In this post we discuss, what is a good air tightness score?
This is a crucial topic of conversation for both new builds and existing dwellings: air tightness. More specifically, we’ll explore what constitutes a good air tightness score. But first we must define air tightness and the metric used to ‘score’ a property.
Understanding Air Tightness
Air tightness, often known and technically called, air permeability, measures the amount of conditioned (heated or cooled) air leaking from a building through uncontrolled ventilation. This is known as air infiltration.
The result is expressed in m³/(h·m²) at 50 Pascals, indicating how many cubic metres of air pass through each square metre of the building envelope per hour under test conditions at an induced 50 Pascals for blower door testing or 4 Pascals for pulse testing.
The lower the result the better the air tightness score. This indicates a lower level of draughts and air leakage.
Design Air Permeability and Air Changes Per Hour
Air permeability and air changes per hour (ACH) are both measures related to air movement in buildings, but they describe different aspects and are used in different contexts.
| Metric | Air Permeability | Air Changes Per Hour (ACH) |
|---|---|---|
| What it Measures | Air leakage through the building envelope | How often indoor air is replaced per hour |
| Unit | m³/(h·m²) @ 50Pa | ACH (air changes per hour) |
| Used For | Building compliance (e.g. Part L regulations) | Passivhaus and Ventilation design (e.g. Part F regulations) |
| Test Method | Blower door or Pulse test | Blower door or Pulse |
| Focus | Energy efficiency and airtightness | Indoor air quality and occupant health |
Scoring Air Tightness
Air Tightness is measured and scored by comparing an onsite test result against a design air permeability.
Scoring for the air permeability test ranges between 0 – 10 m³/hr/m² @ 50 pascals, with zero being the most air tight score and ten being the maximum allowable by Building Regulations.
For air changes per hour (ACH), a result of less than 0.6 ACH is considered very good, while anything above 1.5 ACH indicates that there is significant room for improvement. This is crucial in a Passivhaus project.
The average value of an air tightness test, known as the design air permeability is typically set at below 5. It should be noted however this target is dependant on the site specific design and designer requirements by the client, architect and SAP assessment.
Anything around a 5 would therefore mean the property is efficient enough, and it will pass its test, and a certificate would be awarded, but if it doesn’t achieve this, there is a risk of failure due to non-compliance with the SAP assessment, slowing the process to passing building regulations.
New Builds vs Existing Dwellings
For new dwellings, the Building Regulations Part L specifies that air leakage can be no more than 10m3/h/m2 @50Pa. However, the new maximum target for air tightness is <8m3/ (h·m2)@50Pa, which is a 2 point reduction from the current maximum of 10. This more airtight target allows for low temperature heating systems such as air source heat pumps to be viable and meet the reduced carbon emissions targets.
For existing dwellings, improving air tightness can be more challenging due to the structure and materials used in older buildings. However, it’s still possible to achieve a good air tightness score with proper sealing and insulation.
Regulation Requirements and Part L
The Building Regulations Part L in the UK sets standards for the energy performance of new and existing buildings. It specifies that air leakage can be no more than 10m3/h/m2 @50Pa. However, the new maximum target for air tightness is <8m3/ (h·m2)@50Pa.
ATTMA Accredited Compliance Engineers
The Air Tightness Testing & Measurement Association (ATTMA) is a professional association dedicated to promoting technical excellence and commercial effectiveness in all air tightness testing applications. ATTMA has grown with the air tightness testing industry, creating standards such as TSL1 and TSL2 that are internationally recognised.
All our engineers are at least Level Air Tightness Testers and hold current registration with the Air Tightness Testing & Measurement Association (ATTMA). We have over 10 years on-site experience – you are in good hands.
Our testing is routinely carried out using a blower door. Following the methodology to ATTMA standards in line with CIBSE TM23 or ISO 9972:2015.
Are you looking for an Air Test?
It’s essential to conduct an Air Test for all dwellings including existing homes, new-builds and conversion or extension projects. Air Permeability Testing assesses the level of conditioned (heated or cooled) air leakage from a building through uncontrolled ventilation.
By ensuring that the building meets or exceeds the air permeability rating specified in a design stage SAP or SBEM Calculation, Air Permeability Testing guarantees energy efficiency and performance. This helps achieve the best possible Energy Rating of your Energy Performance Certificate.
Learn more by visiting our Air Permeability service page, by clicking here.
Have a Project in mind? Let’s get to work!
Achieving a good air tightness score is crucial for both new builds and existing dwellings. Not only does it contribute to energy efficiency and lower running costs, but it also helps meet regulation requirements and standards set by Part L and ATTMA. Remember, the key to a good air tightness score starts with proper planning, preparation, and understanding of requirements.
Stay tuned to our blog for more insights into building compliance testing. If you have any questions or need assistance with air tightness testing, don’t hesitate to contact us at Building Compliance Testing Limited.