Airtightness Refurbishment (Blower Door Optimised)

Sealing leaks in the building envelope — window-to-masonry junctions, sockets, cable and pipe penetrations, and vapour barrier laps.

How does airtightness refurbishment work?

Every building loses heat not only through walls and windows but also through air that flows uncontrolled through gaps and leaks in the building envelope. These ventilation heat losses occur at window-to-masonry junctions, sockets in external walls, cable and pipe penetrations, roller shutter boxes and at joints in the vapour barrier in the roof. The key metric is the n50 value, which indicates how many times per hour the building's air volume is exchanged at a pressure difference of 50 pascals. An unrenovated detached house is at 6 to 10 1/h; a refurbished one should achieve below 1.5 1/h, and below 0.6 1/h with a ventilation system. In airtightness refurbishment, a blower door test according to ÖNORM EN ISO 9972 first creates negative pressure, and each leak is located with smoke, thermography or an anemometer. The leaks are then closed with tapes, sealing collars, compressible foam tapes and airtight plasters.

Which buildings benefit from airtightness refurbishment?

The measure is suitable for refurbishment projects of all kinds whose building envelope is fundamentally intact and where individual leaks are the cause of draughts and heat losses. It is particularly useful after window replacement or roof insulation, to connect the new components cleanly to the existing structure. A ventilation concept is a prerequisite, because a more airtight building without controlled air exchange carries a mould risk. In an uninsulated building relying solely on window ventilation, airtightness sealing alone is not sufficient; a comprehensive refurbishment package is needed here.

What does airtightness refurbishment actually deliver?

The saving is 3 to 10 % relative to ventilation heat losses. For a detached house with 420 m³ of heated volume whose n50 value is reduced from 6 to 1.5 1/h, the saving is around 4,500 kWh of natural gas per year. The blower door test including leak detection costs 800 to 1,500 €, the sealing work 1,500 to 5,000 €. With a payback of 5 to 15 years over a 30-year service life, the measure is highly economical. Added to this are draught-free rooms and a significantly lower risk of moisture damage in the structure.

Energy carrier

Erdgas, Heizöl, Pellet, Fernwärme → Erdgas, Heizöl, Pellet, Fernwärme

Savings potential

3-10 % Lüftungswärmeverluste

Worked example

A detached house with 420 m³ of heated volume is conspicuous for draughts at window junctions and sockets. A blower door test gives an n50 value of 6 1/h, typical of unrenovated existing buildings. After leak detection, connection joints, pipe penetrations and roller shutter boxes are sealed, so that the verification test shows 1.5 1/h. For the energy saving, the air volume flow is evaluated using the density of 1.2 kg/m³ and the heat capacity of 0.34 Wh/(kg·K) of air; the factor 0.07 converts the n50 value to the average natural air change rate. With 3,500 heating degree days and gas heating at 90 % efficiency, the calculation is: 420 × 1.2 × 0.34 × (6 − 1.5) × 0.07 × 3,500 × 24 / 1000 / 0.90 = 4,488 kWh/a. The house saves around 4,500 kWh of natural gas per year and gains significantly in comfort.

Investment & payback

Blower-Door inkl. Leckageortung: 800-1.500 €; Abdichtarbeiten 1.500-5.000 € · Service life: 30 a · Payback: 5-15 a

Applicability

Requirements

Exclusion criteria

Typical buildings

Frequently asked questions

How much does a blower door test cost?

A blower door test including leak detection with thermography or smoke costs 800 to 1,500 € for a detached house. A simple measurement of the n50 value without leak detection is cheaper but provides no basis for refurbishment. The subsequent sealing work costs 1,500 to 5,000 € depending on the number and type of leaks. A verification test after refurbishment documents the value achieved.

What is a good n50 value?

An n50 value below 1.5 1/h is considered good for a refurbished building without a ventilation system; with a ventilation system, 0.6 1/h should be achieved, which corresponds to the Passive House standard. Unrenovated old buildings are at 4 to 10 1/h. The value indicates how many times per hour the building's entire air volume is exchanged at a pressure difference of 50 pascals. ÖNORM B 8110-7 governs the requirements in Austria.

Where are the typical air leaks in the building envelope?

The most common leaks are found at the connection joints between window frame and masonry, at sockets and switches in external walls, at roller shutter boxes, at penetrations for cables, pipes and flue pipes, and at joints and junctions of the vapour barrier in the roof. Loft hatches, cellar doors and chimneys are also typical weak points. A blower door test with leak detection makes these points visible.

Does an airtight house lead to mould?

It is not the airtight house but insufficient air exchange that leads to mould. After sealing, uncontrolled air exchange through gaps is eliminated, so moisture from cooking, showering and breathing has to be removed deliberately. A ventilation concept with regular purge ventilation or mechanical ventilation is therefore a prerequisite. Conversely, an airtight envelope prevents moisture damage in the structure, because warm room air no longer penetrates cold components.

Is a blower door test mandatory in Austria?

For refurbishments of existing buildings, there is no general obligation in Austria to carry out a blower door test. For new builds and comprehensive refurbishments, however, OIB Guideline 6 and subsidy programmes such as the Sanierungsscheck (federal refurbishment grant) frequently require proof of airtightness, particularly if a ventilation system is installed. In Germany, the test is mandatory for KfW subsidies for efficiency houses. Regardless, it is the only reliable tool for finding leaks.

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