External shading beats internal blinds for stoppin... | Blog

Updated: Jun 17, 2026

If you've ever pulled the curtains or dropped the blinds on a hot afternoon and found the room still warmed up, you've experienced the central fact of solar shading: where you put the shade matters more than almost anything else. Shading on the inside of the glass helps a bit. Shading on the outside helps a lot. This article explains why, what the numbers look like, and where each approach still earns its place.

The physics, in one paragraph

Sunlight is short-wave radiation, and it passes straight through glass. When it lands on something inside the room - a floor, a sofa, an internal blind - that surface absorbs the energy and re-radiates it as long-wave heat. Glass is far less transparent to long-wave heat than to sunlight, so that re-radiated heat gets trapped inside. This is the greenhouse effect, and it's why a closed car bakes in the sun. The consequence for shading is simple: if your blind is inside the glass, the sunlight has already crossed the glass and become trapped heat before the blind does anything. An external shade stops the energy at the façade, before it ever enters.

What the measurements show

An internal roller blind lowered over a living room window

This isn't just theory. An independent study supported by the British Blind and Shutter Association (BBSA) monitored real rooms and recorded operative temperatures with no shading, internal shading, and external shading:

No shading: 47.5°C
Internal roller blind: 32°C
External venetian blind: 28°C

Across the study, rooms with internal blinds reduced operative temperatures by 8-13°C, while rooms with external blinds reduced them by 12-18°C. Both work - internal shading is far from useless - but external shading delivered a markedly larger reduction, bringing the room close to the 26°C comfort threshold that internal shading alone couldn't reach.

Wider industry figures point the same way. External shading systems can block in the region of 90% of solar gain at the glass. For comparison, even triple glazing - a serious, expensive fabric upgrade - typically cuts solar gain by only around 50%, and guidance for highly glazed spaces consistently identifies external and mid-pane measures as the most effective summer solar control.

The Low-e glazing trap

Here's a misconception worth spending money to avoid. Many people assume that modern energy-efficient (low-emissivity, or "Low-e") glazing will prevent overheating. It generally won't - and can make it slightly worse.

Low-e coatings are designed to keep heat in. That's exactly what you want in winter: warmth generated inside the home is reflected back rather than escaping through the glass. But that mechanism doesn't reverse in summer. When solar gain has heated your room, Low-e glazing helps trap that heat indoors just as it traps your central heating in January. It is a winter efficiency feature, not a summer cooling one. If a salesperson tells you new glazing alone will solve a hot south-facing room, treat the claim with caution.

So why does anyone use internal shading?

A light-coloured internal blind shading a living room window

Because external shading isn't always practical, and internal shading has real advantages of its own. The honest answer is that the best shading is the one you can actually install and will actually use.

A few situations where internal shading makes sense:

Outward-opening windows. Most UK windows open outwards, which physically gets in the way of external blinds or shutters on many openings. This is one of the most common reasons external shading isn't viable on a given window.

Appearance, planning and conservation. External shading changes the look of a façade. On listed buildings, in conservation areas, or where you simply don't want hardware on the outside of the house, internal options keep things discreet.

Preserving the view and the décor. Internal blinds let you fine-tune light and privacy from inside, and they protect furnishings from fading. For mid-terrace or shaded streets that don't get punishing direct sun, internal shading is often perfectly adequate.

Cost and retrofit simplicity. Internal blinds are cheaper and easier to fit than external systems.

If you're limited to internal shading, choose wisely: blinds with a reflective or solar-control coating bounce significantly more heat back out through the glass than standard fabrics, and a light, reflective outer-facing surface outperforms a dark one. It won't match external shading, but it closes the gap.

Where awnings fit

For the windows and doors that catch the most sun - typically large south and west-facing patio doors and living-room glazing - an awning is one of the most practical forms of external shading for a UK home. It sits outside the glass, so it intercepts solar gain before it enters; it retracts when you don't want it, so you keep your winter light and solar warmth; and unlike fixed external blinds it doesn't clash with outward-opening doors. A retractable awning over a patio door does double duty: it shades the room behind it and creates usable shaded space outside.

External shutters and external blinds are the other strong external options where the window arrangement suits them. The right choice depends on the opening, the orientation, and how the room is used - which is exactly the kind of thing worth getting surveyed rather than guessed.

The takeaway

If you can shade the outside of the glass, do - it's the most effective single move against summer overheating, and the measured difference over internal shading is large. Where external shading isn't practical, internal shading with a reflective coating still helps meaningfully. And don't rely on energy-efficient glazing to do a shading job it was never designed for.

In the final article in this series, we put all of this into a practical daily routine for keeping a home cool through a heatwave - without reaching for air conditioning.

GDCG supplies and installs awnings, external shading and patio systems across Lincolnshire, Nottinghamshire, and Leicestershire. If you're weighing up how best to shade a hot room, we can assess your windows and orientation and tell you honestly what will work. Get in touch for a free survey and quote.