Airtightness is a crucial part of any build, and it’s often treated as something that needs to be done at the end. However, that’s not the case.
Airtightness testing should be a continuous thought throughout the process and with new airtightness standards coming into force in England, it could be a costly mistake not to think about them right from the start.
Failing at completion doesn’t just mean a bad number on a certificate – it means ripping back finished walls and chasing trades who’ve already left site.
The Future Homes Standard takes effect on 24 March 2027. While there is a transition window for projects already in the system, we’ve put together a guide to the current airtightness rules and regulations, what’s changing, and five ways you can build airtightness in from day one.
What are the current airtightness rules and regulations?
Current airtightness rules and regulations are found under Part L of the Building Regulations. They state that almost every new build must be air-tested before completion. Here are some key points in the current airtightness standards every contractor should be aware of:
- The limit: the maximum permitted air permeability for a new dwelling is 8 m³/(h·m²) at 50 Pa.
- The reference figure: the reference home in the energy calculation assumes 5 m³/(h·m²) at 50 Pa, so in practice most new homes are designed to achieve around 5 or better.
- Design and as-built calculations: your design air permeability goes into the energy calculation before work starts, and the measured result replaces it once the building is tested. A home that tests worse than its design figure has to make up the shortfall elsewhere in the calculation.
- The test standard: dwellings are tested to ATTMA TSL1, and non-domestic buildings to TSL2.
- The paperwork: test results must be reported to building control within seven days of the test.
What are the new airtightness standards?
The new airtightness standards come with the 2026 edition of Approved Document L, and apply in England only, as Scotland, Wales and Northern Ireland each have their own rules. Here’s what changes for anyone building in England.
- Tighter reference airtightness: the notional dwelling now assumes 4 m³/(h·m²) at 50 Pa, down from 5.
- The backstop hasn’t moved: it stays at 8 m³/(h·m²), but a home that tests close to it will struggle to pass the overall carbon calculation.
- A building-by-building transition: current standards can still apply where a building notice, initial notice or building control full plans application is submitted before 24 March 2027 and work on that building starts before 24 March 2028. Otherwise, the new airtightness rules and regulations apply in full.
- More mechanical ventilation: the notional dwelling assumes decentralised mechanical extract ventilation (dMEV), and many builders choose MVHR, which means more ducting through the airtight layer.
For higher-risk buildings, these airtightness rules and regulations start on 24 September 2027.
1. Draw the airtight line, and keep it unbroken
Meeting the new airtightness standards starts on the drawings. If you can’t trace the airtightness layer around the whole building on your drawings without lifting your pen, the site team won’t be able to build it.
Mark it on every plan and section as one continuous line, and give special attention to the places it’s most likely to break:
- wall-to-floor and wall-to-roof junctions
- window and door reveals
- party wall junctions
- changes of construction type, such as masonry meeting timber frame
Most leakage happens where one element hands over to another, so design the transition detail deliberately. Every junction should be resolved on the drawings, not improvised on site.
2. Plan your service penetrations before the services arrive
Every pipe, cable, duct and flue is a hole in your airtight layer. The more of them there are, and the less thought that goes into them, the harder they are to seal.
At the design stage, group services where you can. Route MVHR or dMEV ducting so it passes through the airtight layer as few times as possible, and specify sealed grommets, collars and airtight-rated fittings up front.
Think about recessed downlights and electrical back boxes on external walls too. A service void, so trades can run cables inside the airtight layer rather than through it, is one of the most inexpensive things you can design in.
3. Specify membranes and laps as carefully as you specify insulation
A membrane is only as good as its joins. Tapes fail on dusty, damp or incompatible surfaces, laps that are too short peel over time, and unsealed fixings quietly undo good work.
Specify compatible membranes, tapes and sealants from the same system where possible. Follow the manufacturer’s lap requirements, and detail how the membrane seals to each adjoining element. Pre-compressed tapes and proper reveal detailing around windows matter more than most people expect.
Then make sure the spec is actually on the drawings, not just in a document nobody opens on site.
4. Sequence the trades around the airtight layer
Airtightness is often compromised not by bad materials but by bad order of work. If the electrician’s first fix comes after the plasterboard is up, or the plumber cuts through a taped membrane the day after it was signed off, the damage is done.
Build the sequence into the programme:
- windows in and sealed before membranes are taped to them
- the airtight layer completed and inspected before it’s covered
- services running in the void, not through the layer
- a hold point before closing up, so nobody boards over an untested seal
Pre-testing while the membrane is still exposed is one of the easiest ways to check the layer before it’s covered. For more information on pre-testing, read our When to Book Your Airtightness Test guide.
5. Brief every trade, and keep the conversation going
The airtight layer is the one part of the building that every trade can damage, and no trade thinks they own.
Put it on the agenda at pre-start, and explain to each trade where the layer is, why it matters and who to tell if they need to breach it. Name one person on site who’s responsible for it, and take photographs of the layer before it’s covered so there’s a record.
The aim is that when an electrician needs an extra cable run, they ask first and seal afterwards. Fixing that conversation costs nothing. Fixing the leak, if the test fails, can cost a lot.
Bring us in early
At Air Tightness Solutions, we’d rather be part of the conversation while the drawings can still change and the new airtightness standards can be designed in from the start.
Our ATTMA-accredited engineers can support you from design-stage drawing reviews and periodic site inspections through to a pre-test while the membrane is still exposed. We’ll then carry out your final pre-completion test, with on-site certification and same-day retesting if you need it.
Speak to our team today to learn more about our airtightness testing services or book your airtightness test today.
Frequently asked questions
Does the Future Homes Standard apply to my project?
It depends on when your building control application is submitted and when work starts on each building. If a building notice, initial notice or building control full plans application goes in before 24 March 2027 and work on the building starts before 24 March 2028, current standards can still apply. If they are submitted after 24 March 2027 and building works don’t start until after 25 March 2028, then the new airtightness standards apply.
When should I book my airtightness test?
The final test takes place before completion, but we recommend a pre-test once the airtightness layer is sealed and before plasterboard, skirting and flooring go in, so any leaks are still easy to fix.
Do all new builds need an airtightness test?
Almost all new build properties need to pass an airtightness test under current airtightness rules and regulations. The result feeds into the energy calculation that shows the building meets its target, so a poor result can cause a compliance problem as well as a leak. If you’re new to the process, our guide to what airtightness testing involves covers the basics.
What happens if my building fails the test?
We can help identify where the air is escaping, so the leaks can be sealed and the building retested. The later a failure is found, the more it costs, because finished work such as skirting boards and plasterboard may need to come off to reach the problem areas. That’s why we recommend a pre-test, and we offer same-day retesting so you don’t lose time.
How long does an air tightness test take?
For a typical home, the test itself normally takes around an hour once the building is set up. You’ll receive an on-site draft certificate, with the full report to follow.