The solar factor Sw expresses the share of incident solar energy transmitted indoors by the complete window, frame included. It gates access to energy saving certificates: scheme sheet BAR-EN-104 requires, other than for secondary glazing, either Uw ≤ 1.3 W/m².K with Sw ≥ 0.3, or Uw ≤ 1.7 W/m².K with Sw ≥ 0.36. A roof window inverts the constraint, with Uw ≤ 1.5 W/m².K and Sw at most 0.36. One thing not to conflate: Sw is assessed to NF P 50-777 and covers the window, whereas the g of EN 410 covers the glazing alone.
Understanding the solar factor (Sw) on glazing, without getting it wrong
Sw, g, LT: the indicators not to confuse on a technical data sheet
On a technical data sheet, these three indicators do not describe the same object, and each is routinely read for the other.
| Indicator | What it characterises | Assessment standard | Required in a grant file |
|---|---|---|---|
| g | The glazing alone | EN 410 | No |
| Sw | The complete window, frame and sash included, mechanically lower since the frame transmits nothing | NF P 50-777 | Yes, and it alone |
| LT | Light transmission, which does not track the Sw: glazing can be very clear and still limit the sun | not specified | No |
What Sw actually measures: direct transmission and re-emitted energy
The Sw adds together the direct transmission of radiation and the "secondary" heat created when the glazing absorbs sunlight and then radiates part of it back towards the interior. It's an indicator of solar gains. It helps anticipate summer overheating, or conversely free gains in winter.
Why Sw varies with the glazing type and treatment (low emissivity, solar control)
Standard double glazing often has a higher Sw than solar-control glazing. A low-emissivity coating improves insulation but doesn't necessarily lower the Sw. Solar-control treatments, tinting, argon fill and triple glazing all cause the Sw to vary by reflecting, absorbing or re-emitting energy differently. Check the value according to the EN 410 standard.
Choosing glazing based on orientation and room use: aiming for the right solar gains
South-facing façade: making the most of winter solar gains without creating overheating
To the south, the idea is simple. Glazing with a fairly high solar factor helps recover winter gains, especially in the living room. For summer, plan for exterior shading (shutters, sunshades, blinds) and effective airing. This is often more useful than "over-insulating" the glass.
East and west: limiting overheating from low sun (morning/late afternoon)
To the east and west, the sun is low and heats up quickly. Solar-control glazing and exterior shading make the difference, especially for bedrooms and home offices. Avoid large unprotected glazed areas. You gain comfort without darkening the room all day.
North: prioritising insulation and comfort, since solar gains are limited
To the north, gains are limited. Priority goes to glazing with a low Uw and good airtightness. Objective: reduce the cold-surface effect and the risk of condensation. This is the façade where installation quality matters as much as the window's performance. To compare solutions, you can rely on the choice between single, double or triple glazing.
Balancing solar gains and summer comfort: the Sw and glazing thermal-performance duo
Sw and Uw: weighing solar gains against heat loss, case by case
The Sw indicates the share of solar energy that passes through the glazing. The higher it is, the more free gains you recover. The Uw measures heat loss. The lower it is, the better the window insulates. The right choice depends on orientation, local climate, glazed surface area and use.
Overheating: when a too-high Sw hurts comfort, even with a good Uw
A very high-performance Uw doesn't block the sun. With a large south- or west-facing bay window, a high Sw can create overheating, especially in a home with little shade or ventilation. Think about summer comfort at the design stage, not after the first summer.
Solar shading: blinds, shutters, sunshades... and their effect on solar gains
Exterior shading is the most effective, because it stops radiation before it reaches the glazing. Blinds, shutters and adjustable sunshades let you modulate gains according to the time of day. An adjustable solution keeps the light while avoiding turning the room into a greenhouse. For a more "passive" approach, passive solar shading can also limit overheating while preserving useful winter gains.
Real cases on site: glazing adjustments that make the difference
Older house: replacing standard double glazing with glazing better suited to the Sw
In a 1930s house, south-facing rooms were overheating despite recent double glazing. The diagnosis pointed to a solar factor that was too high. By switching to more selective glazing (lower g or Sw), without degrading insulation, gains are limited in summer while keeping good light.
Comprehensive renovation: consistency between glazing, insulation and ventilation to manage solar gains
In a comprehensive renovation, very high-performance glazing can trap heat if the envelope is heavily insulated. The right reflex is to align the choice of glazing with shading and well-adjusted mechanical ventilation. Exterior shading often saves more than interior blinds.
Bay windows and conservatories: watch-points to avoid the greenhouse effect
On bay windows and conservatories, the risk is the greenhouse effect. Favour solar-control glazing, openable sashes to create airflow, and possible night-time ventilation in summer. Also check thermal bridges at the base of the frame and the seals.
Grants and 2026 requirements: what to check before proposing a glazing solution
MaPrimeRénov' and CEE: how glazing performance is generally taken into account
Scheme sheet BAR-EN-104 covers replacing a window, roof window or French door that was single-glazed before the works, and fitting secondary glazing over existing single glazing. The Uw and the Sw are required as a pair, never separately.
| Configuration | Uw required | Sw required |
|---|---|---|
| Window or French door, option 1 | ≤ 1.3 W/m².K | ≥ 0.3 |
| Window or French door, option 2 | ≤ 1.7 W/m².K | ≥ 0.36 |
| Roof window | ≤ 1.5 W/m².K | ≤ 0.36 |
| Secondary glazing over an existing opening | ≤ 1.8 W/m².K | ≥ 0.32 |
The roof window is the only case where the Sw is capped rather than floored: in a roof plane the summer sun arrives close to perpendicular, and the overheating risk outweighs the winter gain. The sheet applies to operations committed before 1 July 2028.
RGE and documents to prepare: technical data sheet, marking, proof of performance (Uw/Sw as applicable)
The certification and the proof of completion are checked before assembling the file sent to the CEE scheme provider with its supporting documents.
- The certification categories for vertical glazed elements and for glazed elements in a roof are two distinct scopes, and fitting roof windows under the vertical scope alone does not pass.
- The number of windows and their area on the proof of completion, an area that includes the whole set of profiles, frames and sashes, seals and sealants included.
- The Uw and Sw of the units installed, the same values the file carries.
Energy audit and DPE: how the solar factor influences summer comfort and the work plan
In an energy audit or a DPE, the choice of glazing doesn't only affect heat loss. A too-high Sw can increase overheating. A suitable Sw, combined with solar shading, improves summer comfort and avoids degrading the work plan.
