Do I need external shading, or will internal blinds do?

What this usually means

When sunlight strikes a window, the question is where the heat ends up. With external shading — a shutter, external blind, brise-soleil, overhang or even a tree — most of the sun's energy is reflected or absorbed and re-radiated outside, before it ever reaches the glass, so very little enters the room. With an internal blind, the sun passes through the glass first and is absorbed by the blind inside; that energy is now within the room and largely stays there, re-radiated and convected into the space. The glass effectively traps it, which is the familiar greenhouse effect.

This is why internal blinds, though useful, are weak against overheating: by the time they act, the heat is already inside. They do reduce glare, give privacy, and provide a modest thermal benefit (especially reflective or well-sealed types), so they are not useless — but they cannot prevent a sun-facing room from heating up the way external shading can. People often close internal blinds in a heatwave and are disappointed that the room still bakes, precisely because the heat has already crossed the glass.

External shading should be matched to the window's orientation, because the sun's position differs. High summer sun on south-facing glazing is well controlled by horizontal devices — overhangs, brise-soleil, awnings — that block the steep midday sun while still allowing lower winter sun in. Low morning and especially evening sun on east and west windows comes in at a shallow angle and needs vertical or full-cover shading, such as shutters or external blinds, to be stopped. Where external shading is impractical, the next best options are solar-control glazing or, as a fallback, the most reflective internal blinds — but for genuinely preventing overheating, external shading on the sunniest windows is the measure that delivers.

Common causes

Signs and symptoms

A window admits solar energy as short-wave radiation; once that radiation is absorbed by surfaces or blinds inside, it is re-emitted as long-wave radiation, to which glass is largely opaque, so the heat is trapped indoors — the greenhouse effect. The decisive factor for overheating is therefore where the absorption happens. External shading absorbs and reflects the radiation outside the glass, so the trapped fraction never forms; an internal blind absorbs it inside, where the glass prevents most of the resulting heat from escaping, so the room still warms. This asymmetry is why external shading vastly outperforms internal blinds.

The metric that captures this is the total solar energy transmittance of the window-and-shading system (its g-value or solar heat gain coefficient). External shading can cut the effective solar transmittance dramatically, often by 70–80% or more, because it operates before the glass; internal blinds achieve far smaller reductions because they operate after it, with the exact figure depending on how reflective and well-sealed the blind is. Solar-control glazing reduces transmittance within the glass itself and sits between the two in effectiveness while needing no external device.

Geometry then sets the appropriate device. Summer sun is high in the south, so a horizontal overhang or brise-soleil blocks the midday sun on south glazing while admitting the lower, welcome winter sun. East and especially west sun is low and intense, striking glazing nearly head-on at the start and end of the day, so only vertical or full-cover external shading (shutters, external blinds, awnings) reliably blocks it. Matching the shading type to orientation, and prioritising the windows with the greatest gain, gives the most effective passive control of overheating; an assessment of the solar exposure shows where external shading is essential and where lesser measures suffice.

How to choose shading that actually stops overheating

Prioritise external shading on the sunniest windows, matched to their orientation; use internal blinds for glare and privacy and solar-control glazing where external shading is impractical.

1. 01

   Identify the sunniest windows

   Find the windows gaining the most sun, usually south, east and west facing.

2. 02

   Shade them externally

   Fit external shutters, blinds, awnings or overhangs to stop the sun before the glass.

3. 03

   Match the device to orientation

   Use horizontal shading for high south sun and vertical/full shading for low east and west sun.

4. 04

   Use solar-control glazing where needed

   Where external shading is impractical, specify solar-control glass to reduce transmittance.

5. 05

   Keep internal blinds for glare

   Retain internal blinds for glare and privacy, recognising their limited thermal benefit.

6. 06

   Combine with night ventilation

   Pair shading with night-purge ventilation so any heat that does enter is removed.

How to prevent it coming back

• Shade sun-facing windows externally, prioritising the worst-affected.
• Match shading type to each window's orientation.
• Use solar-control glazing where external shading is not possible.
• Don't rely on internal blinds to prevent overheating.