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Part VI: Staying CoolChapter 10. Passive and Low-Energy Cooling

Shade First: Trees, Awnings, and Exterior Screens

Stopping sun before it hits glass or a roof, which is where cooling projects should start.

Difficulty
Beginner
Cost
$0–250
published

The cheapest cooling is the sunlight you never let into the building. Once sun passes through glass, most of that energy is indoors, and a curtain only decides whether the heat sits at the window or spreads into the room. Shade belongs outside the glass: a tree, an overhang, an awning, a trellis, or a screen.

Start with the west windows and the south windows, in that order for summer comfort. West sun is low and hot at the end of the day, when the house is already warm. South sun is high at noon in summer and can be blocked with a modest overhang that still lets the low winter sun underneath. East sun is a morning problem. North glass, in the northern hemisphere, gets little direct sun.

Do the site survey before you plant or build. A shade structure in the wrong place blocks winter heat you wanted, or misses the west window that was the actual complaint.

Difficulty

Level
Beginner
Time
A day for a small awning or a screen, years for a tree
Skills
Measuring, fastening into sound wood, and knowing where power lines are

Cost estimate

$0–250

A deciduous tree in a pot can be the cheap end and is not shade for several years. Shade cloth and a simple wood frame often land inside this range. A large custom awning, a permit, or a carpenter pushing into the structure of the house costs more.

Materials

ItemQtyNotes
Tape and the site sketch1Window height, and the direction each window faces.
Shade cloth or a wood awning frameFor the worst window firstExterior. Light colors reflect. Dark cloth still shades, and it gets hotter.
Fasteners into solid backingAs the frame needsFascia that is rotten is not backing. Find the rafter or the stud.
A deciduous tree, if the plan is a tree1Planted where the mature crown shades the west or south glass without sitting on the foundation or the sewer.

Why outside wins

Glass lets shortwave sunlight through and then the room's surfaces warm up and radiate longwave heat that the glass does not let back out as easily. That is the greenhouse effect in one window. An interior blind shades the floor and still leaves a hot pocket between the blind and the pane, and a share of that heat ends up in the room. An outside awning never lets the sun reach the pane.

The same outside shade, left in place in January, blocks winter heat you might have wanted. A removable screen or a deciduous tree fits a climate that wants both summer shade and winter sun. A fixed awning has to be sized so the winter sun still gets under it, or it has to come down in the fall.

Overhang geometry, without a simulation

At solar noon the sun's altitude is about 90 degrees minus your latitude plus the sun's declination. Declination is about plus 23.5 degrees at the June solstice and minus 23.5 degrees at the December solstice. At 40 degrees north, noon altitude is roughly 73 degrees in late June and roughly 26 degrees in late December.

A horizontal overhang above a south window casts a shadow down the glass. The vertical reach of that shadow is about the overhang's depth times the tangent of the altitude. To shade a window of height H at that moment, the depth wants to be at least H divided by the tangent of the altitude. At 40 degrees north in late June the tangent of 73 degrees is a little over 3, so a 4 foot tall south window needs on the order of 16 inches of overhang to be in shade at noon. Morning and mid-afternoon sun comes in at an angle and will still strike the glass unless the overhang is deeper, wider than the window, or helped by a side fin.

That noon check is why south windows are the easy ones. West windows fail the same trick. Late-day sun sits low, the tangent is small, and a practical awning cannot be deep enough to block it all without becoming a roof. For west glass, use a vertical screen, a tree, or an exterior roller shade that drops over the whole pane. Do not expect a 16 inch brow to fix a west picture window.

Exterior shade on a windowSection of a window with an overhang. A steep summer ray hits the overhang. A low winter ray passes under it and through the glass. A separate screen is shown on the west side.SummerWinterWest screenShade the glass from outside. An indoor blind lets the heat in first.
A steep summer ray hits the overhang. A low winter ray still reaches the glass. The west window in the same drawing needs a screen, not a deeper version of the same brow.

Trees

A deciduous tree on the southwest or west can be the best shade you will ever have, and it is also the slowest and the easiest to plant in the wrong spot. Look up the mature spread before you dig. Roots and falling limbs need to stay off the foundation, the sewer lateral, and the neighbor's roof. A tree due south of a solar collector or a sunspace will grow into a winter shade problem you measured yourself in the site survey. Put evergreen trees on the north side or as a windbreak, not in front of the winter glazing.

Even a leafless deciduous crown blocks some winter sun. If a collector is planned, keep the future crown out of the southern sky, not merely out of the summer sky.

Call the utility locate service before you dig. A shade tree planted into a buried line is an expensive hole.

Awnings, trellises, and screens

Build the smallest shade that hits the window you actually sit under. A frame of rot-resistant wood or metal, lagged into structure, with shade cloth laced tight, is enough to learn whether the room changes. Extend the cloth past the window on both sides so the late and early rays do not sneak around. Leave a gap at the top or choose a cloth that breathes, so you are not building a sail and a heat trap in one.

Wind is the load. A solid panel awning acts like a wing. Hardware and the wood it screws into have to be sound. If the only fastening is a thin fascia board, stop and find the rafter tails. Removable clips let you take the cloth down before a storm and before heating season.

A vine on a trellis shades like a tree and keeps the leaves off the siding if you give it its own wires. Do not train a vine into the weep holes of a brick wall or under a shingle. The shade is not worth a leak.

Interior curtains remain useful for privacy and for slowing night heat loss in winter. They are a second step, not the cooling strategy.

  1. Name the hot windows

    On a summer afternoon, write down which rooms overheat and which direction those windows face. West first, then south.

  2. Check winter sun before you block it

    If that glass is also your winter heater or your future collector, choose removable shade or a deciduous crown, and sketch the December rays.

  3. Size a south overhang with the noon rule

    Depth at least the window height divided by the tangent of the June noon altitude for your latitude. Make it wider than the window, or add fins, if the ends still glare.

  4. Give west windows a vertical shade

    Screen, roller, or tree. Do not keep deepening a horizontal awning and hoping.

  5. Fasten into structure and watch one hot day

    If the room is still hot, look for roof and attic heat and for windows you missed. If the room is comfortable, you have earned the right to talk about fans.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.