Why You Should Care About the Visible Transmittance of Your Windows

Related: Here Comes the Sun... or why you should care about the Solar Heat Gain Coefficient of your windows.

Sunlight is essential for life: it allows the growth of plants, which produce the oxygen as well as nourishment needed for animal life. Sunlight also keeps us – and our homes – warm. Most people enjoy the feeling of sunlight, and there is a growing amount of scientific evidence that it is essential for our health too. Increased sun exposure is linked to protection from multiple forms of cancer, type 1 diabetes, multiple sclerosis, and many more diseases.

As a species, homo sapiens have most likely originated in equatorial Africa , a place that is about as sunny as it gets. Human migration out of Africa happened fairly recently on an evolutionary scale: modern humans migrated to Europe probably less than 50 thousand years ago , and a new discovery finds their earliest traces in the Americas about 33 thousand years ago . We could still be physiologically adapted for a sunnier environment than most places we currently inhabit — including Canada. No wonder the sun has been at the centre of human life, from being worshipped directly or spurring most monotheistic religions , to feeding the energy needed for our technological progress.

On the health benefits of sunlight

Heliotherapy, or using the sun’s healing power in medical treatment, has been recorded since antiquity . It was widely used in both Europe and North America up until in the modern era, even yielding Niels Finsen the Nobel Prize for Medicine in 1903. Advances like the discovery of vitamin D and antibiotics — and their mass production, while improving life expectancy, may have shifted the public focus towards the negative effects of UV radiation such as skin cancers, thereby reducing the attention to the health benefits of sunlight. Luckily, ‘solar-phobia’ is being isolated out as new research comes to support the health benefits of sunshine, calling for a better balance between sun exposure and protection.

Out on the Terrace. Photo of the Alton Hospital in Hampshire, 1937

Heliotherapy was more widespread before the advent of antibiotics and synthetic medicine. Here, children undergo sun therapy at the Alton Hospital, in Hampshire, 1937. Image: Wellcome Library, London

As a fenestration company, we care about windows that are bright. Apart from providing views outside — which in itself is associated with benefits for vision and mental health, windows should also allow as much natural light indoors as possible. Here are just a few reasons why:

Sunlight helps our bodies produce vitamin D.

Vitamin D is essential for our health, providing numerous benefits such as supporting healthy bones, managing calcium levels, reducing inflammation, supporting glucose metabolism and weight loss, and helping the immune system. Now links between vitamin D deficiency have been made to everything from multiple sclerosis to prostate cancer. Vitamin D has been produced on this earth for more than 500 million years . How is it made? During exposure to sunlight, the sun's UV rays turn a chemical called 7-dehydrocholesterol (a cholesterol precursor) to previtamin D3, which in turn is turned by your body into active vitamin D.

How much sun exposure is needed? Around our latitudes, during the warm months of the year when you can expose enough skin outdoors, about 10–30 minutes of midday sunlight, several times per week is enough. Darker-skinned people have more melanin which blocks out UV radiation, and may need a little more exposure.

A hand holding a Vitamin D softgel pill up to the sunlight.

Vitamin D provides wide-ranging health benefits. It is produced in the skin under sunlight, hence the “sunshine vitamin” epithet, but is also available as a synthetic supplement.

How about winter months? While windows block out a significant amount of UV radiation, they don’t really block out all of it. We tend to keep our homes more comfortable nowadays, allowing us to expose more skin during wintertime. So bask in all the sunshine that makes its way indoors, to help your body replenish on vitamin D, with a much lower risk of developing skin cancers.

Sunlight can lower blood pressure and help prevent stroke.

According to the World Health Organisation , high blood pressure is the leading risk factor for premature death and disease worldwide — more so than cancer. Richard Weller from the University of Edinburgh draws a correlation in epidemiological data between increased sun exposure and reduced blood pressure and cardiovascular mortality.

Apparently, the human skin contains significant reserves of nitrogen oxides, which are converted to nitric oxide (NO) by UV radiation. NO is easily exported to our systemic circulation through the vast network of small blood vessels used by the skin for thermoregulation. Accidentally observed by Robert Furchgott on mice , this pathway was corroborated via in vivo studies on humans 40 years later and was determined to cause arterial dilation — hence reduced blood pressure. It has also been shown that vitamin D supplementation is of no benefit in the prevention or treatment of hypertension , cardiovascular disease, cerebrovascular disease or metabolic syndrome, so we’re looking at a pathway that, like vitamin D production, relies on sunlight, but is completely independent.

