The 0.6 Guarantee — Guaranteed Airtightness Results

The 0.6 Guarantee

We guarantee an airtightness result of 0.6 air changes per hour or lower. Not a target. Not a design intention. A guaranteed result, on your building, verified by test.

Most airtightness products are sold on their properties. A vapour resistance, an air permeability figure, a fire classification. All useful — and none of them tell you whether the building will pass its air test.

That gap is where projects lose money. A failed test on a school or a housing scheme means remedial work on a finished envelope, a second test, and a programme that has already moved on. The materials performed exactly as specified. The building still failed.

So we guarantee the outcome instead. Where we supply and install, we guarantee 0.6 air changes per hour or lower — the Passivhaus threshold — verified by an air test on the completed envelope.

What does 0.078 actually mean?

Wester Hailes High School, Edinburgh. A 27,575 m³ steel-framed school with concrete floor and roof slabs, SFS and sheathing board infill, render and rainscreen cladding, aluminium glazing and a hot melt membrane roof — built by McLaughlin & Harvey Construction.

The design target was 0.600 air changes per hour at 50 Pa. The whole-building pre-test after our works returned 0.078.

That is not our figure. It was measured by EDP Building Sciences (an RSK company, UKAS Testing Laboratory 2546) on 21 November 2025 by an ATTMA-registered test engineer, to BS EN 13829:2001 and ATTMA Technical Standard L4, with the envelope volume certified by the project’s Passivhaus assessor. Correlation on the test run was 0.998.

Read it for what it is: a pre-test. Fixed services were not yet installed and two door openings were temporarily screened — which is the point of a pre-test. It isolates the fabric and proves the airtight line before the building is closed up. The building is tested again after first fix. We publish the pre-test because it is the number that shows what our works did, and because a result this far below target is what makes a guarantee at 0.6 a safe promise rather than a brave one.

How the guarantee works

1. We review the drawings. Send us the design and our technical team marks up where the airtight line runs, where it is at risk, and what each junction needs.
2. We agree the scope. The guarantee applies to the fabric we seal, on the substrates and details we have specified.
3. We install. Spray or brush applied by our certified team, as one continuous membrane across the whole surface — no laps, no tape joints, no fixings through the barrier. Window perimeters and ductwork penetrations are sealed as part of our works.
4. We test it, then ask for an independent test. We air test the building ourselves once our works are complete, so the fabric result is proved while the envelope is still open and accessible. We then ask that the building is independently tested, and that is the result that stands.

What the guarantee covers

Our works: the continuous airtight and vapour control layer we apply to the fabric, the seal at window and door perimeters, and the seal at ductwork and service penetrations present when we are on site. That is the envelope we test and the envelope we stand behind.

How we work with the other trades

An airtight line is only as good as what happens to it after we leave, so we do not leave that to chance. We give toolbox talks on site and we train the follow-on trades — free of charge — so the people drilling, fixing and running services through the building know where the line is and how to make good behind them.

And we run a no-blame culture. If you go through it, tell us straight away and we will come back and fix it, free of charge. Nobody gets a bill and nobody has to hide it — which is exactly why the line stays continuous, and why to date not one building we have sealed has failed its air test.

Why we are able to guarantee it

A guarantee is only as good as the thing behind it. Ours rests on three properties of a liquid-applied membrane that sheet products and tapes cannot match.

There are no joints to fail

Sheet membranes are sealed at every overlap, every penetration and every junction, by hand, on site, often by a trade under time pressure. Each one is a potential leak. A liquid membrane is applied across the entire surface as a single continuous film — the junctions are sealed because there is no interruption to seal.

It is fire rated inside and out

Passive Purple External carries a reaction to fire classification of Class B-s1,d0 to EN 13501-1 (PAVUS classification report PK1-01-20-065-E-1). A fire-rated liquid air barrier is unusual, and it is the reason the system can be specified on facades and in buildings where other airtightness products are refused.

It is certified, not just tested

Property	Result	Standard / source
Air permeability, Passive Purple	0.02 (±0.002) m³/(hm²) — class A	Passivhaus Institut, Darmstadt
Air permeability, Passive Purple External	0.16 m³/(hm²) — class phB	Passivhaus Institut, component ID 2380as00
Air permeance (US)	0.02 L/s·m² at 75 Pa	ICC-ES ESR-4715
Reaction to fire	Class B-s1,d0	EN 13501-1, PAVUS PK1-01-20-065-E-1
Vapour resistance (sd)	0.62 m · 3.1 MNs/g	EN ISO 12572, Kiwa BDA Agrément BAW-21-214-P-A-UK
Crack bridging	Class A3 at 23°C · Class A4 at −10°C	BS EN 1062-7
Water permeability	Class W3	BS EN 1062-3

What it means for your project

Main contractors

Part L risk moves off your programme. You are not carrying the cost of a failed test on a completed envelope, or the delay of remedial work and a retest.

Housing associations and local authorities

A guaranteed envelope result underpins SAP and EPC assumptions on a retrofit programme, and gives a stock team a measured number rather than an estimate.

Schools and education

Summer programmes have no contingency. A guaranteed result means the air test is not the thing that decides whether the building opens.

Self-builders

One test, first time. If you are targeting Passivhaus, 0.6 is the threshold — and it is the number we guarantee.

Proven on real buildings

Project	Result	Build type
Wester Hailes High School (pre-test)	0.078 ACH	Steel frame, SFS infill, render and rainscreen
Croydon Passive House	0.17 ACH	Timber frame
Four-bed detached	0.17 ACH	Timber frame, new build
Everton Heath Primary School	0.3 ACH	Timber frame, partly off-site
Unity Campus, Phase 2	0.43 ACH	Concrete frame and timber SIP, offsite
Bedford Academy expansion	0.43 ACH	Steel interfaces, OSB, beam ends
Ferndale Rise	0.54 ACH	1930s semi, retrofit to brick
Dudley College Animal Studies Centre	0.91 ACH	1950s masonry deep retrofit

Read the case studies · Walk through completed projects in 360° · Watch the installations

Frequently asked questions

Does the guarantee apply if I buy the products and install them myself?

No. The guarantee covers work our certified team installs, because the result depends on the application as much as the material. If you are installing yourself, we will still review your drawings and specify the details — and the products carry their own certifications.

What is 0.6 ACH?

Air changes per hour at 50 pascals — the measure of how much air leaks through a building envelope under test. 0.6 is the Passivhaus threshold. UK Building Regulations Part L works to a different measure, air permeability in m³/h·m² at 50 Pa.

What if the building misses the number?

So far, none has. Every building we have sealed has passed its air test.

The reason is the sequence rather than luck. We test the fabric ourselves as soon as our works are finished, so the envelope is proved while it is still open and accessible. If a later test reads higher, the fabric has not changed — something has been put through it since. So we come back and find it. A little smoke goes a long way: pressurise the building, follow the leak, and it is almost always a duct or a pipe taken through the line. We seal it, and the building is retested.

Tell us early and that visit is free of charge. No blame and no argument about whose fault it was — the building needs to pass, and everyone on site wants the same thing.

Do you work outside the UK?

We supply worldwide and have distributors in the United States. Our installation service, and this guarantee, currently cover projects in the United Kingdom.

Last updated: August 2026