When the heating bill climbs, the reflex reaction is often to blame the boiler or the price per kilowatt-hour. It feels as if you’re heating from the outside, without really knowing where that dearly paid heat is escaping. Many homeowners imagine that everything leaks through old windows or a door that doesn’t seal properly, while the reality of heat losses is quite different.
Because energy experts keep repeating it: the key lies in the building envelope, everything that separates the interior from the outdoor air. According to data compiled in Spain by Contigo Energía and the Instituto para la Diversificación y Ahorro de la Energía (IDAE), the facade and the roof account for about 60 % of a dwelling’s heat losses. This challenges common assumptions and raises a real question: where should action be prioritized?
Heat losses: facade and roof at the forefront
The figures reported by Contigo Energía are based on studies by energy-efficiency specialists. They show that facade and roof form the most critical pair: up to 35% of heat is lost through the walls, about 25% through the roof. The remainder is distributed among the floor, openings, and all the small, invisible air leaks in the home.
- Exterior walls: up to 35%.
- Roof: around 25%.
- Ground floor: about 15%.
- Doors and windows: 15%.
- Air infiltrations: 10%.
The IDAE reminds in its Practical Energy Guide that the building envelope can, on its own, represent between 70% and 80% of a dwelling’s total energy losses. Windows contribute to this, of course, but their surface area remains far smaller than that of walls and roof. In other words, replacing only the glazing without addressing facade and roof often targets the wrong area.
Facade, roof, thermal bridges: where does the heat escape from
From a physical standpoint, an uninsulated wall behaves like a reverse radiator: it transfers heat from indoors to outdoors, especially when the temperature difference is large. For the roof, the effect is compounded by the fact that warm air rises toward the attic. In Spain, more than 58% of homes were built before the NBE-CT-79 thermal standard, with little or no insulation.
On top of this initial weakness, thermal bridges add to the problem—areas where the insulation is interrupted: exposed concrete columns, floor junctions, the perimeter around windows. In these places, heat finds an easier path to escape. They can sometimes be spotted with the naked eye as colder walls, traces of dampness, or mold in the corners during winter, signs that the home is quietly losing energy.
Reducing heat losses through facade and roof
To limit these leaks, experts favor treating the envelope before tackling other items. On facades, they recommend exterior insulation systems of the SATE type, or filling air cavities where the walls have them. For roofs, the approach is to add insulating panels under the roofing or at the attic level. The IDAE notes that the envelope can represent up to 80% of energy losses, i.e., the heat losses from facade and roof totaling about 60%.
Experts place the comfortable temperature range between 19 and 21 degrees, with nighttime lowering. According to the IDAE, “each additional degree of heating increases consumption by about 7%.” The institute adds that, in a typical home, lowering the thermostat from 23 to 20 degrees yields savings of more than 20% on heating.
