Understanding Solar Heat Gain

What Is Solar Heat Gain?

Solar heat gain is the increase in temperature inside a space caused by solar radiation passing through glazing. On a clear summer day, an unshaded south-facing window can transmit more than 600W of heat per square metre. That heat accumulates in floors, walls and furnishings, raising indoor temperatures well beyond comfort long after the sun has moved on.

For homeowners, the consequences are overheating, broken sleep and high air-conditioning bills. For architects and developers, solar heat gain is now a compliance issue.

The g-Value: Your Key Number

Every glazing system and shading product is characterised by its g-value (Solar Factor). It measures what proportion of incident solar energy is transmitted into the building. A g-value of 1.0 means 100% of solar energy passes through; 0.0 means none does.

Product / Glazing	Typical g-Value	Heat Reduction vs Single Glazing
Single glazing	0.85	Baseline
Standard double glazing	0.60	~29%
Low-e double glazing	0.40	~53%
Interior roller blind (openweave)	0.25–0.35	~59–71%
External louvre screens	0.05–0.15	~82–94%
External roller screens (5% openness)	0.08–0.12	~86–91%
MicroLouvre® panels	0.04–0.08	~91–95%

The critical insight: internal blinds intercept solar radiation after it has already entered the building. External shading blocks it before it crosses the threshold.

Why External Shading Outperforms Internal

When solar radiation hits glass, the energy splits between transmission, reflection and absorption. With internal blinds, absorbed heat re-radiates inward. External shading intercepts radiation before it reaches the glass, so even the absorbed component dissipates into the outside air.

CIBSE TM59 — the dynamic thermal simulation standard used for Part O assessments — explicitly distinguishes between internal and external shading. Projects that rely on internal blinds alone frequently fail the operative temperature criterion. External shading consistently achieves compliance.

Orientation and Shading Strategy

Solar heat gain varies significantly by orientation:

• South-facing glazing receives the highest peak intensity but is the easiest to shade with fixed horizontal devices, because the summer sun is high.
• East and west-facing glazing receives intense low-angle sun — better served by vertical louvres or adjustable external blinds.
• North-facing glazing receives minimal direct sun but can still contribute to diffuse heat gain.
• Rooflights and skylights are the most challenging and require specialist products.

Shading Products Compared

External Roller Screens

Fabric roller screens that retract into a box cassette above the window. They offer excellent g-values (typically 0.08–0.15), operate automatically, and allow transparency for views. Available in fabrics from 1% openness to 10%.

Vertical and Horizontal Louvres

Aluminium or glass-fibre louvres that rotate to admit diffuse light while blocking direct sun. When angled correctly, they achieve g-values of 0.05–0.12 while preserving outward views.

Motorised Awnings

Fabric awnings on a roll-out arm, ideal for domestic terraces, hospitality venues and glazed extensions. Modern awnings with wind and sun sensors retract automatically in adverse conditions.

MicroLouvre® Panels

Precision-engineered micro-louvre cells within an aluminium panel system. The geometry is calibrated to block direct solar radiation while preserving views. G-values of 0.04–0.08 make them among the highest-performing products available.