Building Architecture – Sun Path Analysis Part :02

The sunpath analysis helps us to analyze the critical area with high solar radiation and also helps to arrive at the passive design strategy such as shading device to reduce the solar ingress.

The Shading device depends on the orientation and the solar altitude. Horizontal shading device is very useful when the solar altitude is very high. Indian climate is exposed to overhead sun. This solar incidence can be reduced by designing horizontal overhangs commonly known as “chajjas”. Incase of low solar altitude, ideally in the east and west directions where we face solar incidence during early sunrise and late evenings we need to plan for vertical fins to tackle low angle solar radiation.

The design of shading device reduces the requirement of solar heat gain coefficient (SHGC) for glass performance. ECBC – energy conservation building code by law for development of green building provides relaxation in the prescriptive requirements of SHGC.

The Solar factor or the Solar Heat gain coefficient of the prescriptive requirements can be traded off with the use of the overhangs and/or side fins. An adjusted SHGC, accounting to the overhangs and /or side fins is calculated by multiplying the SHGC of the unshaded fenestration product to a multiplying factor(M) factor.

Key Parameters:

M Factor: It is the relative reduction in the annual solar cooling load from overhangs and/or side fins with the given projection factor, relative to the horizontal & vertical dimension.

Projection factor, overhang: The ratio of the horizontal depth of the external shading projection divided by the sum of the fenestration and the distance from the top of the fenestration to the bottom of the farthest point of the external shading projection, in consistent units.

Projection factor, sidefin: The ration of the horizontal depth of the external shading projection divided by the distance from the window jamb to the farthest point of the external.

Shading projection, in consistent units

Three columns represent Overhangs only, vertical fins only & overhang + vertical fins.

STEPS FOR CALCULATION

• Step 1: Determine project location (Latitude)
• Step 2: Determine window orientation
• Step 3: determine Projection factor by ratio horizontal depth to vertical height
• Step 4: Determine “M” factor.

For example

Eg: If a building in Delhi(North Façade) has

• Latitude - 26˚ N
• Shade depth (H) - 1000mm
• Vertical Height(V) - 2000mm
• Projection factor (H/V) - 0.50
• M factor - 0.80
• Effective SHGC - 0.31 (considering the requirement 0.25) (0.31x0.80=0.25)