One of the aspects of passive solar house design that surprised many people is the need for shade. If your heating depends on sunlight, why would you block it? Because not all sunlight is beneficial to a passive solar home – the high summer sun, in particular, can produce far too much heat, requiring some kind of shade to stop your home turning into an oven.
The sunnier your location, the more likely it is that you will need to shade windows. Natural shading comprises all external obstructions that are not part of the home, and comes in two forms: biological and man-made.
Man-made obstructions are things like overhangs and surrounding buildings which provide shade for your home. If you're building external structures – whether they're sheds or ground-based solar thermal collector arrays – bear in mind the effect they will have on the main home and its solar window.
The biological options are more extensive and easier to manage in most cases:
- Trees and vegetation do more work than man-made obstructions because they cool the air (by evaporation) as well as blocking sun and providing shade. Called "transpiration", the process is incredibly effective – a single mature tree is roughly equivalent to five 10,000-Btu air conditioners! There's also no problem with pollution: quite the opposite, in fact. Trees can remove 9°F from the air temperature around a house.
- Deciduous trees are almost always a better choice than evergreens.
- You must consider all aspects of tree growth when you plant. How tall will the tree grow? What shape does it take? How quickly will it grow? What does the branch spread look like?
- South-facing windows will need tall, high-crowned trees. You want the crown to be out of the way of the low winter sun, but in the way of the high summer sun to shade the roof, walls, and windows. When they lose their leaves in fall, the winter sun can pass through the branches. And cool breezes can pass underneath the crown.
- Don't forget to shade east- and west-facing windows, though in most cases you will use bushes, smaller trees, trellises, or arbors. Vines are especially good, as they grow quickly and provide excellent shade – just don't let them get into the masonry!
- Grass also cools the air (by up to 10°F), and doesn't absorb as much sunlight as bare dirt.
- Evergreens work well on the non-solar sides of the house, where the fact that they keep their leaves (or needles) in winter is a bonus. Be careful of planting too thickly, though, as they may block cooling breezes.