Building Fabric 03 – Sunlight and Solar Gain

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Benefits of natural lighting in our buildings

 

Natural light in our buildings is known to improve occupant comfort. Scientists suggest that good natural light can increase occupant productivity and comfort, providing the mental and visual stimulation necessary to regulate the human circadian rhythms. Implementing natural light into a design also leads to substantial energy savings, by way of reducing the electricity requirements for artificial lighting. It is possible to install daylight sensing controls to a lighting design to further reduce energy usage by between 20-60%. 

 

As long as glare is controlled and solar gain accounted for designed appropriately there is no question that natural lighting will improve occupant satisfaction, mood and productivity. Studies suggest that humans in modern cities now spend upwards of 90% of their lives indoors – which makes it more important than ever to ensure we are allowing users to experience the natural outdoor cycles and variation of illuminance levels from within the buildings. 

 

Sunlight

[All data and information is based on a location of latitude 51˚ London.]

 

Sunlight is our main source for the creation of daylight. There is much data, diagrams and mapping to enable us to design a detailed strategy to maximise on the natural benefits of the sun’s rays. 

 

Understanding the path of the sun throughout both the day and the year will help in the design of both daylighting and factoring in solar gain, solar shading and glare. 

 

Sun path diagrams provide us with a visual way of understanding the impact the sun has on our site and our proposed building. Using both traditional sun path diagrams and 3D CAD software we are able to learn a lot about our potential site. Considerations like overshadowing for urban sites, along with potential glare from reflections of other buildings facades are among the factors we can examine. 

Sun position and path

 

The angle of the earths axis is 23.5˚ – which causes the exposure to the sun to vary as the earth rotates around the sun. Therefore the sun appears at different angles and positions throughout the year which can be described by two angles, the azimuth and the elevation angle. 

 

The azimuth is the projection on a plan of the position of the sun measured from north 0˚. 

 

The elevation angle is the projection of the position on the elevation of the sun above the horizon. 

 

Sun path diagrams help us to pin point the position of the sun throughout the year at any given time.

When using these diagrams in design, we can simplify the diagram to extract the key data.

Spring / autumn equinox 21 March and 23 September

Summer Solstice 21 June

Winter Solstice 21 December

Using the simplified sun path diagram we can apply it to a design scheme. The data can be adapted for a whole building design, or to measure the exposure of a particular opening. 

 

To maximise the daylighting of a building design, the building must be in correct alignment with the sun. Sunshine entering the rooms in the autumn and winter, and in the morning is preferable. Sunshine at midday and in the afternoon between June and August is not desirable. Various strategies such as alignment of the building, form of window reveals, glazing bars, shading devices and high level windows all provide control over the penetration of the sun into the building. 

The following diagrams represent the sun path for the summer and winter solstices, along with the autumn and spring equinoxes. The diagrams are used to demonstrate the orientation of a proposed building. 

A North-South building receives sunshine on both sides but minimal windows on the north and south mean that minimal sun will penetrate the building in November, December and January. 

 

An East-West building is a suitable orientation for small flats. The living rooms and bedrooms can be positioned on the south, with stairs, bathroom, lobby and utility areas to the north. 

A North-West to South East block suits larger flats with bedrooms and utility to the north east and living rooms to the south west. 

 

A North-East to South-West building is well suited to 3 and 4 roomed flats with the living and bedrooms to the south-east and utility and subsidiary rooms to the north-west. 

It is worth noting that light from above is brighter and as such the inclusion of roof lights are often preferable too. 100% of light from the sky will reach a roof light, whereas only 33% of the light from the sky will reach the windows at the side of a building. This can be controlled with angled roof lights, shading, up stands and so on. 

Solar Gain

Solar gain occurs when the temperature of an internal space within a building increases due to the absorption of the suns energy by the building fabric. Generally speaking, we design to minimise solar gains, particularly during the summer months. However, during the colder months of the year we often look to exploit the solar gain to reduce heating loads. 

 

The middle of the day is when the sun is at it’s strongest, but it is also at its highest point, so often in the summer, the sun hits a facade at a steep angle in the middle of the day and therefore the heat transfer remains relatively low. It is the time either side of the peak of the day where the angle of the sun is lower, a more direct angle for impacting the facade and transferring a higher proportion of energy to the building. An exception to this is atria and roofs which face the sun more directly at the middle of the day and are especially vulnerable to solar gain when the sun is at its strongest. 

 

To help control solar gain, shading systems are often applied to limit the amount of solar radiation that penetrates the facade. External shading devices are more effective than internal shading. Another strategy is the use of solar control glazing that restricts the amount of solar transmittance through the window. 

 

Solar Shading

 

External shading can reduce the amount of energy required to cool a building during summer months. Internal shading is much less effective and not recommended since the solar energy has already passed through the facade and radiated back into the room. 

 

Generally, in the northern hemisphere is is suggested that horizontal shading devices are best suited to south facing facades which help with the high angle sun, while vertical shading systems are best suited to east and west facades as they are able to deal with the lower angle sun. 

 

When designing shading systems, it is important to find a balance between providing the appropriate shading but maintaining good natural lighting levels within the building. 

solar shadingIt is worth mentioning that natural sun protection is also an option in low level buildings in the form of deciduous trees. The leafy tree will offer natural sun protection in summer, but in winter will allow the sun to pass through the tree letting warmth and light into the room. 

 

Glare

 

Glare can be described as the discomfort or impairment of vision caused by excessive range of brightness in the visual field. Causes of glare can range from bright skies seen through windows, direct views of bright lights, direct and indirect reflection from surfaces and unfavourable contrast in light density. Glare generally causes discomfort which over time will cause annoyance and likely affect efficiency. 

 

Some measures to take to avoid glare include:

  • external shading devices
  • glare protection inside and outside
  • matt surfaces to avoid reflection
  • correct positioning of daylight enhancing lighting

Useful resources and further reading:

 

Neufert – Architects Data 

 

Modern Construction Handbook – Andrew Watts

 

Environmental Science in Building – Randall McMullan

 

Daylighting Pattern Guide by New Buildings Institute – http://patternguide.advancedbuildings.net/

 

Sun Earth Tools [for sun paths and data] https://www.sunearthtools.com/dp/tools/pos_sun.php

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