Passive House Envelope Guide
Airtightness limits uncontrolled air movement through the building envelope. In a Passive House project, it must be designed as a continuous system, documented across every junction and verified through on-site pressure testing.
A building may contain substantial insulation and high-performance windows while still losing conditioned air through gaps between construction elements. Wall-to-floor junctions, window perimeters, roof connections and service penetrations can collectively undermine the intended performance of the envelope.
Passive House design addresses this by establishing a defined airtight layer around the conditioned building volume. The layer may be formed by membranes, boards, plaster, concrete or other suitable materials, but it must remain continuous across changes in construction.
This article focuses specifically on airtight-envelope design and testing. For the wider standard, five principles, PHPP and Australian climate application, see the Passive House in Australia Knowledge Hub.
In Brief
Airtightness Is a Designed and Tested Control Layer
Purpose
Reduce uncontrolled infiltration and exfiltration through joints, gaps and penetrations in the building envelope.
Design Requirement
The airtight layer must be traceable continuously around the conditioned building volume and resolved at every junction.
Verification
A blower-door test measures the completed building’s air leakage under a standardised pressure difference.
Airtightness does not mean eliminating ventilation. It replaces accidental air leakage with a deliberate ventilation strategy that can be designed, balanced and maintained.
Air-Control Layer
What Does Airtightness Mean in a Passive House?
Airtightness describes how effectively the building enclosure limits unintended air movement between inside and outside. Air can pass through small cracks, incomplete seals and interconnected cavities even when those openings are not visible from the finished room.
The airtight layer forms a continuous boundary around the conditioned space. On a drawing, the design team should be able to trace that boundary without lifting the pen. This is sometimes referred to as the red-pencil test.
The material forming the layer may change around the building. A wall membrane may connect to a concrete floor, internal plaster, roof board or window tape. The critical requirement is that each material is suitable for its role and that the transitions between materials are properly resolved.
Airtightness is therefore not one product installed by one trade. It is an envelope-wide design and construction responsibility.
Building Performance
Why Is Airtightness Important?
Energy Balance
Reduced Uncontrolled Air Exchange
Conditioned air is less likely to escape through gaps, while unwanted external air is less able to enter unpredictably.
Comfort
Fewer Draughts
Reducing leakage around floors, windows and junctions can limit localised draughts and uneven internal conditions.
Moisture Transport
Controlled Air Movement
Limiting air leakage reduces the uncontrolled movement of moisture-laden air into or through building assemblies.
Ventilation
Predictable Fresh-Air Delivery
A controlled envelope allows the ventilation system to supply and extract air through intended pathways rather than competing with random leakage.
Acoustics
Reduced Leakage Paths
Closing uncontrolled openings may also reduce some pathways through which external sound enters the building.
Quality Assurance
A Measurable Outcome
Pressure testing provides a measured result that can identify leakage and verify the completed envelope.
Envelope Boundaries
Is the Airtight Layer the Same as Insulation or Waterproofing?
Not necessarily. A building envelope contains several control functions. One material may perform more than one function, but the design team should not assume that insulation, cladding or a vapour-control layer is automatically airtight.
Air Control
Airtight Layer
Limits bulk air movement through the envelope and must remain continuous across joints and penetrations.
Thermal Control
Insulation Layer
Reduces conductive heat flow but may still allow air to pass through or around it when the air-control layer is incomplete.
Rain Control
Weather Barrier
Manages external rain and drainage. Its location and material may differ from the principal airtight layer.
Vapour Control
Diffusion Strategy
Influences vapour diffusion through an assembly and should be selected for the climate, materials and intended drying conditions.
Confusing these functions can create gaps in the air-control layer or inappropriate moisture conditions. The full wall, roof and floor assembly should be reviewed as a coordinated system.
Continuous Detailing
Where Does the Airtight Layer Commonly Fail?
Large uninterrupted surfaces are rarely the most difficult part of airtight construction. The greater risk occurs where materials, trades and building systems meet.
