Double glazing is not a legal requirement as such. No article in the construction code mentions the term “mandatory double glazing.” What the law imposes is a level of thermal performance and decency of housing, which double glazing generally helps to achieve, but not exclusively.
Uw, Ug, and Sw Coefficients: The Real Criteria for Window Compliance
The compliance of a window is not verified by counting the layers of glass. It is assessed through standardized coefficients, listed on the product’s technical label or on the manufacturer’s sheet.
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The Ug coefficient measures the thermal transmission of the glazing alone. For a window to comply with RE2020, this Ug must drop below the threshold of 0.7 to 0.8 W/m².K depending on the type of glazed wall. Single glazing shows an Ug of around 5 to 6 W/m².K, which disqualifies it de facto for any new construction.
The Uw coefficient incorporates the performance of the frame (joinery) and the glazing. This is what thermal engineering offices use in regulatory calculations. A high-performing Uw requires both insulating glazing and a frame with thermal bridge break.
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The solar factor Sw determines the window’s ability to let in solar energy. A high Sw promotes free gains in winter but can cause overheating in summer. RE2020 requires finding a balance between these two parameters, particularly through requirements for summer comfort.
We recommend systematically checking these three values on the quote or product sheet before any replacement. A low-end “double” glazing with a non-reinforced PVC frame may show a mediocre Uw, while an aluminum frame with thermal bridge break and reinforced insulating double glazing will achieve performance close to triple glazing.
To better understand the regulation on mandatory double glazing, it is essential to distinguish the situations: new housing subject to RE2020, old housing rented out, or voluntary renovation. The obligations differ radically from one case to another.
Rental Windows: Housing Decency and DPE, Not Double Glazing
Since January 1, 2022, a rental property must meet enhanced decency criteria. The windows must be air and water-tight, and the property must show energy consumption compatible with DPE thresholds.
A landlord is not required to install double glazing if their single-glazed windows are in perfect condition, with functional seals and a frame without infiltration. In practice, this situation remains rare: a property rated F or G on the DPE will gradually be prohibited from being rented out, and single glazing significantly contributes to this poor rating.
The tenant has recourse if the property does not meet decency criteria. The procedure involves a formal notice to the landlord, followed by the departmental conciliation commission or the judicial court. The judge can mandate the completion of work, including window replacement.
What the DPE Actually Penalizes
The energy performance diagnosis does not check for the presence of double glazing. It measures the overall consumption of the property and its greenhouse gas emissions. High-performing windows do not compensate for poor roof insulation, and vice versa.
A well-insulated property can retain old windows without becoming a thermal sieve. Conversely, we regularly observe properties equipped with recent double glazing but rated F due to an obsolete heating system or lack of wall insulation.
Urban Planning and Co-ownership: Permissions Before Changing Windows
Replacing windows is not just about choosing glazing. In most cases, a prior declaration of work at the town hall is necessary as soon as the exterior appearance of the building is modified: change of color, material (wood to PVC, for example), or dimensions.
- In protected areas (surroundings of historical monuments, classified sites), the architect of the Buildings of France must validate the project. PVC is often rejected in favor of wood or aluminum painted in a mandated shade.
- In co-ownership, replacing windows affects the exterior appearance of the building. Authorization from the general assembly must be obtained, unless a group purchase or renovation program has already been voted on.
- In individual homes outside protected areas, a prior declaration remains mandatory if the modification is visible from the public road. Only a like-for-like replacement (same material, same color, same cut) can be exempted.
Neglecting these urban planning procedures exposes one to penalties: the town hall can require restoration at the owner’s expense, with a constrained deadline.
Co-ownership: Harmonization and Facade Constraints
The co-ownership regulations often set precise requirements for joinery: RAL colors, type of profile, position of crossbars. A co-owner cannot freely choose their windows, even to improve thermal insulation. The property manager checks compliance before and after work.
On-Site Verification: Concrete Methods to Assess Your Windows
Several indicators allow for a quick assessment of the condition and performance of your windows without waiting for a professional diagnosis.
- The lighter or flame test: bring a flame close to the glazing. If you see two distinct reflections, the glazing is single. Four reflections indicate double glazing. A reflection that leans towards purple or green signals a low-emissivity coating (reinforced insulating glazing).
- The thickness of the frame: standard double glazing (4/16/4 or 4/20/4) requires a minimum glazing thickness of 24 to 28 mm. An old frame of 15 mm cannot accommodate double glazing.
- The spacer marking: on the edge of the glazing, between the two sheets of glass, the spacer (metal or composite strip) sometimes bears a marking indicating the composition and performance. This marking often corresponds to CEKAL certification.
CEKAL certification guarantees the durability of double glazing (moisture tightness for ten years). The Acotherm label complements this certification by classifying the acoustic (Ac1 to Ac4) and thermal (Th6 to Th17) performance of the complete joinery.
A glazing that shows condensation between the two sheets of glass has lost its tightness. The insulating gas (argon or krypton) has escaped, and thermal performance is degraded. Replacement of the glazing is necessary, even if the joinery remains in good condition.
Thus, verifying the compliance of your windows involves three levels: the technical coefficients on the product documentation, adherence to urban planning and co-ownership rules, and the physical condition observed on-site. A failure in just one of these three levels is enough to render the installation non-compliant with current requirements.