Technical Advice | Technical Glazing Terms
Air Permeability in Windows and Doors
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Air Permeability refers to the amount of air that will travel through a window or door system in its closed position. Air permeability testing relies on the quality of the systems sealing, engineering and manufacturing to ensure that all opening segments seal together well and fully to stop as much air travel through a system as possible.
It is important in terms of comfort to the internal spaces to ensure minimal wind/breeze intrusion and is also an important factor when it comes to environmental factors, to limit the travel of energy from the internal to the external of the system.
The European standard used to classify air permeability in windows and doors is BS EN 12207:2016. Testing is carried out in accordance with BS EN 1026:2016 on completed window and door assemblies in a factory or laboratory setting by licensed testing facilities such as the Rosenheim Institute in Germany.
The air permeability test has two components; the air permeability over the overall area of the test specimen and the air permeability related to the length of the opening joint. This is measured in units of m3/(h.m2) for the overall area and m3/(h.m) for the air permeability over the panel joint.
A test specimen of the window or door system is fixed into the testing rig and subjected to both positive and negative pressures (blowing force pressure from the front of the elevation and a vacuum sucking pressure from behind). A reference air permeability is taken at 100Pa of pressure which then defines the upper limits of its class. A specimen belongs to a specified class if the measured air permeability does not exceed the upper limit at any test pressure step in that class.
The classes for BS EN 12207 range from class 0 (the lowest) to class 4 (the highest).Class 4 indicates that the window or door system has been tested under pressures up to 600 Pa and had below the class limit of air permeability through the system along the panel joint and overall system area.
How Airtight Glazing Supports Efficient Mechanical Cooling
Air permeability becomes particularly important in buildings where internal temperatures are actively controlled by mechanical cooling. When external air enters through unintended gaps in closed windows and doors, the cooling system has to respond to conditions that have not been introduced through the building's designed ventilation strategy.
During warmer conditions, uncontrolled air leakage can introduce warmer external air into a cooled internal space, making it more difficult to maintain the desired internal temperature. High performance glazing helps limit this uncontrolled air movement by creating an effective seal when windows and doors are closed.
Airtightness should not be confused with ventilation. A highly airtight building still requires an appropriate ventilation strategy to provide fresh air and manage indoor air quality. The objective is to control how air enters and leaves the building, rather than relying on uncontrolled leakage through the building envelope.
Avino Glazing and Air Permeability
The Avino range provides an example of how high performance glazing can contribute to an airtight building envelope. The Avino S-Timber Window, Avino S-Timber Door and Avino S-Timber Sliding Door have all achieved Class 4 air permeability to EN 12207.
This is the highest classification within EN 12207 and demonstrates a high level of resistance to air leakage through the tested window or door system. The Avino systems also combine this airtightness performance with insulated timber frames and high performance glazing, allowing the different elements of the building envelope to be considered together as part of the overall environmental design.
For a mechanically cooled building, reducing uncontrolled infiltration helps the cooling strategy operate under more predictable conditions. It does not, however, translate into a fixed energy saving. The overall cooling requirement will also depend on factors including the building design, solar gain, glazing specification, orientation, occupancy, ventilation strategy and mechanical system.
Glazing Performance and Whole-Building Airtightness
Air permeability testing of a window or door system should also be distinguished from whole-building airtightness.
BS EN 1026 tests air leakage through the assembled window or doorset itself. It does not test the junction between the window or door frame and the surrounding building construction. Correct installation, perimeter sealing and continuity of the building's air barrier are therefore essential.
A high air permeability classification for the glazing can support an airtight building envelope, but the complete building design and installation must work together to achieve the required overall performance.
Specify High Performance Glazing for Airtight Building Envelopes
For projects where airtightness, thermal performance and controlled internal conditions are key design considerations, IQ Glass can provide technical guidance on suitable glazing systems and performance specifications.
Contact our technical team to discuss architectural glazing requirements for your project.
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