External vs Internal Shading: Which Is Right for Your Project?

The Core Principle

Solar heat gain — the energy that warms a room when sunlight passes through glass — is easier to control before it enters the building than after. External shading intercepts solar radiation on the outside face of the glazing; internal shading intercepts it on the inside. The difference in performance is substantial.

A south-facing room with high-performance triple glazing and no shading will still overheat on a summer day. The same room with a well-specified external roller screen or louvre will remain comfortable without mechanical cooling. This is why Part O of the Building Regulations now places external shading at the centre of overheating compliance strategy.

Performance Comparison

Factor	External Shading	Internal Shading
Solar heat gain reduction	70–95% (g-value 0.05–0.15)	30–60% (g-value 0.20–0.40)
Part O compliance contribution	High — primary mitigation strategy	Limited — supporting measure only
CIBSE TM59 modelling	Explicitly modelled, high weighting	Lower weighting, often insufficient alone
Effect on daylighting	Preserves natural light with open-weave products	Can reduce natural light significantly
Glare reduction	Excellent	Good to excellent
View preservation	Good (perforated screens, adjustable louvres)	Variable
Blackout capability	Limited (few external blackout products)	Excellent
Maintenance	Higher — exposed to weather	Lower — protected environment
Cost	Higher	Lower to moderate
Listed building suitability	Often restricted by planning	Generally permitted
Retrofit complexity	Higher (structural fixing, power)	Lower
Motorisation	Standard (wind/sun sensors required)	Optional

When External Shading Is the Right Choice

External shading should be specified when:

• Part O compliance is required — for new-build residential and major refurbishments, external shading is the most reliable route to demonstrating overheating compliance via TM59.
• The building has large areas of south, east or west glazing — particularly with limited thermal mass.
• Year-round comfort without air conditioning is the goal — external shading combined with good ventilation strategy can eliminate the need for mechanical cooling entirely.
• The client occupies or manages the building — and will benefit from automatic operation responding to conditions.
• The project is contemporary new-build or major refurbishment — where external shading can be incorporated into the architectural expression.
• Commercial BREEAM credits are being sought — daylighting and energy use credits both benefit from external shading.

When Internal Shading Is the Right Choice

Internal shading should be specified when:

• The building is listed or in a sensitive conservation area — where external alterations to the façade are restricted or prohibited.
• Blackout is the primary requirement — bedrooms, home cinemas, AV rooms, hospital wards. External shading cannot achieve full blackout.
• Retrofit with minimal disruption is needed — internal blinds and shutters can usually be installed without scaffold or structural work.
• The budget does not support external systems — high-performance internal fabrics with reflective coatings deliver meaningful improvement at lower cost.
• Aesthetic control over interior finish is paramount — curtains, Roman blinds and plantation shutters contribute to interior design in ways that external screens do not.

Hybrid Strategies

The best-performing buildings often use both. A common residential specification: external roller screens on glazed extensions and south elevations for solar gain control, combined with internal blackout blinds in bedrooms and motorised curtains in living spaces for privacy, light quality and thermal performance at night.

For hotels and schools, external shading on facade glazing paired with internal roller blinds on individual rooms is the standard approach — the external layer handles solar load; the internal layer handles privacy, blackout and noise.

The Listed Building Case

Many of our most interesting projects involve listed buildings or conservation area properties where external shading is planning-restricted. In these cases, we specify the best available internal solutions — typically high-reflectance roller fabrics with the lowest possible openness factor, combined with improved glazing if a refurbishment budget allows. We also work with conservation officers on sensitive external solutions — slimline aluminium louvres that respect historic façades have been approved on several projects.