The solar factor is the share of the sun's energy that passes through a glazed element and ends up as heat in the room, expressed between 0 (nothing gets in) and 1 (everything gets in). It comes in two forms: the g, for the glazing alone (EN 410), and the Sw, for the complete window, frame included (NF P 50-777). The Sw 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.
Solar factor: definition and how to read a technical data sheet
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 almost 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.
From the glazing's g to the window's Sw: the simplified calculation
To go from one value to the other without the manufacturer's sheet, an order of magnitude is enough: Sw ≈ g × (glazed area / opening area). The frame being opaque, its contribution is small, and the glazed share of a typical window is around 65 to 75 %. Glazing with g = 0.62 in a frame with 70 % daylight area thus gives an Sw close to 0.43. With identical glazing, wide profiles or a small window lower the Sw, as the frame takes a larger share. The normative calculation remains that of NF P 50-777, and the value declared by the manufacturer is what counts in a file.
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 lowers the solar factor only moderately. Solar-control treatments, tinting, argon fill and triple glazing all cause the Sw to vary by reflecting, absorbing or re-emitting energy differently.
Glazing solar factor: typical values by glazing type
The values below are orders of magnitude taken from common products, not guaranteed values: each make-up (glass thickness, gas, coating position, tint) has its own sheet. The Sw column applies the assumption of a 70 % glazed share (Sw ≈ 0.7 × g).
| Glazing type | Glazing g | Indicative window Sw | LT | Indicative Ug (W/m².K) |
|---|---|---|---|---|
| Clear single glazing 4 mm | 0.85 | 0.55 to 0.60 | 0.90 | 5.8 |
| Clear double glazing 4/16/4 (air) | 0.75 | 0.50 to 0.55 | 0.81 | 2.8 |
| Low-emissivity double glazing, argon | 0.60 to 0.65 | 0.40 to 0.46 | 0.80 | 1.0 to 1.1 |
| Low-emissivity triple glazing, argon | 0.45 to 0.55 | 0.32 to 0.38 | 0.70 to 0.74 | 0.5 to 0.7 |
| Solar-control double glazing | 0.25 to 0.40 | 0.18 to 0.28 | 0.40 to 0.70 | 1.0 to 1.1 |
Triple glazing and solar control come close to the grant minimum of Sw ≥ 0.3, or fall below it: triple glazing in a window with wide profiles (g of 0.45, 65 % daylight area) drops to an Sw of 0.29 despite an excellent Uw, and meets neither of the sheet's two pairs. The choice between double and triple is discussed further in the comparison of single, double or triple glazing.
Solar glazing or solar-control glazing: two opposite families
Enhanced-insulation glazing lets the sun in
What is commonly called solar glazing usually means enhanced-insulation double glazing: a low-emissivity coating keeps the room's heat in while letting most of the radiation through (g of 0.60 to 0.65). It is the default glazing in renovation, which captures winter gains and passes the grant's minimum Sw without difficulty.
Solar-control glazing sends part of the energy back
Solar-control glazing carries a coating (or a tint) that reflects or absorbs part of the radiation before it gets in. Its key indicator is selectivity, the LT / g ratio: around 1.3 for a standard low-emissivity unit, it approaches 2 on selective glazing, which keeps a lot of light for little heat. It is justified where the winter gain does not offset summer overheating: conservatories, glass roofs, large west-facing bays that cannot be shaded from outside, offices. The downside: it also cuts winter gains, all year round, whereas movable exterior shading only cuts them in summer. In a house, exterior shading on low-emissivity glazing often remains the best compromise, and keeps the grant eligibility.
Choosing glazing based on orientation and room use: aiming for the right solar gains
In winter, the low sun mainly strikes the south façade. In summer, high in the sky, it strikes the south little and the east, the west and roof windows much more: this is what makes the solar factor useful at one time and a nuisance at another.
| Orientation | Winter gains | Summer risk | Priority |
|---|---|---|---|
| South | High | Moderate, manageable with an overhang or a blind | Fairly high Sw + exterior shading |
| East / west | Low | High, low sun hard to stop | Exterior shading, or solar control |
| North | Very low | Low | Low Uw, Sw matters little |
| Roof | Medium | Very high | Capped Sw (≤ 0.36 for the grant) + blackout |
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.
Balancing solar gains and summer comfort: the Sw and glazing thermal-performance duo
What 0.1 of Sw changes, in winter as in summer
A window's solar gains are proportional to its Sw, its area and the sunshine it receives. Going from an Sw of 0.45 to 0.30 therefore removes a third of the gains (0.15 / 0.45), and that third is lost in winter just as it is saved in summer. A low Sw protects in summer but costs heating every winter, whereas movable shading only acts when needed.
Sw and Uw: weighing solar gains against heat loss, case by case
The Sw indicates the share of solar energy that passes through the window. The higher it is, the more free gains you recover. The Uw measures heat loss. The lower it is, the better the window insulates: how to read it is detailed in the guide to a window's Uw coefficient. Each added pane or low-emissivity coating lowers the Uw, but also the Sw. The right choice depends on orientation, local climate, glazed surface area and use.
Sw and LT: light without the heat
A high LT with a moderate Sw, in other words selective glazing, lights the room without heating it. To read the two together, see light transmission LTw.
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 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 (shading solutions are covered in the article on roof windows). The sheet applies to operations committed before 1 July 2028.
On the MaPrimeRénov' side, the per-measure flat grant for windows no longer exists for applications filed from 1 September 2026 (decree no. 2026-822): windows can now only be funded within a comprehensive renovation under the supported pathway, as detailed in the article on MaPrimeRénov' for windows.
New build and RE2020: the Sw enters the calculation, not a per-window value
In new build, RE2020 sets no minimum or maximum Sw window by window. The solar factor of the openings, with and without movable shading, enters the regulatory calculation: it weighs on the bioclimatic need (Bbio) and on the summer comfort indicator in degree-hours (DH). A high Sw without shading is therefore paid for on the DH.
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.
