NatHERS Design Factors
Air Leakage and Home Performance
Air leakage can quietly undermine a home’s thermal performance. Gaps, cracks and draughts allow conditioned air to escape and outdoor air to enter, making the home harder to keep comfortable.
Air leakage in brief
Air leakage is uncontrolled air movement through gaps, cracks and junctions in the building fabric. It can reduce comfort, increase heating and cooling demand and weaken the real performance of insulation, glazing and other thermal design features. Good home performance needs both draught control and intentional ventilation.
What air leakage means in practice
Air leakage happens when outdoor air enters the home, or indoor air escapes, through uncontrolled gaps in the building fabric. These gaps can be small, but their combined effect can be significant.
Common leakage points include gaps around windows and doors, service penetrations, exhaust fans, recessed lights, wall junctions, skirting boards, floor edges, roof spaces and poorly sealed construction details.
Unlike intentional ventilation, air leakage is not controlled. It may happen at the wrong time, in the wrong place and at the wrong rate, which can make rooms feel colder, hotter or draughtier than expected.
Why air leakage affects home performance
A home’s thermal performance depends on how well the building fabric separates indoor conditions from outdoor conditions. Insulation, glazing and shading all help manage heat flow, but uncontrolled air leakage can bypass those features.
In cooler weather, warm indoor air can leak out and cold outdoor air can enter. In hot weather, cooled indoor air can escape while warm outdoor air enters. This can increase heating and cooling demand and reduce comfort.
The result is a home that may technically have good insulation or glazing, but still feels uncomfortable because the building envelope is not well controlled.
The practical point
Insulation slows heat transfer through materials.
Air sealing helps stop heat being carried around those materials through uncontrolled gaps and draughts.
Air leakage and NatHERS ratings
A NatHERS assessment estimates how much heating and cooling a home may need to remain comfortable in its local climate. Air movement and infiltration assumptions can influence this performance picture because uncontrolled air exchange affects heating and cooling demand.
The way airtightness is represented depends on the assessment software, project details and applicable modelling settings. Even where a standard assumption is used, the actual built outcome can still be affected by construction quality and sealing details.
This is why air leakage should not be seen only as a modelling issue. It is also a construction quality issue. A well rated design can underperform in real life if gaps and junctions are not properly managed on site.
Common air leakage points
• Gaps around windows and external doors
• Junctions between walls, floors and ceilings
• Service penetrations for plumbing, electrical and mechanical systems
• Exhaust fans, rangehoods and duct penetrations
• Recessed lights and ceiling penetrations
• Gaps to roof spaces, subfloor areas, garages or unconditioned zones
Airtightness is not the same as no ventilation
One of the most common misunderstandings is that airtightness means sealing a home so it cannot breathe. That is not the aim. The goal is to reduce uncontrolled leakage while still providing intentional ventilation.
Uncontrolled air leakage is accidental. It depends on wind, pressure differences, gaps and construction quality. Ventilation is deliberate. It can come from operable windows, trickle vents, exhaust systems or mechanical ventilation where appropriate.
A better performing home usually needs both: a more controlled envelope and a clear strategy for fresh air, moisture management and indoor air quality.
Air leakage, insulation and glazing
Air leakage can reduce the real benefit of insulation and glazing. If air can move freely through gaps, the home may lose heat or gain heat even when the specified materials look strong on paper.
For example, ceiling insulation may perform poorly if air leakage allows conditioned air to escape into the roof space. High performance windows may also underperform if the frames, reveals or surrounding junctions are not well sealed.
This is why air sealing should be considered alongside insulation, window design and construction detailing, rather than treated as a minor finishing issue.
Common misunderstanding
A home can have strong insulation values and still feel draughty.
Thermal performance depends on both material performance and how well the building envelope is sealed and constructed.
Air leakage and comfort
Draughts can make a room feel colder than the measured air temperature suggests. Moving air across the body can reduce comfort, especially in cooler weather or near windows, doors and floor edges.
Air leakage can also create uneven temperatures between rooms. Some spaces may feel difficult to heat or cool, while others may be more stable. This can lead to greater reliance on mechanical systems even when the overall design appears efficient.
Comfort is one of the main reasons air leakage matters. A home that is well sealed, well ventilated and well insulated can feel calmer and more stable through seasonal changes.
Air leakage and moisture
Air leakage can also affect moisture movement. Warm air can carry moisture into colder parts of the building fabric, where condensation risk may increase if the construction is not detailed properly.
This does not mean every home should simply be sealed without a ventilation strategy. Moisture control depends on climate, construction type, vapour control, ventilation, occupancy and building detailing.
The aim is a controlled building envelope, not an accidental one. Good design reduces uncontrolled leakage while providing fresh air and moisture management in a deliberate way.
Design and construction details that can help
• Clear air sealing strategy for junctions and penetrations
• Well sealed window and door frames
• Coordinated service penetrations and ducting details
• Separation between conditioned spaces and garages, roof spaces or subfloors
• Construction sequencing that protects insulation and sealing layers
• Intentional ventilation strategy for fresh air and moisture management
How air leakage connects to compliance
For new homes, air leakage sits within the broader building fabric and construction quality conversation. While many NatHERS discussions focus on star ratings, glazing and insulation, the real performance of the home also depends on how the building envelope is built.
Projects targeting or required to achieve a 7 Star Rating may need stronger attention to construction details, especially where the design already has thermal pressure points such as large glazing, exposed floors or complex roof forms.
In NSW, the thermal performance pathway may also need to align with BASIX documentation, while Whole of Home considers broader energy systems and equipment.
Design considerations for Australian homes
Air leakage should be considered early enough that sealing details can be built into the design and construction documentation. This is especially important around windows, doors, service penetrations, roof spaces and transitions between conditioned and unconditioned areas.
The right approach also depends on climate. Cooler climates may place more emphasis on reducing heat loss and draughts. Warmer climates may focus on controlling hot air infiltration while still supporting ventilation and moisture management.
The best outcomes usually come from treating airtightness and ventilation as connected design decisions, rather than leaving draught control to the end of construction.
Working with Certified Energy
Certified Energy provides NatHERS assessments for new homes, townhouses and multi residential projects across Australia. Our team can help project teams understand how the building fabric, documentation and design assumptions affect the thermal performance pathway.
Where needed, we can help identify whether the NatHERS result is being influenced by glazing, insulation, shading, orientation, roof colour, climate zone, floor construction or broader envelope design. We can also help connect the assessment with related requirements such as NatHERS, BASIX, 7 Star Rating and Whole of Home.
For a broader explanation of the rating framework, visit our NatHERS Knowledge Hub.
FAQ
Does air leakage affect home performance?
Yes. Air leakage can affect home performance because uncontrolled gaps and draughts allow warm or cool air to escape and outdoor air to enter, increasing heating and cooling demand.
Does NatHERS consider air leakage?
NatHERS modelling includes assumptions about air movement and infiltration. The way airtightness is treated depends on the software, project details and assessment pathway.
Is airtightness the same as ventilation?
No. Airtightness is about reducing uncontrolled air leakage through gaps and cracks. Ventilation is about providing intentional fresh air through windows, vents, fans or mechanical systems.
Can a well insulated home still be draughty?
Yes. Insulation and air sealing do different jobs. A home can have good insulation values but still feel draughty if gaps, junctions and penetrations are not properly managed.
When should air leakage be considered?
Air leakage should be considered during design and construction documentation, especially around windows, doors, penetrations, junctions and transitions between conditioned and unconditioned spaces.