Wall-to-floor junctions. The wall air barrier must connect reliably to the slab, subfloor or lower-level envelope.
Wall-to-roof junctions. Top plates, trusses, rafters, parapets and ceiling systems can interrupt the intended layer.
Windows and doors. Frames must connect to the surrounding wall layer around the complete opening perimeter.
Service penetrations. Pipes, conduits, cables, ducts, exhausts and drainage lines require durable and accessible seals.
Structural penetrations. Beams, columns, brackets and balconies may pass through or interrupt the envelope.
Material transitions. Membrane, plaster, concrete, board and window systems require compatible connection methods.
Opening Installation
How Do Windows and Doors Affect Airtightness?
A high-performance window is not automatically airtight at the wall junction. The frame, opening mechanism, perimeter seals and installation interface all influence the completed result.
The internal air-control connection may use suitable tapes, membranes, sealants, plaster or proprietary transition systems. The selected detail must account for movement, substrate preparation, fixing methods and the sequence in which the window and wall layers are installed.
Window installation must also coordinate with insulation, thermal-bridge control, external weatherproofing and drainage. One seal should not be expected to perform every envelope function.
Large sliding doors, multiple-panel systems and complex opening arrangements may require particular attention because their seals, tracks and thresholds differ from simple fixed glazing.
Services Coordination
How Should Service Penetrations Be Managed?
Every penetration through the air-control layer creates a connection that must be sealed. The simplest strategy is usually to reduce the number of penetrations and coordinate their positions before construction.
A dedicated internal service cavity can allow electrical wiring, switches and smaller services to be installed without repeatedly puncturing the principal airtight layer. Larger ducts, pipes and structural penetrations still require documented connection details.
Penetrations should not rely on an informal application of expanding foam or sealant without considering the substrate, expected movement, service temperature, durability and access for inspection.
The architectural, structural, hydraulic, electrical and mechanical documentation should show a consistent envelope boundary so that trades understand when their work crosses it.
Controlled Fresh Air
Does an Airtight Passive House Still Receive Fresh Air?
Yes. Airtightness and ventilation perform different functions. Airtightness limits accidental leakage through the building fabric. The ventilation system introduces and removes air through deliberate, designed pathways.
In a typical residential Passive House, fresh air is supplied to living spaces and bedrooms while extract air is removed from kitchens, bathrooms and utility areas. Heat or energy recovery may reduce the conditioning energy otherwise associated with that air exchange.
The system must still be appropriately designed, installed and balanced. Airtight construction does not compensate for incorrect airflow, poor duct routing, excessive noise or inadequate maintenance access.
Occupants may still open windows. Window opening is an operational choice rather than the building’s only source of routine ventilation.
Moisture Management
Does Airtightness Cause Condensation or Mould?
Airtightness alone should not be treated as the cause of condensation. Moisture performance depends on the complete assembly, climate, internal humidity, temperature conditions, vapour-control strategy, weatherproofing, thermal bridges and ventilation.
Uncontrolled air leakage can transport significant moisture into cold parts of an assembly. A properly designed airtight layer helps limit that convective movement, but the surrounding materials must still be able to manage vapour and drying appropriately.
The correct location and vapour characteristics of membranes can vary between cool, temperate, hot-dry and humid climates. A detail copied from another climate may not provide an appropriate Australian moisture response.
Complex assemblies, internal insulation, cold surfaces or unusual material combinations may require separate condensation or hygrothermal analysis.
Australian Application
Does Airtightness Work in Warm Australian Climates?
Yes. Uncontrolled air leakage can increase heating demand in cool climates and cooling or dehumidification loads in warm climates. Airtightness supports a deliberate response in either case.
What changes between climates is the complete envelope and services strategy. Shading, cooling, humidity control, ventilation equipment, vapour management and material drying potential may require a different response from one Australian region to another.
In humid locations, external moisture conditions and cooling operation require careful coordination. In colder locations, winter heat loss and the temperature of internal surfaces may be more prominent design considerations.
