating sustainability rather than incentivising,” concludes Simko. 

Sustainability reporting marks generational change for property.

Photography and graphs supplied.

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