# SHGC: The Glass Number That Decides Your Cooling Load

> Solar Heat Gain Coefficient is the fraction of solar radiation striking a glazed unit that ends up as heat inside the building — an SHGC of 0.25 means a quarter of the sun's energy gets through. In a hot climate it is the single most important number on a glass specification.

## Why it matters on a facade project

SHGC is where the facade decision and the MEP decision meet. Glass is chosen by
the architect, usually on appearance; the cooling load it creates is then
inherited by the services engineer, who has to size chillers around it.

Because the facade is normally ordered before the HVAC is finalised, a poor
glass choice is locked in before anyone sizes the plant — and then gets paid for
twice: once in larger equipment, and again in electricity for the next 25 years.

## How it is specified in practice

| Parameter | What it measures | Typical commercial range |
| --- | --- | --- |
| SHGC | Solar radiation transmitted as heat | 0.25 – 0.40 |
| U-value | Conductive heat flow (W/m²K) | 1.8 – 3.3 |
| VLT | Visible light transmitted | 0.40 – 0.70 |
| LSG ratio | VLT ÷ SHGC — daylight per unit of heat | Higher is better; above 1.25 is good |

ECBC 2017 sets maximum SHGC and U-value for the building envelope, varying by
climate zone and by how much of the wall is glazed. A building in composite
Delhi and one in hot-dry Jaipur are held to different envelope targets, which is
why glass cannot be specified nationally and then reused everywhere.

## Common mistakes

- **Choosing glass on a sample board.** Appearance and reflectivity are what get
  reviewed; SHGC, U-value and VLT are what determine running cost. Both belong in
  the same conversation.
- **Increasing the glazed area without adjusting the glass.** ECBC tightens the
  envelope requirement as the window-to-wall ratio rises. A fully glazed
  elevation needs better glass than a punched-window one to reach the same
  target.
- **Optimising SHGC and ignoring VLT.** Very low SHGC glass can leave floors
  needing artificial light at midday, adding electrical load and harming the
  space.
- **Forgetting the spandrel.** Spandrel areas are opaque assembly, not glazing,
  and insulating behind them is often the cheapest way to recover an envelope
  shortfall.
- **Treating the whole facade as one specification.** North and south elevations
  see very different solar exposure. Zoning the glass specification is often
  cheaper than upgrading every panel.

## Standards referenced

Envelope performance targets, including maximum SHGC and U-value by climate
zone, are set by **ECBC 2017**. Sustainability approach sits in **NBC 2016, Part
11**, and architectural safety glass requirements in **IS 2553 (Part 1)**.

## Frequently asked questions

### What is the difference between SHGC and U-value?

SHGC measures solar radiation coming through the glass — sunlight becoming heat. U-value measures conductive heat flow driven by the temperature difference across the glass. In most of India, cooling dominates and solar gain far outweighs conduction, so SHGC is the number that matters most. In cold-climate projects in Himachal or Kashmir, that priority flips towards U-value.

### Can I just specify the lowest SHGC available?

Not without checking daylight. Pushing SHGC down usually pulls visible light transmission down with it, and a very dark facade forces artificial lighting on all day — which adds back electrical load and hurts the occupant experience. The measure worth optimising is the ratio of light transmission to solar gain, not SHGC alone.

### How much does SHGC actually change the HVAC size?

On a glass-heavy commercial floor it is substantial. Solar gain through glazing is frequently the largest single component of the cooling load, so halving SHGC can remove a meaningful share of the peak load — often enough to drop a chiller size. That capital saving usually exceeds the extra cost of better glass.

## Sources

- [ECBC 2017](https://beeindia.gov.in/) — Building envelope — glazing
- [NBC 2016, Part 11](https://www.bis.gov.in/standards/national-building-code/?lang=en) — Approach to sustainability
- [IS 2553 (Part 1)](https://standards.bis.gov.in/website/published-standards/department-wise) — Safety glass — architectural

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Source: https://hagerstone.com/glossary/shgc