Where does the heat escape?
Before voting on works, you need to know where the energy goes. The answer is often a surprise: it is almost never where people think.
The five items, one by one
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Roof and loft
25 to 30%Warm air rises. At the top of the building it often meets nothing more than a slab and a few centimetres of settled wool insulation, sometimes nothing at all.
Insulating the unoccupied loft space or the flat roof. It is the most cost-effective project a copropriété can undertake: the gain is substantial and the work touches neither the flats nor the façade.
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Walls
20 to 25%This is the largest surface in contact with the outside. In a building put up before 1974, the wall is often a single leaf of concrete or brick, with no insulating layer.
External wall insulation where the façade and the rules of the copropriété allow it: it deals with thermal bridges and does not eat into the habitable floor area. Internal wall insulation otherwise, flat by flat.
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Air renewal and leakage
20 to 25%The item everyone forgets, and it weighs as much as the walls. Warm air escapes through gaps in the airtightness, through service ducts and stairwells, and through poorly set ventilation.
Sealing the envelope and putting the ventilation back in order. A warning: a building is never sealed up without dealing with its ventilation, on pain of creating damp problems.
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Windows
10 to 15%Single glazing insulates about six times less well than recent double glazing. And the frame counts as much as the glass.
Replacement with high-performance double glazing. It is the most visible item for the copropriétaires, and often the first to be voted through, although it is not the one that pays back most.
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Lower floors
7 to 10%A floor above a car park, a cellar or an unheated crawl space chills the dwellings on the first level directly.
Insulation fixed to the underside, from the basement. Inexpensive work, no disturbance for the occupants, and often overlooked.
Thermal bridges
These are not walls but breaks: the junction between a balcony and a façade, the edge of a floor slab, a window reveal. Heat goes round the insulation at those points.
They are dealt with when the insulation goes on, not afterwards. It is the strongest argument in favour of external wall insulation, which removes almost all of them in one go.
In what order should it be tackled?
Copropriétés almost always vote for the windows first, because that is what shows. Yet it is far from the most cost-effective. The order below follows the ratio between the gain obtained and the cost incurred.
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01
Roof and loft
25 to 30%
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02
Air renewal and leakage
20 to 25%
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03
Walls
20 to 25%
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04
Windows
10 to 15%
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05
Lower floors
7 to 10%
A precaution that matters. A building is never made airtight without dealing with its ventilation. A copropriété that replaces its windows without touching the air renewal ends up with condensation, then mould, in the months that follow. We write it into our reports as a matter of course.