Insulation : the heating goes up because the air is escaping
Hello, it’s Claude. Heating increases especially when air escapes due to poor airtightness: even a good insulator isn’t enough if air circulates freely. To maintain real thermal comfort, you must act quickly on air leaks, often invisible but costly.
To be done immediately
- Check the classic leak points: attic hatches, windows, electrical outlets, and baseboards.
- First insulate the roof, because up to 30 % of the heat loss occurs through the roof.
- Use suitable air-tight membranes to prevent drafts.
- Have a blower-door test performed to precisely locate the leaks.
- Opt for a controlled mechanical ventilation to maintain good indoor air without uncontrolled air exchange.
To learn more about controlling heat loss or discover how to react to a poorly insulated house, you have good leads.
Why a good insulator isn’t enough against air leaks
The air that escapes lowers heating efficiency. An insulation such as mineral wool or cellulose fiber works by trapping still air. As soon as air circulates, this mechanism is broken and you lose between 15 and 30% of efficiency, sometimes more. It’s convection and the wind-washing (wind sweeping the insulation) that ruin your performance.
Marc, near Rennes, spent €2,400 to insulate his attic, but his boiler still ran just as much. The diagnosis revealed major leaks due to a poorly sealed hatch and unprotected electrical conduits. His insulation was good, but the air tightness was not ensured. This case is unfortunately not rare.
Areas where air escapes most often in a poorly insulated house
A few specific points account for the majority of air leaks:
- Ceiling penetrations: attic hatches, recessed spots, electrical ducts.
- Junction of wall and intermediate floor where air circulates freely.
- Around window and door frames, especially if expanding foam is poorly applied.
- Old roller shutter housings, often poorly insulated and permeable.
- Outlets and switches on exterior walls that are not sealed.
In simple terms, these leaks can cause up to 25 % of the heat produced to be lost, or 30 % in some cases, reducing the actual effectiveness of your thermal insulation.
Concrete steps to fix air leaks
- Thermal diagnosis and blower-door test to detect major leaks.
- Installing an airtight membrane, hygro-variable vapor barrier or vapor barrier depending on the area and local climate.
- Targeted repair of technical penetrations, joinery, and additional attic insulation.
A high-performance insulation combined with good air tightness can reduce your heating energy consumption by 20 to 30%, i.e., about €450 per year in savings on average.
Comparative table of airtightness solutions in 2026
| Solution | Average installed price (€ / m²) | Life span | Recommended use |
|---|---|---|---|
| Polyethylene film (vapor barrier) | 8 – 14 | 20-30 years | Finished attics, humidity-free zones |
| Hygrovariable vapor barrier | 14 – 25 | 40-50 years | Roof renovation, bio-based insulations |
| OSB3 boards joined + adhesive | 25 – 40 | 50+ years | Timber frame, braced walls |
| Continuous plaster or mortar coating | 18 – 30 | Building lifetime | Masonry, monomur brick, cellular concrete |
Mistakes that push up the bill despite the work
A punctured membrane, unsealed hatches, or badly bonded sheets can turn your project into a giant air leak. The best insulation will never compensate for a lack of airtightness. It’s a bit like having a door open in the middle of winter. Remember: “It’s not the insulation; it’s the airtightness that really makes the difference.”
You also need ventilation that is perfectly suited, otherwise indoor air becomes damp, with increased risk for health and equipment. Insulating better without proper ventilation is sealing the house to humidity.
The benefits of good insulation with controlled air-tightness
In 2026, a well-insulated and airtight house offers:
- A real gain in thermal comfort, no more cold rooms in winter and stuffy in summer.
- Significant energy savings, which ease the heating bill and reduce your ecological footprint.
- Increased durability of the structure, as moisture and mold are better controlled.
For example, 70% of a household’s energy consumption heats the living space. Poor insulation is like throwing money out the window. Recognizing quickly the signs of a bad insulation can save you many troubles.
How can I tell if my home is losing too much heat?
A heating bill that rises for no reason, cold walls, drafts, and signs of humidity are clear signs. The blower-door test allows you to precisely locate the losses.
Should I perform a blower-door test before or after the works?
It’s best to do one before installing the vapor barrier to correct defects as early as possible, then a final test to validate the performance.
Is it possible to improve airtightness without redoing all the insulation?
Yes, by targeting specific points such as the attic hatch or the window joints. But the continuous barrier is always preferable for an optimal result.
What type of ventilation for a well-sealed house?
Humidity-regulated mechanical ventilation (single-flow VMC) is recommended below Q4Pa-surf 1.0. For better performance, the two-flow VMC is ideal, especially if your envelope is very airtight.
What financial aid for insulation and airtightness works?
Insulation-related works can benefit from MaPrimeRénov’, Eco-PTZ, reduced VAT and CEE, subject to using certified RGE professionals.
Thanks for your interest; Claude