The requirement for a continuous air-control layer remains, but its location, materials and connections should be developed for the actual project climate and construction system.
Pressure Testing
How Is Passive House Airtightness Tested?
A blower-door test uses a calibrated fan installed temporarily in an external opening. The fan creates controlled pressure differences between the inside and outside of the building while airflow measurements are taken.
The standardised result is commonly expressed as n50: the calculated air changes per hour at a pressure difference of 50 pascals. For a new building seeking Passive House certification, the applicable limit is normally no more than 0.6 air changes per hour at 50 pascals.
The 50-pascal test condition is used to create a repeatable measurement. It is not intended to represent the building’s normal everyday pressure condition or the amount of routine fresh air supplied to occupants.
The test boundary, building preparation, temporary sealing and reporting method should be agreed for the project and certification pathway before the final test occurs.
Testing Strategy
When Should Airtightness Testing Occur?
Construction-Stage Review
Interim Test
Completed while the air-control layer remains accessible, allowing leakage locations to be identified and repaired before linings or finishes conceal them.
Completed Building
Final Test
Measures the completed envelope after relevant doors, windows, services and sealing work have been installed in their final condition.
An interim test does not replace the final result required for certification, but it can reduce the risk of discovering extensive leakage only after the building has been completed.
Leakage Investigation
How Are Air Leaks Located?
The blower-door fan creates a pressure difference that can make leakage easier to detect. The testing team may use several techniques depending on the building stage and conditions.
Physical inspection and detection of air movement around joints, seals and penetrations.
Smoke or other suitable visualisation methods used in controlled conditions to reveal air paths.
Thermal imaging where sufficient temperature difference and suitable conditions make leakage patterns visible.
Targeted investigation of recurring risk areas such as window perimeters, thresholds, top plates and service routes.
The measured result identifies the overall leakage rate. Locating and repairing the individual leaks remains a separate diagnostic and construction task.
Delivery Process
How Is an Airtight Envelope Delivered?
Stage 01
Define the Envelope Boundary
Confirm which spaces sit inside the conditioned volume and trace the proposed air-control layer around plans and sections.
Stage 02
Select the Air-Control Materials
Identify the membranes, boards, plaster, concrete and connection products forming the layer in each part of the building.
Stage 03
Resolve Junctions and Penetrations
Develop details for floors, roofs, windows, doors, structure and building services before work reaches those locations.
Stage 04
Coordinate Construction Sequence
Confirm which trade installs each layer and connection and when the work remains accessible for review.
Stage 05
Inspect and Record the Work
Review critical connections before concealment and retain the construction records needed for the intended project pathway.
Stage 06
Test and Rectify
Complete interim and final testing through the appointed testing provider and address identified leakage while repair remains practical.
Project Responsibilities
Who Is Responsible for Airtightness?
Responsibility should be established explicitly. Without a coordinated plan, each trade may complete its own work correctly while leaving gaps between separate packages.
Design Team
Defines the envelope boundary and documents materials, transitions, openings and penetrations.
PHPP Consultant
Uses the appropriate design-stage or measured airtightness input within the developing building energy model.
Builder
Coordinates sequencing, trade responsibilities, protection, inspection and repair of the installed layer.
Trades
Install the relevant systems without damaging completed air-control work and seal approved penetrations correctly.
Testing Provider
Prepares and performs the agreed blower-door testing and issues the relevant test documentation.
Building Certifier
Reviews the accepted final test result and associated evidence where formal Passive House certification is pursued.
Certification Boundary
Does Passing the Airtightness Test Certify the Building?
No. Airtightness is one part of the Passive House assessment. A passing test does not independently establish compliance with heating or cooling demand, primary-energy, comfort, window, thermal-bridge, ventilation or wider documentation requirements.
Formal certification requires independent review of the complete project evidence by an appropriately accredited Passive House Building Certifier. The final airtightness result is incorporated into that wider assessment.
Similarly, a building that performs well in an airtightness test should not be described as a certified Passive House unless the full certification process has been completed successfully.
