Airtightness, vapour control and windproofing explained
Airtightness limits uncontrolled airflow through the envelope. Vapour-control layers moderate moisture movement by diffusion. External windproofing limits air washing through insulation and supports weather protection. One product may perform multiple functions, but each function needs continuous, project-specific design.
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Three functions, three questions
An air leak can carry warm, moist indoor air through gaps, affecting comfort, energy use and assemblies. Diffusion is the slower movement of vapour through materials. A vapour-control product is therefore not automatically airtight at its edges, while an airtight joint alone does not prove a safe diffusion design.
The external windproof layer reduces air movement through insulation and contributes to weather defence. It is not the internal airtight layer. Functional separation makes reviews clearer; its limitation is that the actual layer sequence depends on climate, materials and project conditions and must be designed professionally rather than copied as a universal recipe.
- Request the airtight line on plans and sections
- Ask which layer controls vapour in each assembly
- Review wind and rain junctions separately
- Do not select products without the full moisture assessment
Definition: convection and diffusion
Convection carries moisture with moving air through leaks; diffusion passes vapour through materials under a vapour-pressure difference. They require different checks.
Continuity is won at junctions
Sheet areas are rarely the whole challenge. Windows, bases, floors, roofs and service penetrations interrupt layers. The airtightness plan should show how each interruption is bridged and who owns each trade handover.
Late service changes create unplanned holes. A service cavity can reduce penetrations but cannot replace coordination. Ask when details remain visible for inspection, whether substrates are suitable and how repairs are recorded before linings conceal them.
- Maintain a penetration schedule across services
- Approve sample junctions before repetition
- Assign trade-boundary responsibility
- Locate pre-close photographs precisely
Test as a process, not a ceremony
An airtightness test is valuable while leaks remain accessible; an agreed final test records completion. The designer defines project targets and method. One result does not locate every cause or validate vapour and weather functions.
An owner worksheet should list test stages, building preparation, leak investigation, corrective action and final report. A tight envelope still needs ventilation, but the air change is intentionally planned rather than left to random gaps.
- Book an accessible-stage test
- Agree leak log and retest route
- Record temporary seals
- Keep reports and detail images in handover records
Hypothetical junction issue
At a fictional pre-lining test, airflow is found around a cable bundle. The design team identifies the airtight layer, the responsible trade remakes the approved junction and records it. They also confirm that external wind protection and vapour-control functions remain intact. The owner receives a traceable verification path, not an improvised tape recipe.
Pitfalls to avoid
- Using airtightness and vapour barrier as interchangeable terms
- Choosing field materials while leaving bases, windows and penetrations undrawn
- Testing only after finishes make leaks inaccessible
Frequently asked questions
Does an airtight home still need ventilation?
Yes. Airtightness removes accidental leakage; healthy air change needs a planned ventilation approach.
Must vapour control be a plastic sheet?
No. The function depends on the assessed complete assembly.
Does an airtightness test prove all moisture safety?
No. It assesses air permeability under defined conditions, not every diffusion or rain-control function.
When should testing happen?
An accessible interim stage is useful, followed by the completed state when specified; exact stages belong in the test plan.
Related guides
Sources
These sources explain general planning principles, mainly in the German context. They do not confirm project scope, permission, funding or technical suitability. Local requirements need separate professional assessment.
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