Karratha Solar Analysis

Karratha is a coastal town in the Pilbara region of Western Australia. The town experiences tropical weather conditions, with hot humid summers and warm winters. Temperatures are high for most of the year, with 232 days per year over 30C. From September until April, average temperatures rarely fall below 30C, and can reach up to 48C (21st Jan 2003).

March is generally the hottest month of the year in Karratha, and it is not unusual to experience up to 4 days over 40C during this time. July on the other hand, is the coolest month with an average maximum temperature of 26.2C.

Due to these extreme temperatures, building design in Karratha requires additional design measures so as to not allow unwanted heat build-up or ingress into the house. The most effective of these is shade. Simply put, direct solar heat gain will occur to buildings and their surrounds unless adequate shading is provided.

In addition to shading windows and walls, it is beneficial in climates such as this to provide shade to any areas of thermal mass at ground level, such as paving and/or concrete (driveways and paths). Materials such as these, with thermal mass, have the capacity to absorb and store large amounts of heat energy over a period of time and re-release that stored heat once the heat source is removed. The re-radiated heat can then ‘blanket’ the house long after the sun has set.

Our Design Guidelines for Baynton West require an 800mm eave to all external walls on new residential properties. This requirement delivers additional shade to all walls and windows compared to standard (450mm or 600mm) eave lengths, and allows shade to surrounding ground surfaces to prevent heat build-up.

To demonstrate the benefits of an 800mm eave, CODA have conducted a solar study of the effect of an 800mm eave compared to a 100mm eave on a house with an eave height of 2700mm. For the purpose of the study, a lot size of 525sqm (15m x 35m) has been modelled, with a 265sqm (11.5m x 23m) rectangular house built on it. Note, the modelled house does not represent a form which is compliant with either the R-Codes or Design Guidelines with regards to setbacks etc, but simply demonstrates the shade provided by a 100m or 800m eave on each of the elevations. The solar study was conducted with a north-south house orientation and an east-west orientation.

 

Our findings are as follows:

1. A North-South orientated house (long-axis running from north to south) with an 800mm eave, is provided:

Summer Solstice: 100% shade to northern and southern elevations all day, with morning solar exposure to the eastern elevation and afternoon solar exposure to the western elevation. Full shade to house at midday.

Winter Solstice: 15-30% shade to northern elevation for most part of day, with 100% shade to southern elevation all day. Morning solar exposure to the eastern elevation and afternoon solar exposure to the western elevation.

Spring and Autumn Equinox: Both northern and southern elevations in full (90% or above) shade all day, including ground shade to the southern side of the house. Morning solar exposure to the eastern elevation, and afternoon solar exposure to the western elevation with both eastern and western elevations in full shade at midday.

 

2. A North-South orientated house (long axis running from north to south) with a 100m eave, is provided:

Summer solstice: 100% shade to northern elevations all day. Varying shade to Southern Elevation throughout the day, peaking at 80% shade at mid-day. Significant morning solar exposure to the eastern elevation and extreme afternoon solar exposure to the western elevation from early afternoon (15% shade at 3pm).

Winter Solstice: Little to no shade (max 5%) to northern elevation all day, 100% shade to southern elevation, including ground shade all day. Morning solar exposure to the eastern elevation and afternoon solar exposure to the western elevation.

Spring and Autumn Equinox: Southern elevations in full shade all day, including ground shade to the southern side of the house. Northern elevation shaded no more than 15% until late evening. Significant morning solar exposure to the eastern elevation lasting until early afternoon, and afternoon solar exposure to the western elevation from mid-day onwards.

 
3. An East-West oriented house (long axis running from east to west) with an 800mm eave, is provided:
Summer Solstice: 100% shade to northern and southern elevations all day, with morning solar exposure to the eastern elevation and afternoon solar exposure to the western elevation. Full shade to house at midday.
Winter Solstice: 15-30% shade to northern elevation for most part of day, with 100% shade to southern elevation all day. Morning solar exposure to the eastern elevation and afternoon solar exposure to the western elevation.
Spring and Autumn Equinox: Both northern and southern elevations in full (90% or above) shade all day, including ground shade to the southern side of the house. Morning solar exposure to the eastern elevation, and afternoon solar exposure to the western elevation with both eastern and western elevations in full shade at midday.

 
4. An East-West oriented house (long axis running from east to west) with a 100mm eave, is provided:
Summer Solstice: 100% shade to northern elevations all day. Varying shade to Southern Elevation throughout the day, peaking at 80% shade at mid-day. Significant morning solar exposure to the eastern elevation and extreme afternoon solar exposure to the western elevation from early afternoon (15% shade at 3pm).
Winter Solstice: Little to no shade (max 5%) to northern elevation all day, 100% shade to southern elevation, including ground shade all day. Morning solar exposure to the eastern elevation and afternoon solar exposure to the western elevation.
Spring and Autumn Equinox: Southern elevations in full shade all day, including ground shade to the southern side of the house. Northern elevation shaded no more than 15% until late evening. Significant morning solar exposure to the eastern elevation lasting until early afternoon, and afternoon solar exposure to the western elevation from mid-day onwards.

 
Conclusion:
While a 100mm eave will provide similar shade to an 800mm eave at midday on December 21st (summer solstice), the additional shade provided by an 800mm eave in the hours prior to and following mid-day is significant: 10% increase in shade to eastern walls at 9am and 35% increase in shade to western walls at 3pm.
An 800mm eave also provides superior solar protection during the spring, autumn and winter months, when the sun is much lower in the sky. Most significantly, a 75% increase in solar protection to northern walls in March, when the temperatures are often the hottest (up to 450C)and the sun is much lower in the sky.
An 800mm eave also provides significantly more ground shade year round, preventing heat build up and re-radiation from external thermal mass surfaces.