Why You Should Care About the Solar Heat Gain Coefficient of Your Windows.

The US National Renewable Energy Laboratory provides one astonishing fact: in one hour, the Earth receives more energy from the Sun than all the energy used by everyone in the world in a whole year. Per Vaclav Smil from the University of Manitoba, the total solar energy received by Earth in a year is 3,850,000 exajoules , equivalent to over 1 billion terawatt-hour — compare that to the mere 160 terawatt-hour a year humans consume globally.

Honestly, these numbers are hard to fully grasp. How about we bring them to a more digestible scale, using familiar units like square metres and kilowatt-hour? Let's run some math: 3.85×10⁶ exajoules per year converts to 1.07×10²¹ watt-hour per year; dividing this to the Earth's surface ( 5.10×10¹⁴ m² per Michael Pidwirny / UBC ) yields an average of 2100 kWh per square meter a year.

Now, this figure does not take into account cloud cover, latitude, exposure or orientation, all of which will reduce solar potential further. A good resource on insolation levels in Canada is provided for photovoltaic applications, and you can see in this neat solar potential map that South Ontario and most of Canada has 1000 kWh and more per square metre. Coastal areas in British Columbia and the Maritimes get less due to accrued cloud cover, while the Prairie provinces are the sunniest because of their dry continental climate.

A map showing the Annual Photovalic Potentials: South-facing with Latitude Tilt.

In the most populated areas in the South, Canada has over 1000 kWh per square meter in photovoltaic potential. Source: Natural Resources Canada

1000kWh is actually comparable to the energy used annually per household in Canada, which StatsCan says was about 25700 kWh in 2015 . Running some more math will give you more potential energy available on your property than your household actually uses

Technology has evolved, allowing us to convert more sunlight to energy usable in buildings. We can immediately think of modern technologies like photovoltaics that generate electricity. While wind turbines, hydro and tidal generators are driven by air and water currents, that energy too originates in heat exchanges due to the sun — via convection, evaporation, and condensation.

Passive house design, a breakthrough keyword closely related to insulation and efficient space heating and cooling, actually relies on a principle used in our human dwellings since the earliest history – use all the free energy you can get, reuse what you can, and reduce all wasted energy where possible.

Where do Canadian homes use their energy?

A pie chart of Energy Spent in 2018 by end use in Canadian homes.

Canadian households spent more than half (54.4%) of their energy on space heating in 2018. In contrast, space cooling accounted for barely over 1.1% of the overall energy use. Data: StatsCan

When you hear home energy, one thing that comes to mind may be electricity, and all the appliances and gadgets that you rely on to make your home, well, homely. However, the term encompasses everything, including fossil fuels, like natural gas or heating oil. According to Natural Resources Canada, over 54% of all the household energy was still used for heating in 2018 , surpassing all other end uses combined. In contrast, the same source estimates cooling energy cost at barely over 1%. How's that for a heating/cooling cost ratio - fifty to one! This is where the effects of our Northern climate really show.

Where do windows fall in the equation?

When making your home energy-efficient, insulating the heck out of it would be top of mind. That goes hand in hand with making it as airtight as possible. Think of it like your down jacket on a cold winter day. If you keep the jacket zipped up, you stay warm, but if you unzip the jacket, you lose heat quickly. Now, you'd think that windows and doors go against everything insulated and airtight stand for. No hole in the wall, no matter what it's covered with, could possibly insulate as well as a wall. And you would be right!

However, we all know that's not why we make those openings in our walls. We need them to enter and exit the home, ventilate it, provide exterior views, and bring natural light inside. Yet so many people don't realize that windows are also passive energy sources. Letting sunlight in actually allows us to heat our homes, and who wouldn't want their energy bills lowered in winter?

Sunlight is a heat source. Use your windows to capture it.

We all know the sun heats up everything, but how does it work? Sunlight is actually energy that propagates as waveforms. Most of the light that reaches the surface of the Earth is electromagnetic radiation in wavelengths corresponding to the visible light spectrum (yes, the rainbow), with a bit more in the near-infrared and ultraviolet wavelengths invisible to humans.

You can think of infrared light as radiant heat. However there isn't as much energy in it as there is in the visible and the ultraviolet spectrum. The cooler the light colour, or the shorter the wavelength, the more energy is embodied in each photon. That is why ultraviolet light can be so damaging - its high energy can break chemical bonds in the DNA, which in turn causes mutations (read: cancer) or death of the living cells. Luckily, most of the ultraviolet radiation is reflected back into space by the ozone layer, and fenestration helps further reduce the UV radiation levels.

So, your best source of passive heating from the sun is visible light. This is where windows with a high solar heat gain coefficient are important. They will help allow more light through, and retain the generated heat better.

What to look for in your new windows

What makes an energy-efficient window is actually a fine balance between its three key characteristics:

  • Low air infiltration/exfiltration rates. If your window is leaky, your money is literally flowing out the window, together with the heat. Simple as that. You want your windows as airtight as possible. What about ventilation? This should be done when the weather is mild. At large temperature differences between the interior and exterior, external ventilation is best done through heat recovery systems, while air recirculated internally should be filtered.

