French renovated buildings

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Your building will potentially fall under the existing building thermal regulation called "RT Existant" (RT stands for Réglementation Thermique). In order to determine whether you need to abide by it, you need to know whether you will need a work permit or not. Considering you said it's a major renovation, I'm going to assume yes.

Under the "RT Existant", there are two paths:

• "RT élément par élément": this is a prescriptive by element path. For each element you are affecting, you will need to match a specific performance criteria (called "garde-fous" <> safeguards). Eg: if you are replacing the wall insulation, the R-value needs to be higher than 2.3 m².K/W (except exceptions). If you are replacing a boiler, you will need an efficiency higher than a specific value.

• "RT globale": this is both a prescriptive by element and a global performance path. Each element you will change (and only those) will have to meet the same specific performance criteria as above, BUT you will ALSO have to demonstrate a performance for the building as a whole. Using a specific calculation methodology (see [1]), you will need to demonstrate that you are below a specific consumption in source kWh/m².year that matches both your climate (location) as well as the type of heating system you have (that's for residential, for commercial it's 30% below baseline). You'll also need to check that a summer indoor temperature is below a reference temperature (that varies a lot too).

In order to determine whether you fall under one or the other:

• If your building is smaller than 1000 m² you're under "RT par élément"
• If your building is larger than 1000 m² but the cost of the work is lower than 25% of the value of the building (value being defined in a specific way and not representative of its costs on the market), you're under "RT par élément"
• If your building is larger than 1000 m², the cost of the work is higher than 25% of the value of the building, but the building was built before 1948, you're also under "RT"
• Basically, the only way you're under "RT globale" is if your building is larger than 1000 m², the cost of the work is higher than 25% and the building was built after 1948

see: Site RT Bâtiment (in french)

[1] Note about the calculation methodology: this is a normative calculation. IT DOES NOT REPRESENT THE REALITY OF HOW YOUR BUILDING IS USED. The idea is that all entry parameters are pre-defined: occupancy, indoor temperature, etc. It allows apple-to-apple comparison of two buildings but completely forgoes the intended usage. If for a given reason you know right from the start that a given building will be used at indoor temperatures of 24°C in the winter for example, you could end up making bad choices just to abide by the law... This doesn't tell you how much you will actually ... (more)