🏡 Part L 2026: Airtightness Becomes a Core Compliance Driver
Part L 2026 pushes air tightness, technical known as air permeability, to the forefront of building performance, tightening expectations for both new-build and retrofit projects. The goal is simple: reduce uncontrolled heat loss, improve SAP outcomes, and ensure homes are genuinely ready for low‑carbon heating. With airtightness now directly tied to energy efficiency, comfort, and compliance, the quality of detailing on site matters more than ever.
Airtightness targets are also becoming more realistic and performance‑led. For new homes, achieving around 4 m³/h·m² @50Pa is increasingly expected to meet design-stage SAP requirements. For existing homes, anything around 5 m³/h·m² @50Pa is considered very good—especially when you consider that RdSAP 10 assumes a default of 15, meaning many homes are performing far better than the software gives them credit for. Testing is therefore essential to avoid unnecessary penalties in EPC ratings.
📐 Airtightness in New Dwellings
The 2026 guidance sets out a detailed, design‑led approach to airtightness. The emphasis is on continuity, clarity, and durability.
🧩 Clear and Continuous Air Barrier
- Drawings must show the exact position and continuity of the air barrier.
- Designers and installers should review these together to ensure materials and junctions are fully specified.
🔌 Service Penetrations Managed Properly
- Group incoming services to minimise penetrations.
- Use grommets, collars and air‑sealing tapes around all service entries.
- Core‑drill internal service penetrations and seal with proprietary systems.
🧱 Structural and Fabric Detailing
- Seal all structural penetrations; consider joist hangers to avoid inner‑leaf breaches.
- Masonry walls require fully filled mortar joints and, where dense blocks are used, internal parge coats or liquid membranes.
- Plasterboard alone is no longer acceptable as an air barrier.
🌲 Timber Frame Requirements
- Vapour control layers must overlap, be taped, and repaired if damaged.
- Sheathing boards used as air barriers must be taped at all junctions.
🚪 Openings and Access Points
- Windows and doors must connect directly to the primary air barrier and be taped to structural openings.
- Loft hatches must be designed and installed to achieve airtightness at ceiling level.
🧱 Preventing Air Movement Within Thermal Elements
To maintain insulation performance:
- Insulation must abut the air barrier, or
- The void between insulation and barrier must be fully filled.
🛠 Airtightness in Existing Homes
Retrofit airtightness focuses on reducing uncontrolled leakage while maintaining safe ventilation.
🔧 Practical Measures
- Seal around new or existing service penetrations.
- Draught‑proof and fill gaps when renovating thermal elements.
- Ensure replacement windows, doors, roof windows and rooflights are well fitted and reasonably airtight.
⚠️ Ventilation Considerations
Improving airtightness must be coordinated with:
- Approved Document F (ventilation)
- Approved Document J (combustion appliances)
This ensures moisture and combustion safety are not compromised.
📊 Why Airtightness Testing Matters More Than Ever
With Part L 2026 tightening expectations, airtightness testing is no longer a box‑ticking exercise. It directly affects:
- SAP scores
- Heating system sizing
- Energy bills
- Comfort and draught control
- Compliance evidence for Building Control
For new builds, aiming for around 4 m³/h·m² @50Pa is becoming the norm. For existing homes, achieving around 5 m³/h·m² @50Pa is excellent—and far better than the RdSAP 10 default of 15, which can significantly under‑represent real performance unless a test is carried out.
Airtightness is now one of the most cost‑effective ways to improve EPC outcomes, reduce heat loss, and future‑proof homes for low‑carbon heating.
🤝 How Building Compliance Testing Can Help
We support developers, architects and contractors through every stage of the new standards:
- SAP 10.3 modelling & Part L testing
- Ventilation testing & commissioning (Part F)
- Airtightness testing to 2026 expectations
- Low‑carbon system verification
We make compliance simple, predictable, and stress‑free.
Have a project in mind? Let’s get to work.
Need help with your retrofit renovation project or PAS-2035 compliance advice? Get in touch with our team today.