Shading

The principleStop the sun before it reaches the glass

Once sunlight passes through a window, most of its heat stays in the room, whatever you hang behind the glass. An internal blind or curtain catches the light, warms up and releases that heat indoors. External shading stops the sun outside, where the heat blows away. External devices are roughly 50% more effective than internal ones at blocking the sun's heat.

Internal blinds and curtains still matter. They control glare and privacy, and on winter nights a close-fitting curtain with a pelmet cuts heat loss through the glass. They just aren't the answer to summer sun.

NorthSizing the eave

North glass is the easiest to shade, because the summer sun is high when it faces north and the winter sun is low. A fixed horizontal eave can tell the two apart.

The rule of thumb for southern Australia: the eave should project about 45% of the height from the window sill to the underside of the eave. Here is how that works on a typical Canberra living-room window:

In practice, an eave sized this way fully shades the window at noon from about mid-October to late February. From late May to late July the whole window gets the noon sun. In between, the shadow moves up and down the glass with the seasons.

Two details make it work:

• Keep the eave close above the window, around 500 mm or less. An eave much higher than that, such as a raking ceiling with a high fascia, lets summer sun in under it.
• Extend it past the window on both sides by at least its own projection. Summer sun on a north window comes in at an angle in the morning and afternoon, and a narrow eave misses it.

East and westLow sun needs a different answer

Morning and afternoon sun comes in low, almost horizontally, so an eave of any sensible depth barely touches it. West glass gets this sun at the hottest time of day, in the late afternoon when the air has been warming since morning.

What works:

• External blinds or louvres that cover the whole window when lowered. Adjustable versions let you keep the view and the breeze when the sun isn't on the glass.
• Deep verandahs or covered balconies, which shade the glass from a steep angle as well as providing outdoor space.
• Vertical screens or fins, angled to block sun from the north-west or south-west. Fins facing straight out do little against sun that's directly west.
• Planting, such as trees and shrubs to the west, which shade the wall as well as the glass.
• Less glass. The cheapest shading for a west window is not having it, or moving it to the north.

A west-facing bedroom shows why this matters. The room absorbs heat all afternoon and gives it back just as people try to sleep, long after the air outside has cooled.

PlantingTrees and vines

Deciduous trees and vines look like the perfect shade: leaves in summer, bare in winter. They help, but bare branches still block more sun than people expect, typically around a third and sometimes half. A big deciduous tree directly in front of north glass will cost you winter heat.

Place planting where its shade is wanted and its winter shadow does least harm:

• Deciduous trees to the north-east and north-west, far enough away that the low winter sun passes under or beside them.
• Evergreens to the west and south-west, for afternoon summer shade and shelter from winter wind.
• Deciduous vines on a pergola over north glass. Unlike a solid roof, they shade the terrace in summer and let in most of the light once the leaves drop.

Planting takes years to grow, so it works best alongside built shading rather than in place of it.

Outdoor roomsShelter without darkness

A covered terrace outside the main living room is a popular request. It's also one of the easiest ways to wreck a house's winter performance. A solid roof deep enough to sit under is deep enough to block the low winter sun from the windows behind it for most of the year.

Better options:

• Put the covered area beside the living room rather than in front of its main north windows.
• Use an adjustable louvre roof that opens in winter and closes in summer.
• Build a pergola with a deciduous vine, or with shade sails that come down for winter.
• Add high windows above the terrace roof to bring winter sun past it.

This has to be decided with the house plan. Added later as landscaping, the terrace roof usually ends up in the worst place.

CheckingSeeing it before it's built

The 45% rule is a starting point. The site decides the rest: the roof pitch, window heights, neighbouring buildings and trees all change where shadows fall. A 3D model of the house on its site shows the sun and shadows on any date and at any hour. Checking midwinter, midsummer and the equinoxes, at morning, noon and afternoon, catches most problems while they're still lines on a drawing.

The energy rating also models eaves, verandahs and neighbouring buildings, so a change to the shading changes the result. In NSW, BASIX has its own rules for what counts. For example, under the simpler DIY method, an eave more than 500 mm above the window head only counts for half its depth.