Equivalent English definitions relating to thermal transmittance do not make an issue of what is and what is not a "courant" ("standard") part of a wall or roof, etc. If no special features are mentioned, it is assumed that special features are precisely that - special, and not covered by the general calculation rules. But the French are Cartesian AND used to Napoleonic codes in which anything that is not expressly allowed is disallowed, so they go into excruciating detail ...

It seems to be too that they are not comfortable with the term themselves, for the definition falls short of what it really means. The most important point of that definition is "thermiquement homogène". The business about "couches" is strictly irrelevant. What they mean is that if you have a roof made of slates, say, with a given type of insulation and underfelt, with roof windows in it, the "courant" part refers to the roof, not the windows, at least to the extent that the roof components are identical throughout. Similarly, if you have a double-glazed atrium, say, comprising panels of insulated opaque materials, then the "courant" part is the double glazing (or not, depending on which is used in the greater proportion).

If all else fails, "the standard (or routine) part of the floor/roof".

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Note added at 1 hr (2007-04-10 15:00:29 GMT)
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Note that while "paroi" generally IS a wall, when discussing insulation in particular it can also refer to floors and, especially, roofs. Just as a shoebox has 6 "parois", so does a building, as a rule (especially when on stilts or standing over some other vacant or otherwise uninsulated space).