What is an Air Tightness Test & Why Do New Builds Need One?
What is an Air Tightness Test & Why Do New Builds Need One?
Modern new-build homes are designed to be more energy efficient than ever. Better insulation, high-performance windows and improved construction methods all help reduce heat loss, but another important part of the building envelope is air tightness.
An air tightness test, sometimes called an air pressure test or blower door test, measures how much uncontrolled air leaks through gaps in the building fabric.
For new builds, this can play an important role in demonstrating the performance of the completed building, identifying unwanted air leakage and helping ensure that heating and ventilation systems can operate as intended.
So, what exactly happens during an air tightness test, what does the result mean, and how can you improve air tightness before the test takes place?
What is an Air Tightness Test?
An air tightness test measures the amount of air that escapes through unintended gaps and openings in a building.
During the test, a specialist fan is normally fitted temporarily into an external doorway. The fan creates a pressure difference between the inside and outside of the property, allowing the tester to measure how quickly air moves through leaks in the building envelope.
This can reveal air leakage around areas such as:
- Windows and external doors
- Service penetrations
- Pipework and ductwork
- Electrical sockets and cables
- Wall-to-floor junctions
- Roof and ceiling junctions
- Loft hatches
- Membrane joints
- Poorly sealed openings through external walls
The test provides a measured air permeability result, giving the project team a much clearer indication of how airtight the finished building actually is.
Why Do New Builds Need Air Tightness Testing?
Air tightness forms part of the overall energy-performance approach used within modern Building Regulations. Exact testing requirements and target figures vary depending on where the property is being built, the applicable regulations and the stage of the project, so requirements should always be confirmed with the relevant building-control professional or energy assessor.
For example, Approved Document L provides the current energy-efficiency guidance for new dwellings in England, with the applicable edition depending on which regulatory standards the project falls under.
However, air tightness testing is about much more than completing a compliance exercise.
1. Reducing Uncontrolled Heat Loss
Small gaps throughout a building can add up to a significant amount of unwanted air movement.
During colder weather, heated indoor air can escape through these gaps while cold outdoor air enters elsewhere. This uncontrolled infiltration means that the heating system may have to work harder to maintain a comfortable indoor temperature.
Creating a more airtight building envelope helps reduce these unwanted air paths and supports the overall energy performance of the property.
2. Checking the Quality of the Building Envelope
A building can look completely finished while still containing hidden gaps.
An air pressure test provides a way of checking the as-built performance of the property rather than relying solely on drawings, specifications or assumptions made during the design stage.
Where excessive leakage is discovered, further investigation can help locate weak points so that they can be sealed.
This is why paying attention to airtightness throughout construction is much easier than waiting until the building is completely finished.
Where Does Air Leakage Commonly Occur?
Air can escape through surprisingly small gaps.
Common problem areas include junctions between different construction materials, window and door frames and any point where services pass through the building's airtight layer.
Ventilation installations also create penetrations through walls and ceilings, making careful sealing around ductwork particularly important.
Using appropriate airtightness products during construction can help seal gaps, joints and penetrations before they become difficult to access.
At BPC Ventilation, our airtightness range includes tapes, membranes, primers, sealants, foil tapes and liquid membrane solutions suitable for a range of construction and ventilation applications.
Does an Airtight Home Still Need Ventilation?
Yes. Airtightness and ventilation are not the same thing.
Airtightness is about reducing uncontrolled air leakage through the building fabric.
Ventilation provides planned and controlled movement of air through the home.
As homes become more airtight, relying on random gaps around windows, floors or walls for fresh air becomes increasingly unsuitable. Instead, the property needs a ventilation strategy designed to supply or introduce fresh air while removing moisture and stale air where required.
This relationship is particularly important with Mechanical Ventilation with Heat Recovery (MVHR).
An MVHR system is designed to provide controlled supply and extract ventilation. Stale, moisture-laden air is extracted from areas such as kitchens and bathrooms while fresh filtered air is supplied to living spaces and bedrooms. Heat from the outgoing air can then be transferred to the incoming air through the heat exchanger.
A well-sealed building envelope gives the ventilation system greater control over where air enters and leaves the property. BPC's own airtightness guidance notes that airtightness is particularly important for MVHR because these systems work best within a controlled building envelope.
In other words, the aim isn't to build a home that cannot breathe. The aim is to control how it breathes.
How Can You Improve Air Tightness Before the Test?
