Overheating & Solar Control

When solar radiation strikes a glazing unit, three things happen to it: some is reflected straight back, some is transmitted directly through, and some is absorbed by the glass and then partly re-radiated and convected inward and outward. The solar heat gain coefficient (SHGC) - called the g-value in Europe and India - is the single number that captures the net result: it is the fraction of the incident solar energy that ends up inside the space, counting both the directly transmitted part and the inward share of what the glass absorbed. It runs from 0 to 1.

The numbers are stark. Ordinary clear single glazing has an SHGC around 0.82 - it lets in over four-fifths of the sun's heat. A good solar-control low-e double glazing can sit near 0.25-0.30 - roughly a third as much heat for the same sunshine. That is the difference between a west room that is unusable at 4 pm and one that is merely warm, from a component that looks almost the same.

Two things it is not. SHGC is not the U-value: U-value governs heat conducted through the glass because of the air-temperature difference (the winter-night, summer-day steady flow), while SHGC governs the solar gain when the sun shines. A window has both, and in a hot, sunny climate SHGC usually matters more. And SHGC is not the same as how bright the glass looks - that is a separate property, visible light transmittance, which is the crux of the next section's trade-off. Get the SHGC right and you have controlled the largest, most concentrated summer load a facade carries.

Choosing glass is choosing an SHGC (and a U-value, and a light transmittance) to suit the climate and orientation - and simulation is how you test whether the choice actually holds comfort.

In a cooling-dominated climate - most of India - the instinct is a low SHGC to keep solar heat out, especially on east, west and unshaded south glass. Solar-control coatings and tints get you there; the best are spectrally selective low-e coatings that reject the near-infrared (heat) part of the spectrum while passing the visible (light) part, so you drop SHGC without collapsing daylight. In a heating-dominated climate the logic can invert on the equator-facing wall, where a higher SHGC lets in welcome winter solar gain - the passive-solar move - though you then lean on shading to handle summer.

India's codes put numbers on this. The residential Eco Niwas Samhita works through a Residual Envelope Transmittance Value (RETV) limit that bundles the envelope's conduction and solar gain, effectively pushing designers toward lower window SHGC and better shading in hot zones; the commercial ECBC sets maximum SHGC (and U-value) values by climate zone and window-to-wall ratio. These are compliance floors - a simulation lets you go beyond them and check real comfort. A worked instinct: halving a west facade's SHGC from 0.8 to 0.4 roughly halves its solar gain, often the difference between meeting a cooling target and missing it, and the model quantifies exactly how much.

Remember shading and glazing are partners: shade stops the direct beam, SHGC handles the diffuse and residual gain. Specify them together, not one instead of the other.

The tension that makes glazing selection interesting is that heat and light come in through the same window, and crude solar control sacrifices both. Visible light transmittance (VLT / Tvis) is the fraction of visible light the glass passes - high VLT means a bright, daylit room; low VLT means a dim one that needs electric light even at noon, quietly undoing the energy you saved on cooling.

The metric that captures the trade-off is the light-to-solar-gain ratio (LSG = VLT / SHGC). A high LSG means the glass is good at its real job: letting light in while keeping heat out. A simple dark tint or reflective coating lowers SHGC but lowers VLT just as much, so its LSG stays around 1 - it makes the room darker without making it much cooler in daylight terms. A spectrally selective low-e coating can reach an LSG of nearly 2 - a bright room with half the heat. Plotting glazing options as VLT against SHGC, the good products sit toward the top-left (much light, little heat), the crude tints toward the bottom.

This is where solar control and the daylighting of Module 5 meet. Push SHGC too low with the wrong glass and you win the cooling battle but lose daylight autonomy, drive up lighting energy, and get a gloomy interior - a false economy. The design goal is not minimum heat; it is the best balance: enough VLT to daylight the space and cut lighting, low enough SHGC to hold comfort, which points you at high-LSG spectrally selective glazing rather than the darkest glass on the shelf.

Solar control has a pass/fail test: does the space actually overheat? For an air-conditioned building, uncontrolled solar gain shows up as cooling energy and peak cooling load - the chiller works harder, the plant grows. But for a free-running or mixed-mode building - common and desirable in much of India - there is no chiller to hide the problem, so overheating shows directly as hours the space is too hot, and it needs its own metrics.

The standard approach counts exceedance: how many occupied hours the indoor operative temperature rises above a comfort threshold, or how far and how long it overshoots. The UK's CIBSE TM52 (non-domestic) and TM59 (homes) formalise this with criteria based on the adaptive comfort model - people in naturally ventilated buildings accept warmer indoors when it is warmer outside - flagging a space as overheating if it exceeds the adaptive limit for too many hours, by too large a margin, or on the worst day. Related measures include degree-hours above a set point and simple 'percent of occupied hours over 28 or 30 degrees C'. Whatever the flavour, the idea is the same: put a number on the discomfort so you can test a design against it.

Simulation is what produces these numbers. An energy model (EnergyPlus and its interfaces, from Module 4) runs the design against the weather file and reports indoor operative temperatures hour by hour, from which the overheating hours fall out. Change the glazing SHGC, the shading depth or the ventilation, re-run, and watch the overheating hours move - the comparative, option-versus-option reading this whole course argues for. In a warming climate, checking overheating against a future weather file is fast becoming good practice.

Pulling the module together, controlling solar gain in a hot climate follows a clear order of leverage - and glazing SHGC, though vital, is not the first move.

First, form and orientation. The cheapest solar gain to control is the one you never invite: minimise unshaded east and west glass, keep the largest openings on the shaded and cooler faces, and let the radiation map of Lesson 6.1 steer the massing. Second, shade the beam - external shading sized to the sun angles (Lesson 6.2) kills the direct component before it reaches the glass, and it is cheaper per unit of heat rejected than premium glazing. Third, choose the glazing SHGC to handle the diffuse and residual gain, reaching for high-LSG spectrally selective glass so daylight survives. Fourth, manage internal and residual gains - lights, equipment and people add heat too, and in a well-controlled envelope they can dominate; efficient lighting and appliances (and daylighting to switch lights off) matter. Fifth, remove what is left with ventilation and thermal mass (night purge, Module 8) before, finally, mechanical cooling.

The honest closing note is the one the module keeps making: simulation ranks these moves and tells you how much each buys, under a stated climate and assumptions, but it is decision-support, not a comfort guarantee or a compliance certificate. Statutory compliance (ECBC, Eco Niwas Samhita) is confirmed with the authority's own procedures, and the real building will drift from the model as occupants open blinds and prop doors. Use the metrics to compare options and get the order of moves right; that is where the leverage - and the comfortable, low-energy building - actually comes from.