A doctor checks a patient's blood pressure.

Increased sun exposure appears to be correlated to reduced blood pressure and lower cardiovascular mortality.

So, per Richard Weller, sunlight could bring significant health benefits beyond vitamin D synthesis, and the UV-induced release of NO in the skin may have more widespread cardiovascular actions than BP reduction. Considering that cardiovascular disease is the largest risk factor worldwide, sunshine for cardio is yet to be weighed to its proper importance!

Sunlight regulates the circadian rhythm.

The circadian rhythm , or circadian cycle, is a process that virtually all living organisms are subject to — from animals to plants and fungi. It is driven by a kind of internal circadian clock in the hypothalamus region of our brains, and responds to changes in the external environment. It is designed to match the day-night cycle, allowing the organism to anticipate external changes and adjust its activity level for optimum balance. While the sleep-wake cycle would be the first thing we can think of, disturbances to our circadian cycle go beyond sleep disorders: hormone regulation, reaction time, and behavior are all affected. In fact, under optimal conditions, most cells in our body run in a cyclic pattern, and this pattern is connected to the internal clock.

A woman stretches her arms and happily wakes up in the morning.

The internal circadian clock relies on daylight as the primary cue to keep everything inside us functioning in unison. Image: Cottonbro/Pexels

We have photosensitive cells in our eye retinas, connected to the pituitary gland in the brain. Their stimulation is done by sunlight, regulating the levels of serotonin and melatonin produced in the body. Dr.’s Turner and Mainster of the University of Kansas School of Medicine state that “these photoreceptors play a vital role in human physiology and health.” Light-sensing cells in the eye directly affect the hypothalamus region in our brains, which is in charge of our biological clock. Therefore, without sun stimulation, any part of the organism can falter. Per Turner and Mainster, “ensuing circadian disturbances can have significant physiological and psychological consequences,” including increasing risk of disease such as cancer, diabetes, and heart disease.

Sunshine combats seasonal affective disorders.

Winter blues is a real thing, especially in Canada and other countries at high latitudes. A seasonal affective disorder (SAD) is a form of depression, or mood disorder, that happens every year at about the same time . It is said to be more prevalent in women .

While the exact causes of SAD are unknown, a few scientists think that certain hormones produced in the brain cause behavioural changes at specific times of year. They believe that these hormonal changes could be responsible for SAD. One theory is that reduced sunlight in fall and winter time makes the brain produce less serotonin , a hormone linked to internal mood-regulating pathways. When these brain cell pathways are compromised, a feeling of depression can ensue, together with related symptoms like fatigue, weight gain, irritability, sleepiness, and desire to be alone.

A person hunkers down in their bed feeling the affects of seasonal affective disorders.

The darker days of winter may bring about darker moods. Seasonal Affective Disorders significantly affect more than 10% of the population in Northern climates. Image: Cottonbro/Pexels

Research suggests that getting some sun may help shake winter blues : light hitting your skin, not just your eyes, helps reverse seasonal affective disorder (SAD) . A walk outside, or basking in some sunshine in the morning could have a beneficial effect.

Sunlight helps keep your vision healthy.

Sunlight is good for your eyes — after all, seeing it is what we have those eyes for. Did you know that syntonics, or photo-syntonic light therapy , has been used by optometrists for decades? Nevertheless, myopia, the condition of being nearsighted, has become more common than it was before. Since the early 1970s, the prevalence of myopia in Americans has soared by 66 percent , according to a 2009 study by the National Eye Institute. In many East Asian countries, as many as 90 percent of recent high school graduates are thought to have it. A lot of the causes are thought to be due to lifestyle changes, with people spending more time indoors and glued to electronic devices.

Some recent epidemiological evidence suggests that children who spend more time outdoors are less likely to be or become myopic. One mechanism for this protective effect is the release of dopamine from the retina by visible light stimulation, which inhibits excessive axial elongation — the main cause of myopia. New research suggests that the shorter wavelengths in visible light (violet, 360 nm to 400 nm, very abundant in sunlight) suppress myopia progression, in both animals and in children provided with violet light-transmitting lenses.

A group of glasses-wearing employees sit around a table with laptops during the workday.

Not just a fad: more young people require prescription eyewear today than a generation ago. Image: Fauxels/Pexels

Sunlight provides full-spectrum lighting, which is as close to black-body radiation as our atmosphere allows. It also means that you get to see real colours across the entire visible spectrum, without specific tones being dimmed out as it happens with artificial light sources. You are probably familiar with the colour rendering index (CRI), it is advertised on some of the more expensive light bulbs, with values as approaching 100 as much as possible. Yet none of them match the light quality that the sun provides. We may need to rethink our 24/7 filtering UV from sunlight. Early morning natural sunlight can render all artificial incandescent, fluorescent, and warm LED bulbs obsolete. And the sun’s been there all along, free for all.

