For Aggressive Efficiency, Choose Passive Solar

As we discussed in the November/ December issue, low-hanging solar-responsive design strategies reduce a building’s energy consumption and typically cost less than the high-hanging solar-collecting technologies — or nothing at all. (See Choose the Low-Hanging Fruit by Lechner.) In this issue we focus on passive solar heating techniques.

Most passive solar space-heating systems are of three major types: south windows, also called direct gain; Trombe walls, where south-facing glass covers a mass wall and sunspaces.

A Trombe wall system delivers no light, only heat into a space. Although sunspaces are the least efficient and most expensive, they are popular. Besides their basic construction cost, they are expensive to insulate and shade. The most sustainable sunspaces are those that have no east, west or overhead glazing. In effect, they are south-facing building spaces that are not mechanically heated or cooled. Their south façade would be all glass and may be more than one story high.

There is no truth in the idea that solar energy is too expensive to collect. It all depends on the system. Remember, basic passive solar heating is free if implemented during design, and the enhanced systems usually pay for themselves in a few years. Only the sunspace is hard to justify on financial grounds.

Any building that requires space heating should use passive solar design, not only because it is environmentally favorable but also because it can cost as little as nothing.

Every south-facing window is a net gainer of energy in the winter because it collects more energy during the day than it loses over a 24-hour period. Thus, every south-facing window is a basic passive solar heating system.

The Earth-sun relationship is such that vertical south windows receive the most sun in the winter and the least in the summer — just what is needed (see figure 1). On the other hand, horizontal glazing (horizontal or near- horizontal openings such as skylights) and east and west glazing get the most sun in the summer and the least in the winter — just the opposite of what we need.

Consequently, the most cost-effective strategy, which is actually free during the design phase, is to move some of the east and west windows to the south façade. Keep in mind that the more south windows in a building, the more heat is collected during the winter. Because passive solar heating will fail if the windows are not exposed enough to the winter sun, the designer can use a site-evaluation tool to ensure sufficient solar exposure. Both commercial and do-it-yourself site-evaluation tools are available. (Some commercial tools advertise in SOLAR TODAY, and two DIY tools are described in my book, Heating Cooling Lighting: Sustainable Design Methods for Architects, 3rd edition.)

Because the challenge with passive solar is to reduce a building’s energy use as much as possible but at a reasonable cost, it is important to consider passive solar enhancements such as a thermal mass, high R-value windows, night insulation and summer shading. Let’s examine each of these in turn.

Storing Excess Heat

On a mild sunny winter day, the amount of heat collected by south windows can easily overheat the passive solar spaces. Ideally we’d like to avoid having to open the windows, instead storing some of that excess heat for nighttime. Thermal mass can do that very effectively. Any building with concrete floor slabs has this mass, if the slab is merely left free of carpets or other insulating covers. Acid staining a concrete slab creates a beautiful and most sustainable finish. Any part of the concrete slab that receives direct sunlight should be left exposed, and, for best results, leave all of the slab exposed.

Essentially, the more mass the better, but adding mass beyond a slab-on-grade must be carefully considered because other mass is expensive. The first few inches into the mass area store most of the heat during the day and then release most of the collected heat during the night. Thus, mass area is much more important than mass depth.

Minimizing Loss, Maximizing Gain

Because south windows lose heat as well as collect it, it is desirable to minimize the heat loss and maximize the solar energy collected during the day.

Although windows with a high R-value reduce heat loss, they also reduce solar gain. Each glazing layer blocks about 10 percent of the sun. Furthermore, windows with the highest R-values are expensive. The ideal solution is to have a moderately high R-value window with high solar gain along with night insulation. The night insulation can also be used during summer days when no one is home, to keep the heat out.

To stop heat loss by conduction and convection, a tight seal is needed at the perimeter of the night insulation. Such a system is available commercially as a roll-down window quilt. However, reducing heat loss by radiation — roller shade with a metallic reflective coating or venetian blinds with shiny metallic slats, for instance — is simpler. A radiant barrier in a window or wall can reduce heat loss/gain as much as R-4 insulation. Night insulation is an energy-efficiency technique much underutilized. Here is an opportunity for inventive people to develop more practical low-cost night insulation systems.

Controlling Glare, Excessive Light Levels

Important concerns with passive solar heating include glare and pigment-damaging light levels. In commercial and institutional buildings, glare can be controlled with sets of stacked venetian blinds on each window. The upper (above-eye-level) blinds would be white and adjusted so that the sun is reflected to the ceiling. The lower venetian blinds should have slats that are white on one side and dark on the other.

In the winter, this lower venetian blind is adjusted to either reflect the light to the floor just inside the window, or with its dark side out so that it absorbs the sun and reradiates the energy into the space as heat instead of light. In residential buildings, it is wise to avoid placing objects that can fade where they may be exposed to high light levels.

If that is not possible, a good alternative, whether for a residential or commercial building, is a Trombe wall (see figure 2). The thickness of the mass wall determines the time lag (i.e., how many hours the heat is delayed in entering the space). In most cases, a mix of direct gain (south windows) and a Trombe wall gives the best results.

Shading for Year-round Performance

Although south-facing windows collect less sun in the summer than the winter, that amount is still too much. Thus, to improve the necessary year-round performance, south as well as any east and west windows must be shaded.

It is a common myth that a fixed overhang can be effective on south-facing windows. The fact is that a fixed overhang will either block too much of the winter sun by being too long or allow too much of the summer sun to enter by being too short. At best, an intermediately long fixed overhang will both block much of the desirable winter sun and allow much of the unwanted summer sun to enter (see figure 3). The solution is to use a movable overhang like an awning, a design element popular before air conditioning was available. A twice-a-year adjustment of a movable overhang will allow 100 percent of winter sun to enter and block 100 percent of the direct summer sun.

Everyone who has ever entered a closed car during the summer knows that passive solar works better in the summer than in the winter. This often-ignored phenomenon can offset any winter benefits. Thus, it is imperative to shade all glazing, including any Trombe wall and sunspace glazing, from the summer sun. In all climates, buildings must be designed for the whole year and not just one season. For this reason, the next article in this series will be on shading. Look for it in the March issue.

Norbert M. Lechner (lechnnm@auburn.edu) is an architect, professor emeritus in the College of Architecture, Design, and Construction at Auburn University, LEED-accredited professional and ASES Fellow. He is an expert in energy-responsive architectural design with an emphasis on solar-responsive design. Lechner’s book, Heating, Cooling, Lighting: Design Methods for Architects, is used by more than a third of all architecture schools in the United States and in architecture schools worldwide. He is also a sought-after speaker, giving keynote lectures and workshops at universities and conferences around the world.