For the complete sequence and project-role boundaries, see the Passive House Certification Process Guide.
Existing Buildings
Can an Existing Home Be Made Airtight?
Potentially, although existing buildings can make continuity more difficult. Retained floors, roof structures, internal walls, chimneys, adjoining construction and concealed cavities may interrupt the proposed layer.
A retrofit may establish the air-control layer internally, externally or through a combination of systems. The selected approach affects finishes, room area, façades, junctions, moisture conditions and construction staging.
Existing leaks should not simply be sealed without understanding ventilation, combustion appliances, moisture sources and the broader building condition. The retrofit strategy must consider the complete building rather than isolated cracks.
For the complete existing-building pathway, see the Passive House Retrofit and EnerPHit Guide.
Common Delivery Risks
What Commonly Prevents an Airtight Result?
No clearly defined boundary. Drawings show products but not one continuous air-control strategy.
Unresolved material transitions. Each individual surface is airtight, but the connection between surfaces is incomplete.
Late service changes. New penetrations are introduced after the principal sealing work has been completed.
Incompatible substrates or products. Tapes and sealants are applied to dusty, wet, uneven or unsuitable surfaces.
Damage after installation. Completed membranes or seals are punctured by later trades and not repaired.
Testing only at completion. Leakage is discovered after access has been restricted by internal and external finishes.
Common Misunderstandings
What Airtightness Should Not Be Confused With
Airtight does not mean unventilated. Fresh air is provided through the designed ventilation system.
Insulation is not automatically airtight. Air can move through or around many insulation products.
A vapour barrier is not always the air barrier. The control layers may coincide, but their functions should be identified separately.
The blower-door result is not a ventilation rate. It is a standardised measurement of envelope leakage.
Passing one test is not building certification. Airtightness is one input within the wider Passive House assessment.
More sealant is not a complete strategy. Durable performance depends on design, substrate preparation, movement, sequencing and compatible connections.
Frequently Asked Questions
Passive House Airtightness FAQs
What is an airtight building envelope?
It is a continuous air-control boundary around the conditioned building volume that limits uncontrolled air movement through gaps, joints and penetrations.
What airtightness level does a Passive House require?
A new building pursuing Passive House certification normally requires an n50 result no greater than 0.6 air changes per hour at 50 pascals. The applicable requirement should be confirmed for the project and certification pathway.
What does n50 mean?
It expresses the calculated number of air changes per hour through the tested building envelope at a standardised pressure difference of 50 pascals.
Can windows still be opened in a Passive House?
Yes. Airtightness relates to leakage through the closed building envelope. Occupants can still open suitable windows and doors when desired.
Does an airtight building need mechanical ventilation?
A Passive House uses a planned mechanical ventilation strategy to provide and remove air deliberately. Airtightness should not be used without an appropriate ventilation response.
Is the airtight layer always a membrane?
No. Suitable membranes, boards, plaster, concrete and other systems can form part of the air-control layer. The material and connection method depend on the construction.
Does airtightness cause mould?
Not inherently. Moisture risk depends on the complete envelope assembly, climate, ventilation, humidity, weatherproofing, vapour control and thermal conditions. These elements must be designed together.
When should the blower-door test be completed?
An interim test is useful while repairs remain accessible. A final test is completed in the agreed finished condition to provide the result required for the project assessment.
Does Certified Energy conduct blower-door testing?
Blower-door testing is a separately appointed specialist service. The testing scope and provider should be confirmed for the individual project.
Does passing the airtightness test make the building a certified Passive House?
No. The test result is one part of the evidence. Formal certification requires independent review of the PHPP model, technical documentation and wider construction information.
Related Guidance
Explore the Connected Passive House Pathways
Passive House Envelope Review
Need to Clarify the Airtightness Strategy for a Passive House Project?
Send the available plans, sections, construction build-ups and Passive House objectives for an initial scope review. Certified Energy can help clarify the envelope information and PHPP inputs requiring coordination. Detailed architectural design, blower-door testing and formal certification remain separately appointed roles.
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