Can an Airtightness Test Improve Your EPC Rating?

Why EPCs Assume the Worst by Default

When a Domestic Energy Assessor (DEA) produces your EPC and no measured airtightness test exists, RdSAP falls back on a standard default value rather than your property's actual performance. That default is set deliberately conservatively - commonly cited at around 15 m³/(h·m²) @50Pa (air permeability measured at a 50-pascal pressure difference) - because assuming the worst case is the safe option when there's no real data to work from.

In practice, most existing homes perform considerably better than that. Industry sources report typical measured results in the 6–10 m³/(h·m²)@50Pa range for ordinary existing housing, and well-sealed properties testing as low as 3–4. That gap between the conservative default and what a real test usually finds is exactly why this matters: a meaningful number of landlords are currently being scored against an assumption that's actively worse than how their property actually performs.

What "Air Permeability" Is Actually Measuring

Air permeability measures uncontrolled air leakage - draughts escaping through gaps around floorboards, skirting, loft hatches, service penetrations, and construction joints - expressed as the volume of air (in cubic metres) that escapes per hour per square metre of the building's external envelope, at a standardised 50 pascal pressure difference. It's not measuring your intentional ventilation (trickle vents, extractor fans, chimneys left open) - those are accounted for separately in the EPC calculation. It's specifically about the leakiness of the building fabric itself.

Two test methods are currently recognised for this: the traditional blower door test and the newer, less intrusive pulse test. Both produce a comparable air permeability figure, but they work quite differently and suit different situations - covered in full in our companion guide, Pulse Test vs Blower Door Test. The rest of this section walks through what actually happens on the day, for either method.

How the Test Actually Works

The Blower Door Test

This is the longer-established method, and the one most people picture when they hear "air test."

1. Preparation. Before the tester arrives, all external windows and doors need to be closed and locked, except the one that will hold the test equipment. Internal doors, by contrast, are left open, so the whole building is tested as a single connected space. Any gas appliances - boilers, water heaters, pilot lights - are turned off to avoid dangerous back-drafting once the pressure inside the building drops, fireplace dampers are closed, and any mechanical ventilation, extractor fans, and HVAC systems are switched off. A loft hatch or crawl space access stays open only if that space is meant to be part of the building's heated envelope.
2. Setup. The tester mounts a temporary frame with a calibrated fan into the chosen external doorway - this is the "blower door" itself - along with a manometer, the instrument that measures the pressure difference between inside and outside the property. Installation typically takes 5–10 minutes.
3. The test. The fan depressurises the building, drawing air out until a steady 50 pascal pressure difference is reached - roughly equivalent to the effect of a 20 mph wind hitting every side of the building at once. The tester takes an outdoor baseline pressure reading, runs the fan, and the manometer measures how much air the fan has to move to sustain that pressure - which is a direct measure of how much air is leaking in through gaps elsewhere in the building.
4. Optional diagnostics. With the fan running, a tester can walk the property with a smoke pen or their hand to physically locate specific leaks (around loft hatches, skirting, service penetrations) - useful for a landlord who wants to fix leaks before retesting, though this adds time beyond the basic measurement.

A basic blower door test - install, preparation, and the core measurement - typically takes around 30 minutes or less; adding diagnostic leak-hunting on top can extend a visit to an hour or more, depending on the size and complexity of the property.

The Pulse Test

This is the newer, less intrusive method, and increasingly the more practical choice for a tenanted rental.

1. Setup. A single, portable air receiver unit is positioned on the ground floor, along with a small compressor and a touch-screen control unit. There's no need to seal a doorway or install a large fan rig.
2. Charging. The compressor fills the air receiver with compressed air, up to around 10 bar.
3. Measurement. The tester measures the building's internal volume and external envelope area - the inputs the calculation needs - then triggers one or more short "pulses" of air from the receiver into the building, raising the internal pressure by a much smaller amount than a blower door test (around 4 pascals, rather than 50). Sensitive sensors track how quickly that small pressure rise dissipates as air escapes through the building's leakage paths, and the system calculates the equivalent air permeability figure, extrapolated up to the same 50 Pa basis used for blower door results, so the two methods remain directly comparable.
4. Result. Results are calculated and displayed on-screen within the same visit.

A typical pulse test takes under 15 minutes in total - no large equipment to install, no need to identify and close off a specific doorway, and less disruption to a property that's still being lived in.

Which One Fits a Tenanted Rental Better

Because the pulse test doesn't need a doorway sealed off, doesn't rely on a large visible rig, and takes a fraction of the time, it's generally the more practical option where a tenant is in situ and a landlord doesn't want to arrange a lengthy access visit. A blower door test remains equally valid for the EPC calculation, and may still be preferred where a landlord specifically wants the diagnostic leak-hunting a blower door test allows - useful if the plan is to seal specific leaks before improving the fabric standard, rather than just to obtain a number for the EPC. Our Pulse Test vs Blower Door Test guide covers this trade-off in more depth.