How much sunlight are our homes getting?

Let’s face it: most of us live in homes that someone else built. Most often, your home was built by a developer who cared about fitting as many housing units in an acre as possible, dressing the picture-perfect facade and kitchen for a quick sale, and just about meeting building code requirements. The cookie-cutter approach meant placing the home designs on the street with a complete disregard of orientation to the sun (which could help maximise passive heating) or indoor natural light levels (for your health). Per building code, unobstructed glazing in bedrooms in Ontario must be a minimum 5% of the floor area , and not many builders want to go beyond minimums. Even those who do get to build custom homes may have to comply with regulations limiting the glazed surface on the walls to no more than 15%.

Older homes in coveted heritage neighbourhoods, like the ones brandishing the distinctive Toronto Bay & Gable style , are most often narrow row houses with only the front and back having any fenestration. No wonder the Industrial Revolution brought about, along with high-density urbanization, soaring levels of rickets in children .

In our Northern climate, we spend a disproportionate amount of time indoors. And our homes, schools, work and other places could use more natural light. It is true that energy costs money, but so do health problems. And an energy-efficient building does not necessarily need to be a dark one.

Where do windows fall in the equation?

Windows actually serve multiple purposes, and have gone a very long way in their technological evolution to better suit every one of them. Here are just some reasons why we put windows in our walls:

Windows bring natural light inside your home.

This is the number one reason people made holes in the walls for. Bringing light inside means not relying on fire or contemporary lighting to be able to see inside your home. Sunlight is free, and you want to use it as much as possible. Moreover, by being a full spectrum light source, sunlight is better than any artificial light. And it makes you healthier, as covered above.

Windows provide exterior views.

Of course, we need to be aware about what’s happening outside our walls - be it time of day, the weather, neighbours, what your kids are doing in the backyard, or just watching your garden or the bids. Seeing potential threats, like dangerous animals or criminals, is also important. However, one benefit to seeing outside is often overlooked: your eyes’ ability to focus on distant objects. We have come to rely on manipulating objects – tools, food, electronics and other things – most of our time, and it is often done at arm’s length or less. To the modern human, this causes a disproportionately long time focusing on near objects, especially when compared to other animals. By providing distant objects to focus on, windows literally help prevent myopia. Seeing out in the open can also help against claustrophobia.

Windows help thermoregulate your home.

Windows that are operable means they can be open, and this is super important for ventilation. Creating an air flow to and from the exterior helps release excess heat and cool your home, improving comfort levels. Across most of the densely populated areas in Canada, exterior temperatures during the night in summer are optimal for sleeping, allowing one to get a better rest for a more energetic next day.

Windows improve indoor air quality.

Our homes are closed environments, so naturally the inside air quality deteriorates over time. We need to release the built-up carbon dioxide, volatile organic compounds from our home and the objects inside, as well as any other air pollutant — including radon, excessive humidity, or unpleasant smells. Unless one lives in an extremely polluted environment, outside air will always be cleaner and contain more oxygen than indoors. Ventilating will also help bring nice scents inside - be it from your garden, fields and forests, or even distant seas, and who wouldn’t prefer that to the smell of asphalt?

Windows help seal and insulate your home.

While windows do not match the insulation levels provided by a standard wall, they still help keep heat where it belongs - inside in winter and outside in summer. A window will also be built to protect your home against inclement weather by deflecting strong winds and driving rain, as well as by preventing water ingress inside. Finally, a window should also provide some protection against intrusion, whether from animals or humans.

What to look for in your windows

We want a lot of things from our windows: to be bright, well insulated, air- and watertight, strong, secure, and easy to operate. While making a window excel in any of this is pretty straightforward, it is getting it to work well across the board that takes the bulk of the R&D work. Let’s look at what it takes to make a bright, but also energy-efficient window.

The Visible Transmittance (VT)

When thinking about the primary roles of windows in your home - bringing natural light inside and offering views outside, you want to make sure that the window actually performs these functions best by letting as much light through as possible. There is a metric for this, and it is called the Visible Transmittance (VT). This is a factor of how much light in the visible spectrum actually passes through a transparent object. As with any factor, the maximum value it can take is 1 or 100%, and the lowest is zero. Anything that does not let any light through will have a visible transmittance of 0, and only an invisible object could have a visible transmittance of 1.

