Passive Solar Systems for Sustainable Heating

Welcome to round two of the five-part series exploring the foundations, so to speak, of sustainable building. As a recap, our featured protagonist, environmental advocate, and locale Andy Hill is walking us through the important features of energy-efficient design and using his real-time retirement home construction to demonstrate. Two weeks ago, we explored the “Perfect Wall,” a systematic design previously coined by the Cold Weather Institute in Fair Banks, Alaska, that Hill incorporated into his own project.

Now, we move forward in the building process, graduating from walls to windows. The sunshine that pours warmth and joy through our windows is, at times, a force to be reckoned with. It has the power to heat but equally to overheat, to gift comfort and taketh away, to fill rooms with light that is just right, but overflow when light transforms to heat. These forces, however, are not impervious to human control. Passive solar is a mechanism which allows sustainable designers and contractors like Hill the opportunity to use nature’s resources to their fullest extent and, in turn, decrease energy consumption.

In order to more acutely define passive solar, let us compare it to its counterpart- active solar. Active solar exploits the sun’s natural energy for alternate forces, premium for human comfort, like electricity. Passive solar, rather, uses a combination of site location, window placement, and varying window compounds to regulate how the sun’s rays themselves interact with our homes. One of the most vital acknowledgements in understanding this process is that the sun’s rays will have a much more dramatic affect in heating our homes when angled through a window, or a series of windows, and penetrate the floor. Here, insulation absorbs heat and in turn raises the home’s indoor temperature. Thus, says Hill, “the square footage of the windows correlates directly to the square footage of the house where heat can be absorbed.”

However, in Earth’s complex place in space, revolving around the sun and tilting on its axis, the sun’s angle isn’t stagnant year-round, particularly in areas further south and north of the equator. Our region’s name, the Northwoods, hints that we may be in for a bit more geometry in calculating when, and where, the sun’s rays will seep indoors.

“You’re playing with the sun’s angle,” says Hill. “Maximum light equals maximum heat.” In the northern hemisphere, the most direct sunlight comes from the south. Therefore, south-facing windows most effectively utilize sunlight. The summer sun sits higher in the sky than the winter sun, and as a result will enter homes through the window at a sharper angle and reflect directly onto a home’s floor. In the summer, though, this result- significant heat absorption- is undesirable. A rooftop overhang is a simple solution to this problem; an overhang can extend the roof’s surface area to create a barrier between the summer sun’s angle and the windows.

The winter sun sits lower in the sky, its most prevalent rays penetrating south-facing windows and extending across the length of a home’s floor, eventually striking the interior north wall. This setup allows sunlight to extend all the way through a home, which is a defining aspect of passive solar design. So, says Hill, “when choosing our site location, we were looking for a property that sat far back against the north side with dominant southern exposure.” Hill also describes the importance of considering flora in site location. If a home has nearby deciduous forest, for example, residents can count on more sun exposure from that direction in winter, when those trees aren’t flaunting their greenery. Hill calls this “working with nature,” as could we describe his endeavor overall.

After site location and window placement, the second most important consideration is window composition. Passive solar design involves window glazing, a method that allows windows to either gain or repel light and heat dependent upon the gas utilized. “Changing the gas filling changes the performance of the window,” says Hill. Another composition tactic for defending against heat penetration is the use of energy-efficient glass coatings, one type of which is called Low-e. Low-e glass limits the amount of infrared light, or heat energy, entering a home; it is an all-in-one for illumination plus climate control.

In summary, Hill’s new gas-filled, low-e, triple-paned windows aid in maintaining a constant indoor temperature. The windows’ glazing mixture, he says, “is designed to let heat in and keep heat in. A house in Florida may still elect passive solar, but their mixture will let light in and repel heat.”

But, what would design be without style? The European-style windows Hill selected are unique in that their opening mechanism angles inside (think pull versus push), and also tilt open in order to further control the angle at which the sun is entering the home.

So, open your Northwoods home up to the south, choose the windows best for your site location, and let the light in. With a smile, Hill makes a final, less scientific recommendation to place a window facing east near your dining room table. “Folks in the Northwoods want a sunny breakfast spot,” he says. “This makes for a cheery home.”