Achieving good air tightness usually starts long before the final air pressure test.
It should be considered throughout the design and construction process.
Create a Continuous Airtight Layer
The airtight layer should form a continuous barrier around the heated area of the building.
Breaks in this layer can create leakage paths, so connections between walls, floors, ceilings, windows and doors all need careful attention.
Seal Service Penetrations
Every pipe, cable or duct that passes through the airtight layer creates a potential leak.
These penetrations should be properly sealed using products suited to the material and application rather than simply being filled at the end of the build.
Use Suitable Airtightness Tapes and Sealants
Dedicated airtightness products are designed for sealing joints, membranes and building connections.
Depending on the application, this may include:
- Airtight sealing tape
- Vapour-control or airtightness membranes
- Airtight sealants
- Primers for difficult or porous surfaces
- Window sealing tapes
- Liquid airtight membranes
Check Details Before They Are Covered
One of the biggest advantages of dealing with air leakage during construction is accessibility.
A poorly sealed connection is much easier to correct before plasterboard, flooring or other finishes are installed.
Making airtightness part of regular site checks can therefore prevent much more difficult remedial work later.
When Should an Air Tightness Test Be Carried Out?
The final air tightness test is normally carried out when the building envelope is sufficiently complete for an accurate measurement to be taken.
However, for projects targeting particularly good levels of airtightness, carrying out checks earlier in the construction process can also be valuable.
Identifying leakage while key construction details are still accessible gives the installer or builder a chance to correct problems before final finishes are applied.
The exact testing process and timing should be discussed with your air tightness tester, energy assessor or building-control professional.
What Happens if a New Build Has Too Much Air Leakage?
If the measured result does not meet the required performance for the project, the source of the leakage will need to be investigated.
Some leaks may be relatively straightforward, such as an unsealed service penetration or loft hatch. Others can be hidden behind finishes or occur across several junctions throughout the property.
Depending on the issue, remedial work could involve:
- Resealing windows or doors
- Sealing around pipes and cables
- Repairing or reconnecting airtightness membranes
- Sealing gaps around ductwork
- Treating difficult junctions with airtight sealant or liquid membrane
- Correcting gaps around floors, ceilings or wall connections
This is another reason why airtightness should be considered as part of the construction process rather than something to think about only when the test date arrives.
Plan Airtightness and Ventilation Together
Airtightness is only one part of creating an energy-efficient new home.
As uncontrolled air leakage is reduced, having the correct ventilation strategy becomes increasingly important.
If you're planning an MVHR or MEV system for a new build, BPC Ventilation can help you determine the ventilation requirements for the property before installation begins.
With our Free Estimate + Layout Guide, you can send us your plans and project details for review by our design team. The service can provide room-by-room airflow figures, valve requirements and guidance on unit location, valve positions and intake and exhaust routes.
Planning the ventilation system alongside the building envelope helps ensure that both elements are considered as part of the overall project rather than independently.
Frequently Asked Questions
What does an air tightness test measure?
An air tightness test measures uncontrolled air leakage through the building envelope while the property is subjected to a controlled pressure difference.
Is a lower air tightness result better?
Generally, a lower measured air permeability indicates a more airtight building because less air is leaking through unintended gaps.
However, the required target should always be confirmed against the energy calculations and Building Regulations applicable to the individual project.
Is air tightness the same as insulation?
No, insulation is primarily used to reduce heat transfer through elements such as walls, roofs and floors.
Air tightness focuses on reducing uncontrolled air movement through gaps, cracks and penetrations.
Both contribute to the overall thermal and energy performance of a building.
Can a house be too airtight?
A highly airtight home requires an appropriate ventilation strategy.
The solution isn't to deliberately leave uncontrolled gaps within the building fabric. Instead, ventilation should be properly designed so that fresh air enters and stale, humid air is removed in a controlled way.
Is MVHR suitable for airtight new builds?
MVHR is particularly well suited to modern, well-sealed properties because it provides controlled supply and extract ventilation while recovering heat from outgoing air.
The ventilation system still needs to be correctly sized and designed for the individual property.
Getting Your New Build Ready
Good air tightness comes down to planning, suitable materials and attention to detail throughout construction.
By creating a continuous airtight layer, carefully sealing penetrations and coordinating the building fabric with the ventilation design, you can reduce uncontrolled air leakage and create a home that is easier to ventilate efficiently.
