The SHGC is the fraction of incident solar radiation admitted through a skylight, both directly transmitted and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a skylight's solar heat gain coefficient, the less solar heat it transmits.
However, unfortunately the SHGC is directly proportional with visible light transmittance (Tvis%) or full light transmittance , The lower a SHGC, the less light transmittance it transmit Tvis%. Therefore, in hot climate countries like GCC/Middle East, what is more important than the other, the saving in HVAC produced from lowering heat gain or losing the brilliant visible light transmittance (specifcallyclearer sky view)?
To tackle this question efficiently we must think of two questions and their answers as a start, then after that we will think how to resolve an optimized solutions.
1- Why we need plenty of visible daylight transmittance even in hot climate countries? The answer is simple,
a- Higher duration of daylight transmittance during the days of year simply reduces (dramatically) the need for electrical lighting, approximate 0.23 USD/ft2 /year saving were found in California retail buildings, ref . A 0.16 USD/ft2/year in educational buildings ref . California climate is not exactly as Middle East one, however this is only just to demonstrate the case as it is applicability is present in Middle East too.
b- Human finds comfort in full daylight transmittance, in California retail sales workers 6% more efficiency was correlated to full day light ref. 21% higher education scores was also correlated to full day light in schools ref .
2- The other question is why we need low SHGC in hot climate countries? The answer is simple too,
a- Lower SHGC significantly helps in reducing HVAC load needed to run the space at acceptable comfort levels of ambient temperature. However, this is only true in summer (approximate 6 months in a year in Dubai / UAE as an example)
b- As a product of lowering SGHC you achieve glare reduction which is in itself annoying to building users. However, this can be controlled with light diffusing (Haze) which is now a day’s property of all standard glazing materials including ETFE (at least in commercial buildings). Therefore, we can safely assume that lowering HVAC load and cost on investors and environment is the major reason.
After knowing the why questions, lets start with how can we optimize between the two? The target is to maintain as much as possible of Tvis% and at the same time make sure that heat gain does not exceed the threshold of users comfort. The easier way to approach this is to take United Arab Emirates (UAE) as example.
1- In UAE there are 6 months of high temperature, or high sun altitude which form the major concern, the other 6 months are not hot or the sun altitude is not high, therefore they are not of a concern in this article. See below figure-01 and Figure-02. It is interesting to look at the flip side of winter in other cold countries (low winter sun altitude) you find that UAE must be luckier to get all full day light during the full year especially in winter.
Figure-01 Typical Sun path at winter and at summer ref
Figure-02 Sun angles on a flat horizontal skylight ref
2-Investigation of climate and project location in relation to all year sun altitude and the south direction (Azimuth)-refer to figure-02- is very important to calculate how much Tvis% and heat you will gain. Before, you decide that SHGC and Tvis% going to be following the manufacturer data sheet based on 0 degrees incident angle of energy try to think how far is this from reality that sun never behave in such way and our planet is tilted around its N-S axis, it will always come with altitude angle depends on project geographic location on earth and that makes incident angle always greater than 0. The point here is that you might be able to allow high visibility in skylight material and keep lower SHGC by manipulating the orientation of skylight with south, the shape of skylight(flat or non-flat, non flat such as domed , due pitch shape, arched, etc) and taking into consideration any existing shade like buildings or natural obstruction nearby or adding some obstruction around the sky light plan to reflect some part of solar energy before skylight .
3- Investigation of the well height of skylight does impact the Tvis%, the well is the distance between bottom of skylight and the ceiling/plenum (-see figure-3 below) . The deeper the well the less Tvis%. Perhaps this won’t reduce total solar gain but it will contribute to less light transmission if intended. The point here is that you may design the building internal space design to form light well keeping habitat far enough from high light transmission if needed while keeping acceptable light transmittance and visibility in other parts. If this is not enough, you can always add frits(print) on outer layer of ETFE and reduce heat gain and subsequently the visibility or Tvis% to some specific parts of building to have more shade than others or in other words if it is only some part or parts which are directly impacted more than other part(s) then it is enough to design those parts only to have frits (less Tvis%) and you don’t need to reduce the entire skylight light transmission because of some areas which could be limited (usually the north part of skylight in northern hemisphere).
Figure-03 Skylight with well ref
The SHGC and Tvis% are strongly related to the geographic location of project, climate and local obstructions, besides that, the shape & orientation of skylight is of significant importance too. Before you sacrifice big amount of beautiful full daylight transmission or Tvis%, think wisely of different passive ways to control SHGC and keep the sky clear view above you. In commercial retail buildings saving in HVAC by lowering SHGC must be carefully assessed commercially with the benefits of day light transmission on retail workers efficiency and coolness of place( returning customers), this could alone counter act the saving if not proved higher impact. Not to mention finally that for UAE the 50% of year winter is when all events are happening and shoppers / tour are focusing. Consultants and Architects shall carefully specify SHGC and Tvis% to the purpose intended in terms of feasibility taking into consideration to elaborate all above factors of impact, the other way around may lead in some situations to a contradictory direction of results.
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 Visible light transmittance of skylights by Jon McHugh, Rocelyn Dee & Mudit Saxena Heschong Mahone Group March 2004