How to Choose South-Facing Windows

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There are two parts to maximizing solar gain through windows: direct gain and indirect gain. The more obvious and common of the two is direct gain – using the sunlight that comes through the windows and heats the air and thermal mass in the home by falling directly on objects and walls.

As a rule of thumb, window surface area for direct gain should range between 7% and 12% of the total area of the home's floor space. Depending on where you live and how much solar energy is available , this could provide as much as 50% to 80% of your home's annual heating needs. If you design well, it could provide even more.

You know by now that you should have more glazing facing true south than on the other sides of the home, as that's where the sun's light comes from (in the northern hemisphere). But should you go for vertical or tilted windows?

In most passive solar home designs, vertical windows work sufficiently well that they are the standard choice. Tilted glass does offer some advantages, though.

  • The sun travels at right-angles to the glass surface in winter. Less energy is reflected away and lost than for vertical windows.
  • There is no overhang to shade tilted glass. It allows more light in throughout the rest of the year, so can be a good choice for colder climates.

The problem is that tilted glass has some significant disadvantages.

  • Because tilted windows let more light in, they can cause serious overheating in the warmer seasons (especially summer) unless the home has a way of venting the extra heat.
  • Any window that is not vertical will eventually leak, no matter how well it is installed. This means your home will suffer from extra air infiltration and possibly even water leaking in from tilted windows.
  • Many building codes demand the use of tempered glass in any non-vertical window installation, which is more expensive than normal glass.
  • Non-vertical windows are harder to shade than their vertical counterparts, creating design difficulties in areas with strong sunlight in the non-heating season(s).

With all that in mind, it is understandable that opinions are split on using tilted windows, though most designers and builders go with the vertical standard. Careful design and use can make angled windows very effective additions to a home, and produce a beautiful aesthetic that vertical alternatives cannot provide. On the other hand, the gains in solar heating are negligible when there is snowfall, which reflects extra light through vertical windows and makes them as efficient as a tilted option.