Side by side flower vases with shadows coming from sunlight through a window.

No transparent material is fully translucent: they always cast shadows. Each new layer reduces the visible transmittance. Photo: Jill Burrow/Pexels

The visible transmittance has a direct effect on the translucency (how well light passes through, or how little of it is lost in the process) and the transparency (how little distortion happens to the light rays, for the purpose of getting a clear image on the other side).

This may sound strange, but it is true: no piece of glass is fully translucent! In fact, most regular clear (aka float) glass used in fenestration will have a VT in the 0.8-0.9 range. It means that each sheet of this glass accounts for 10%-20% of light lost. You can see it for yourself by looking at the shadow the glass projects on the ground, comparing it with the fully-lit surface. You will see the difference!

The U-factor

With Canada’s cold winters, we really care about how well a window insulates. The most used insulation metric in fenestration is the U-factor. It shows how much heat passes from one side of the window to the other. Contrary to the visible transmittance, the best U-factor is the lowest one. Lowering the U-factor is achieved through a series of means, such as using multiple panes of glass, special coatings that reflect heat, high-performance warm-edge spacers between the panes, filling the insulated glass unit with an inert gas, making the window frames with as many hollow chambers as possible, injecting foam inside frames, etc. Unfortunately, well insulated windows can end up a lot darker than anticipated:

  • More glass panes mean darker windows.

    You can multiply the visible transmittance of each pane to get the maximum overall transmittance of the insulated glass unit. Glass with a visible transmittance of 0.85 will yield a maximum 0.72 VT in double-pane, and 0.61 in triple-pane - that’s almost 40% of light lost.

  • More glass panes mean a smaller glass surface.

    Glass is a heavy material, accounting for most of the mass in a window. To support more weight, the window frames, sashes, and even the hardware have to be made thicker, giving you more plastic and less glass in the window.

  • Glass coatings make it darker.

    Thermal insulated glass units use low-emissivity coatings. These coatings help reflect radiant heat so that most of it is trapped where it originated. Since these coatings are usually metallic, a lot of visible light is reflected or absorbed by them, rendering the glass darker. The heavier the coating, the darker the window.

We’ve seen quite a few homeowners who’ve shopped for replacement windows based on U-factor alone, and had the unpleasant surprise of ending up with really dark windows, with a visible transmittance of barely 0.3. Window replacement is a significant investment in your home, and you don’t want to end up living in a place that you can’t wait to move out of. For those who care about the brightness of their homes, the visible transmittance (and solar heat gain coefficient!) of their new windows should also be carefully considered when shopping around.

How do we make our windows bright AND well-insulated?

At Nordik Windows and Doors, energy efficiency goes together with brightness in our efforts to make our windows the best value on the market. Making windows that are well-insulated and bright at the same time, without costing an arm and a leg, takes a significant chunk of our R&D. Here are a few ways our RevoCell windows achieve top energy efficiency, while being brighter than alternatives:

Microcellular PVC helps achieve triple-pane efficiency in double-pane

In regular PVC windows, more hollow chambers means less heat loss through the frames and sashes. In our own microcellular PVC windows, there are billions of ‘chambers’ inside, functioning as a closed-cell insulation that eliminates the heat leaks due to convection and condensation. Plus, microcellular PVC is stronger than the liquid PU foam injected into hollow windows, also retaining the material recyclability. The result? Our dual-pane windows are as insulated as many competitors’ triple-panes, bringing the benefit of fewer sheets of glass — hence more brightness — for the same overall energy efficiency.

Stronger, leaner frames mean larger glass

The strength of microcellular PVC allows for leaner frames, sashes and mullions, which means larger glass and less plastic in your window. On an average 2-lite casement window, it can add up to as much as 20% extra in glazed surface.

We use brighter glass in windows

Not all glass is the same. We use low-iron, super-clear glass in windows that’s at least 10% brighter than regular green float glass. Our advanced low-e coated glass also strikes a better balance between heat retention and visible transmission.

The bottom line:

When it’s time to replace the old windows, don’t forget why you have windows in your walls to start with — for natural light and all the health benefits it brings along. Keep your interiors bright with windows that hit the sweet spot on insulation level (U-factor), passive heat potential (Solar Heat Gain Coefficient), and brightness (Visible Transmittance). If that sounds too hard, go with the highest Energy-Rating (ER) value available, it is pretty well balanced out for you.

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