How Much Difference Can It Actually Make

Industry reporting on RdSAP 10 suggests a measured result can shift an EPC score by somewhere in the region of 1 to 3 points, depending on how far your actual result beats the default, and how the rest of the property's fabric performance is scoring. That's not usually transformative on its own, but on a borderline property - one sitting just below a band boundary - a few points can be the difference between passing and not.

It's worth being realistic about this rather than treating it as a guaranteed win:

• The improvement only happens if your actual result is better than the 15 m³/(h·m²)@50Pa default. If your property genuinely is that leaky (older, poorly maintained, draughty floors and doors, no meaningful sealing work ever done), a test could in principle produce a result close to, or in rare cases worse than, the assumed default - meaning there'd be little or nothing to gain from paying for one.
• It works best as one part of a broader fabric performance picture, not a substitute for insulation, glazing, or draught-proofing measures. A very airtight but poorly insulated property still won't score well overall.
• It's genuinely optional - a DEA doesn't need a test to issue a valid EPC. This is something a landlord chooses to commission, not something required of them.

The properties most likely to benefit are ones a landlord already has reason to believe perform better than a "worst case" assumption - recently draught-proofed, well-maintained doors and windows, sealed floor voids, or simply built to a reasonable standard for their age without ever having been formally tested.

Who Can Carry Out the Test - and What Your Assessor Needs

Not just any test will do. For a DEA to actually use a measured result in place of the RdSAP default, the test needs to come from a recognised route:

• A tester accredited under the Elmhurst Airtightness Scheme (EAS) or the Air Tightness Testing & Measurement Association (ATTMA) can provide a certificate that's straightforwardly accepted.
• An independent tester outside those schemes can still be used, but the assessor will need the full test report plus proof of equipment calibration - within the last 12 months for a blower door device, or 24 months for pulse test equipment.

Once your assessor has valid evidence, the measured figure replaces the default in the calculation, and the resulting air permeability value appears in the EPC's performance breakdown alongside the rest of your fabric data.

Does the Cost of Testing Count Toward Your MEES Budget?

This connects to the broader PRS MEES cost cap, though with one caveat worth being upfront about: the government's January 2026 policy response confirms that the cost of specialised retrofit advice can be included within the overall £10,000 cost cap (with the exact eligible amount to be confirmed in legislation), and that the cost of the EPCs commissioned before and after improvement works can also count. An airtightness test arguably sits in that "diagnostic and specialist advice" category rather than being an installed fabric measure itself - but the report doesn't name air permeability testing specifically, so treat this as a reasonable, not confirmed, interpretation until the detailed guidance is published.

HEM Is Coming - and Testing May Stop Being Optional

This is worth knowing before deciding whether to bother: RdSAP 10, the methodology in use today, treats air permeability testing as optional and falls back on a default when no test exists. Independent technical guidance on the Home Energy Model (HEM) - the methodology set to replace RdSAP under EPC reform - indicates that HEM will treat a measured air pressure test result as a required input rather than an optional improvement, using it to model infiltration far more precisely (splitting leakage across different parts of the building envelope and modelling it against actual local wind conditions, rather than RdSAP's single simplified estimate). We haven't been able to confirm this directly against GOV.UK's own HEM documentation, so treat it as likely rather than certain - but if accurate, it changes the calculus meaningfully: testing now, while it's still optional and can boost a RdSAP 10 score, also gets a landlord ahead of what may become a mandatory input once reformed EPCs arrive.

That has an obvious connection to grandfather rights: if a measured test result is what pushes a property over the line to an EER of C before 1 October 2029, that property locks in compliance under today's system for up to 10 years - sidestepping the new metrics, and the mandatory testing that may come with them, altogether.

Practical Steps

1. Consider whether your property has good, if unproven, reason to test better than a "worst case" assumption - recent draught-proofing, well-sealed floors, decent doors and windows - before paying for a test speculatively.
2. Book a test with an EAS or ATTMA-accredited tester where possible, to keep the evidence straightforward for your assessor.
3. A pulse test is generally quicker and less disruptive for an occupied rental than a blower door test - see our comparison guide for which suits your situation.
4. Provide the certificate (or report plus calibration evidence) to your assessor when the EPC is commissioned, so the measured figure is used from the outset rather than requiring a re-assessment later.

Key Takeaways

• RdSAP 10 defaults to a conservative air permeability assumption (commonly ~15 m³/(h·m²)@50Pa) unless a measured test overrides it.
• Most existing homes measure tighter than that default, which is why testing can improve a score - typically by an estimated 1–3 points, though this varies by property.
• The test only helps if your property genuinely performs better than the default - it isn't a guaranteed win, and a genuinely leaky property has little to gain.
• Only tests from EAS/ATTMA-accredited testers, or independent testers with valid calibration evidence, can be used by your assessor.
• HEM may make this kind of testing a required input rather than an optional one - worth treating current testing as preparation, not just a one-off score boost.

Frequently Asked Questions