A building’s airtightness is not merely an add-on to insulation, but one of the key factors that actually determine energy consumption. Even well-insulated partitions will not be fully effective if heated air constantly escapes through the building envelope or cold air from outside flows in. In such a situation, some of the energy is lost not through the wall or roof itself, but through uncontrolled air exchange.
This is precisely why airtightness is increasingly viewed not merely as a construction detail, but as part of a building’s overall energy strategy. The better the building envelope is designed and constructed, the easier it is to minimise heat loss in winter, overheating in summer and unintended deviations in ventilation performance. This is particularly evident in modern buildings, where improvements in the insulation of partitions mean that air infiltration and the quality of workmanship are becoming increasingly important.
What is a building’s airtightness?
Air tightness refers to the ability of a building envelope to limit uncontrolled air flow through leaks. This includes all areas where air can enter through the joints between partitions, around windows and doors, through service penetrations, the roof, ceilings, timber-framed walls, or the joints between structural elements. The more such areas there are and the poorer the workmanship, the greater the uncontrolled air exchange.
This must be clearly distinguished from ventilation. Ventilation is designed to be controlled. Airtightness is not intended to ‘seal off’ a building without a supply of fresh air, but rather to eliminate unintended air flows. This ensures that air reaches where it is needed, in quantities determined by operational requirements and design specifications, rather than through unintended gaps in the building envelope.
How does airtightness affect the energy balance?
The impact of airtightness on the energy balance is straightforward. If heated air escapes from the building through leaks, the heating system must constantly make up for this loss. The greater the air infiltration, the greater the energy required to maintain the desired indoor temperature. The same mechanism applies in summer, only in the opposite direction. Warm air from outside enters the building and increases the cooling load on the building.
In modern buildings, the problem becomes even more apparent, as the building envelope itself now boasts increasingly better thermal performance. In such cases, the difference between a well-sealed building and one constructed carelessly begins to be felt not only in calculations, but also in actual use. Heat loss through air infiltration is no longer just a minor issue. They start to become one of the main reasons for higher energy consumption.
Air-tightness and mechanical ventilation
A well-sealed building performs best when combined with a properly designed ventilation system. This arrangement allows for a clear distinction between two objectives: on the one hand, minimising unintended air leakage; on the other, ensuring controlled air exchange where it is needed. This is precisely why airtightness is so important in buildings equipped with heat recovery and other mechanical ventilation systems.
If the building envelope is not airtight, the ventilation system becomes less predictable. Air begins to circulate not only through the ducts and diffusers, but also through uncontrolled gaps in the walls and ceilings. This worsens the energy balance, makes it harder to regulate the system, and reduces the effectiveness of heat recovery. The heat recovery unit alone will not fix the problem of an air-leaky building envelope. It can only function properly if the building is genuinely airtight.
Airtightness is not the same as ‘no draughts’
One of the most common mistakes is to think of airtightness solely in terms of perceived comfort – in other words, whether there’s a draught somewhere or not. This is too narrow an approach. Admittedly, draughts and localised cold spots are symptoms of air leakage, but the problem is broader. An air-leaky building envelope also affects temperature stability, the performance of the heating system, moisture levels in the walls and the long-term durability of the building.
The opposite mistake is also made: treating airtightness as an end in itself, without considering ventilation and building physics. Simply ‘sealing everything up’ does not solve the problem if the building does not have adequate air exchange. In a well-designed building, airtightness and ventilation do not compete with one another. They complement each other.
Where do leaks most commonly occur?
The biggest problems usually don’t stem from large wall or roof surfaces, but from joints and details. Critical areas include the surroundings of window and door frames, the junctions between walls and the roof or ceiling, pipe and cable penetrations, junction boxes, points where different materials meet, and any sections where the waterproofing layer is interrupted or poorly joined.
This is precisely why the issue of airtightness is so closely linked to the quality of workmanship. You can have a sound design on paper, yet still lose much of the intended benefit during the construction phase. In practice, the outcome is rarely determined by a single major error. More often than not, it comes down to the accumulation of minor inaccuracies which, taken individually, seem insignificant, but together create a real energy problem.
Air-tightness and durability of partitions
A draught doesn’t just mean higher heating or cooling bills. It also poses a risk of adverse moisture-related issues within building envelopes. If warm, moist air enters the structural layers unchecked, condensation can form in places where it wasn’t intended to. Over time, this leads to dampness in the insulation, a deterioration in the performance of the materials and a reduction in the durability of the entire wall.
Air-tightness should therefore be regarded as a factor that contributes to both energy efficiency and the preservation of building materials. A well-executed air-tight layer not only helps to minimise heat loss but also maintains more stable operating conditions for the building envelope. In buildings with high energy standards, this is no longer simply a matter of ‘it’s worth doing’. It is one of the prerequisites for the entire system to function as intended.
Why is airtightness becoming increasingly important in new construction?
In older buildings, a large proportion of energy loss was simply due to very poor building envelopes and outdated installations. Today, the situation is different. Walls, roofs and windows are becoming increasingly efficient, and heating systems are more effective. In this context, factors that were once considered secondary are becoming increasingly important. One such factor is airtightness.
The more energy-efficient a building is, the less room there is for construction errors. A leak then begins to act like a leak in a well-built system. It doesn’t have to be huge to spoil the result. That is why, in new buildings, airtightness is increasingly no longer treated as a secondary consideration, but is becoming one of the key quality indicators for the entire project.
What does this actually mean for investors and designers?
The key point is simple: it is impossible to accurately assess a building’s energy balance without taking airtightness into account. Insulation alone is not enough. Nor is a good U-value alone sufficient. If the building envelope is not continuous and airtight, some energy will inevitably be lost beyond our control.
From a design perspective, this means that airtightness must be considered from the outset, rather than only at the end of construction. From a construction perspective, it means paying close attention to details, joints and service penetrations. From a user perspective, however, this translates into lower energy consumption, better interaction with ventilation and more stable indoor conditions. This is precisely why a building’s airtightness is now one of the key pillars of energy efficiency.
Summary
A building’s airtightness has a direct impact on its energy balance, as it limits uncontrolled air exchange through the building envelope. A well-executed airtight layer reduces heat loss in winter, limits the inflow of hot air in summer and allows the ventilation system to operate in a predictable manner. In a modern building, this is not merely a construction detail, but one of the prerequisites for genuine energy efficiency.
FAQ – Building airtightness and energy balance
Yes, because draughts increase heat loss and force the heating system to work more frequently. The greater the air infiltration, the harder it is to maintain a stable temperature whilst keeping energy consumption low.
Not always, but in highly airtight buildings, mechanical ventilation complements the building envelope very well. It allows you to control air exchange rather than leaving it to random draughts.
Most often in details and joints, rather than on large wall surfaces. Problem areas include the vicinity of windows, doors, service penetrations, the roof, the junction of walls and ceilings, and where different materials meet.
No. Even very good partitions will not be fully effective if heated air is constantly escaping through leaks. Insulation and airtightness should work together.
Yes, because in summer, air leaks increase the inflow of warm air from outside and can increase the building’s cooling load. This means higher energy consumption by air conditioning or reduced indoor comfort.