  • Low U-factor. The U-factor, or U-value, is a measure of how much heat is transmitted through the surface of the window. You want heat to stay where it belongs — inside in winter and outside in summer. A good overall U-factor is achieved by good insulation in the window frame as well as in the insulated glass unit. A window construction with cellular insulation is the most efficient. A glass unit filled with an inert gas like Argon or Krypton will lose less heat via convection, while quality cellular spacers will reduce direct heat transfer.

  • High Solar Heat Gain coefficient. This is a measure of how much heat from sunlight can be gained and stored inside your home, and is usually achieved with a combination of brighter glass and quality low-emissivity glass coatings. Brighter glass has higher visible transmittance, allowing more light in the visible spectrum through. Low-e coatings serve as virtually invisible mirrors for light in the infrared spectrum, reflecting radiant heat back where it came from — outside or inside. In our heating-heavy Canadian homes, a high solar heat gain coefficient helps heat the interior passively using free energy from the sun and reduce active heating costs.

When choosing windows for new home construction or for improving an existing home, one should strike the right balance between all three characteristics above. That is no easy feat, even for seasoned professionals.

Luckily, Natural Resources Canada also calculates the Energy Rating (ER) of a window, which is a weighted measure of the three performance metrics above. It is the most comprehensive overall indication of a window's energy efficiency. Choosing windows with the highest available Energy Rating is the simplest way to get good windows for your home.

How do we make our windows insulated, airtight, and bright?

At Nordik Windows and Doors, we put most of our research and development efforts into maximizing the ER value for our standard product configurations. We achieve it by:

  • Minimizing the U-factor: our RevoCell windows feature a microcellular PVC construction, which insulates like closed-cell foam, while the Argon-filled glass units use high-performance warm-edge spacers to further minimize heat transfer and lower the U-value;

  • Minimizing air leaks: we favour fusion-welded frames and sashes, structural construction with integrated cross-mullions, and using as much weatherstripping as possible between moving parts;

  • Maximizing solar heat gain: we build our windows with stronger but slimmer frames, which maximize the glass surface. We source super-clear, bright low-iron glass, as well as low-e coated glass featuring the optimum balance between brightness and heat reflection.

As a result, our RevoCell windows come standard with the Energy Star Most Efficient rating (even with dual-pane glass), a designation reserved for the crème de la crème in fenestration.

What else can you do to improve passive heating in your home?

To maximize passive heating, apart from letting light inside, you also need to be able to store more heat and release it slowly during nighttime. Choosing the right materials can help.

Bright objects reflect more light, so you may want darker surfaces if you want them to heat up faster. Incident light will be absorbed by the molecules of the material, exciting them and raising its temperature. This heat will build up inside the object, and you want to store as much of it as possible. Finally, stored heat will be released to surrounding air and neighbouring objects.

Light-absorbing materials should have a high thermal mass. Thermal mass is a measure of the amount of heat that can be efficiently absorbed, stored, and released by a material. There are three key requirements that materials serving as thermal mass should meet:

  • High density. In a nutshell, denser objects are heavier, they have more mass, and can usually store more heat. There are exceptions to the rule, so read on.

  • High specific heat capacity. This is the amount of heat that can be stored in a unit of mass. Materials with compound molecules tend to have a higher specific heat capacity than elemental materials. For instance, iron (Fe) is eight times denser than water (H₂O), but water can store ten times more heat per unit of mass.

  • Higher thermal conductivity. This is a measure of how fast heat can travel through an object, from the warmer to the cooler part. High thermal conductivity is the inverse of the material's insulating property. It means that a material with a lower R-value has a higher conductivity and acts more as a thermal bridge rather than a break.

Examples of common construction materials that offer good thermal mass are stone, concrete, ceramics, and brick. Unfortunately, these are mostly used on the exterior of a home, but they can be used creatively in your interior as well. Drywall is gypsum-based (and has a good specific heat capacity), but it is usually too thin (has insufficient mass) to store a lot of heat. On the other hand, ceramic tiles with the mortar bed underneath will store more heat than wood or floating floors. Steel and other metals have a low specific heat capacity, while insulating materials and carpets have low thermal conductivity and density, neither make good thermal mass. While water is the best at storing heat among common materials, its containment and circulation system is expensive — you don't want leaks in a timber-framed house.

What about summer heat — how do you control that?

The answer is simple: ventilate to let out excessive heat. You'll also get fresh oxygen inside, and hopefully nice smells from your garden. Then, block out excessive sunlight using blinds or awnings. Planting deciduous trees at South-Western exposures will do you and your home tons of good. Trees also make awesome windbreaks and can help reduce noise levels.

While most Canadians will likely heat their homes up to half a year, those two weeks of heat waves in summer could make you seriously consider moving into the fridge. And if a beach, pool, or lawn sprinkler party is not an option — hey, that's what air conditioning is for.

The bottom line:

Just remember — most Canadians spend nearly 50 times more on heating than on cooling. Being a smart Canuck, favour windows with a high solar heat gain coefficient on sun-facing exposures. Or choose the highest Energy Rating all-around. You'll be splitting the bill with Mother Nature, and doing both of yourselves a favour.

Continued Reading: Here Comes the Sun Part II... or why you should care about the Visible Transmittance of your windows.

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