Every option has its downside.
None of them is good in the abstract. Here is what each one delivers, and above all what it demands, because that is what you are rarely told before the vote.
External wall insulation
The most effective on walls
The building is wrapped in insulation, then given a new outer finish. The façade changes appearance.
What it delivers
It removes almost all the thermal bridges, which no other option does. It does not eat into the habitable floor area, and the flats stay occupied while the work goes on.
What it demands
It alters the external appearance: planning permission is required, and it may be refused in a protected heritage area. It also assumes the copropriété will accept scaffolding for several months.
Internal wall insulation
The fallback when the outside is not possible
The walls are lined from the inside, flat by flat.
What it delivers
No planning permission, a lower cost per square metre, and each copropriétaire can decide for their own flat.
What it demands
It costs several centimetres on every wall, it does not deal with the thermal bridges, and it means emptying the rooms. The real gain is markedly lower.
Loft insulation
The best gain for the cost
The floor of the unoccupied loft is insulated, or the flat roof from above.
What it delivers
It is the largest heat-loss item and the simplest job: no scaffolding, no work inside the flats, no change to the façade.
What it demands
The ventilation of the loft has to be checked, on pain of condensation under the roof.
Dealing with the ventilation
Inseparable from everything else
The air renewal system is put back in working order, or replaced.
What it delivers
Air renewal weighs as much as the walls in the heat losses. And it is what protects the building once it becomes airtight.
What it demands
It is never the item the copropriétaires want to vote for: it does not show. Yet it is the one whose omission brings the mould.
Replacing the windows
The most visible, not the most cost-effective
The windows are replaced with high-performance double glazing.
What it delivers
Immediate and noticeable comfort: no more cold wall surface, less noise. It is what wins people over at the assemblée générale.
What it demands
The item accounts for no more than 10 to 15% of the heat losses. Replacing the windows of a building whose loft is not insulated means dealing with the fourth problem and leaving the first.
Renovating the communal heating
Acts on generation, not on the losses
The boiler plant is replaced or brought under proper control, and the distribution circuits are balanced.
What it delivers
An immediate effect on the bill, without touching the flats or the façade.
What it demands
Sizing a boiler plant before insulating leads to oversizing, and so to paying twice. The order matters: insulate, then resize.