The Sun and its Impact on Home Design – Design Tips

Looking for North

It is no rocket science that the sun rises in the east, moves towards the north in the southern hemisphere and sets in the west. Yet, a surprising number of home-owners are unsure of the orientation of their property, sometimes after as many as twenty years in the one house! Not everyone is aware of the solar movements or has north-facing windows to keep track of the sun: the fact is that too many houses still only enjoy the sun as an outside view but never get its warming rays inside. Adding to the not-so-uncommon disorientation is the fact that the path of the sun changes slightly everyday, with its rising and setting directions varying dramatically over the course of a few months. The only constant is that the sun shines from the same direction everyday of the year at midday – ignoring for now the question of daylight savings which complicates the issue a bit.

Summer sun explained

Everywhere in Australia, the sun rises south of east in the summer and sets south of west, covering in effect two thirds of the 24 hour dial. The sun stays in the sky for an average of 14 hours out of the 24 and travels at high altitude in the middle of the day, creating short shadows and barely penetrating through north-facing windows – a blessing which can be further helped by a 450-600mm wide roof eave. In the morning and afternoon however, the sun is low enough on the horizon to penetrate deeply through east and west-facing windows but its heat can be fierce at both ends of a summer day. The amount of radiant heat generated by the sun is in fact the same in the morning and the afternoon, the only difference being that it hits a cool house in the morning and a hot house later in the day – a recipe for turning it into an oven. In all climates of Australia, it is desirable to keep a house cool in summer which calls for a dramatic reduction of both glazing and wall area on the rising and setting directions. A reflective membrane can be installed in the cavity of the walls facing these directions, and if any window needs to be located on the western side, a low-emissivity coating can help the glazing fend off unwanted radiation. As an alternative, shading with trees planted east and west of the house can prove helpful for an existing house as the solutions mentioned above are better implemented on a new house than retrofitted to an existing one.

Winter sun 101

In winter, the sun rises north of east and sets north of west, covering in average just over one third of the 24 hour dial – these data vary somewhat from Hobart to Darwin, but the same principle applies everywhere. By the time the sun’s gathered any useful heat, it is already midday and north-facing, rendering any east and west-facing windows useless as far as passive solar gains. As the sun only has an average of less than 10 hours to start and complete its travel in the sky, it stays at relatively low altitude the whole day, basking north-facing rooms in warmth for about 4 hours (10:30am to 2:30pm in average). In all climates where winter solar gains are desirable – basically all regions bar sub-tropical ones – it is best to maximize the length of north-facing walls and north-facing glazing.

Ideal window location

We’ve established that windows facing east or west are not desirable to achieve high efficiency because they create overheating in summer and are useless in winter – as far as passive solar gains that is. And because windows are so weak thermally – the best ones on the Australian market still only offer a third of the thermal insulation of a R2.0 wall – the east and west-facing windows promote heat loss that is not balanced by solar heat gain… what north-facing windows are so good at doing. Ideally therefore, a perfect passive solar design will have all its windows on the north face of the building, and none on the east and west. The conventional head height of windows is 2.1 metres or 7 feet which entails that in average, the winter sunrays penetrate a house 4 metres into a north-facing room. This promotes in turn the idea of a shallow rather than deep floor plan with its long side facing north – and south – and its short side facing east and west. An ideal floor plan depth is 7 or 8 metres, which allows for a row of living areas to face north, a hallway at the back of these and a row of buffer zones (entry, laundry, garage etc…) to face south. Note that window heads at 2.4 metres or 8 feet are a good idea on the north side as they allow greater winter sun penetration and capture more of the available views year round.

Impact on building massing

In principle, a long and skinny building facing north is very much designed like a passive solar collector, a guarantee of energy efficiency. In practice, considerations of the views to be captured, land fall, lot size and orientation as well as aesthetics will also play a part in determining the house shape.  Nevertheless, it is always possible to moderate those factors where achieving proper orientation is a strong focus. For example, where the views are to the west, locate only one or two strategic rooms on this side, design picture windows rather than window walls and make sure the glazing features reflective coating. Where north is at the front of the block, locate your living areas on the street side and build a good privacy fence, or locate them at the rear and work with roofs to include significant amount of high-level north-facing windows to illuminate the back of the house. Another strategy again is to create internal courtyards so that the parts of the house that are further away from the street still have some solar frontage.

Typical home extension plans

The ranch style house is typically a long building with verandah all around, usually facing the view and parallel to the street and giving little consideration to solar orientation. If you are lucky enough that its long side faces north, your first and foremost move is to get rid of the north-facing section of the verandah which cuts all passive solar gains in winter. A horizontal folding-arm blind or pergola with deciduous vine will do as good a job at summer-shading without excluding winter sun. Now consider your typical suburban house with a more compact rectangular shape. If the house faces either east or west at the rear, the solar frontage is along the side boundary of the block and a long and slender extension located close to the south boundary will be facing north and maximise solar frontage. In contrast, the solar frontage of a house with a north-facing backyard is usually shorter, being the width rather than length of the block. Extending to the sides to use the full available width is usually a good option.

Choosing a block of land wisely

As options for siting a house on the typical size suburban block are limited, properties with an east or west-facing backyard boasts the greatest solar frontage and are ideal. If the block is narrow, watch out for overshadowing from the double-storey house next door however. Next best choice is the north-facing backyard which, again, works well where the block is wide enough (say 13 metres and up). Avoid south-facing backyards as privacy issues pop up when large north-facing windows face the street. Don’t leave your compass at home when hunting for a house or land and make sure that any construction makes the most of the property solar potential by exposing the long side of the house or extension to the north. This has to be one of the most important things you can do to improve both the energy efficiency of the house and your enjoyment of living there.