Self-Build & Passivhaus - Airtightness Made Achievable

How and where does a house actually leak?

Leakage is not spread evenly over your walls. It concentrates where one material meets another, or where something passes through the envelope. Almost every disappointing air test comes back to the same short list.

Floor-to-wall junction. Where the sole plate sits on the slab or beam-and-block floor, there is a joint between two materials laid by two trades on two different days, running round the whole perimeter of the house.

Intermediate floors. On a two-storey timber frame the first floor joists interrupt the wall lining. Air leaves the room, crosses the floor void along the joist zone and escapes at the perimeter, past joist ends, hangers and rim boards. Invisible once the ceiling goes up.

Eaves and the wall head. The ceiling line, wall plate, and insulation all have to come together in the least accessible part of the building, often with a ventilated cold roof void on the other side.

Window and door reveals. Two reveals, a cill and a head at every opening, plus the gap between frame and structure that was packed and foamed. Foam is an insulator, not an air barrier.

Service penetrations. Consumer unit tails, water and waste, heat pump pipework, extract and MVHR ducts, soil vent pipes, cables to outside lights, sockets on external walls. Each is a hole made by someone whose job is not airtightness.

Loft hatches and downlights. A hatch is a purpose-made hole into a cold void; a run of recessed downlights is dozens more.

Passive Purple is a liquid. It is brushed, rolled or sprayed on and cures into one continuous film, so those junctions are not sealed one at a time with separate pieces of tape and sheet; they are part of the surface being coated. There are no laps, because there are no sheets.

Will my house be stuffy?

This is the question almost every self-builder asks, usually phrased as living in a sealed box. An airtight house is not an unventilated house; it is the only kind of house whose ventilation you actually control.

Leakage is not ventilation. A draughty house exchanges air through gaps whose size and position nobody chose: on a still mild day it barely ventilates, on a cold windy night it over-ventilates the rooms you least want cooled, and none of that air is filtered or measured. Seal the fabric, and you can put the air where you want it, in the quantity the design calls for, filtered on the way in and with the heat recovered from the air going out.

There is a direct regulatory link between the two, and it is the reason the airtightness figure has to be settled early. Approved Document F, Volume 1 permits natural ventilation, background ventilators and intermittent extract, only for what it calls "less airtight dwellings": those with a design air permeability above 5 m³/(h·m²) at 50 Pa and an as-built result above 3 m³/(h·m²) at 50 Pa. Build tighter than that and continuous mechanical extract or mechanical ventilation with heat recovery is required instead.

So your air permeability target chooses your ventilation strategy. Fitting trickle vents to a house that then tests at 2 is how self-builds end up with a system that does not match the fabric. Design the two together.

When in the build does this happen?

Airtightness is a sequencing problem more than a product problem. The material goes on in a window that opens once and closes when the plasterboard arrives.

1. At design stageDraw the airtight line as one unbroken pen stroke round a section through the house, and decide where it changes plane, which material carries it across. Fix the air permeability target at the same time as the ventilation strategy - Approved Document F ties the two together.
2. Frame up, roof on, weathertightThe substrate needs to be dry and the building shut in. Coat before insulation, battens or service voids cover the junctions you need to reach.
3. Before and during first fixDetail the junctions with the brush grade first - floor-to-wall, joist ends, wall head and eaves, reveals - then coat the field areas. Seal each service penetration as it is made rather than hunting for them afterwards.
4. Before plasterboardThe last honest inspection. Walk the whole airtight line with the drawing in your hand and look for anything piercing it. Once boards are up, a fix means taking them down.
5. Before the official testDepressurise the house with a fan and walk the junctions with a smoke stick to find leakage paths while you can still reach them. Fix what you find, then book the test.

How much will I need?

Take the areas off your own drawings, the internal face of every external wall, the ceiling plane, and any floor area carrying the airtight line, then apply the coverage rate for the substrate.

Worked through: 300 m² of wall and ceiling in OSB and plasterboard is 240–300 kg of Passive Purple, roughly 25 to 30 tubs at 10–12 m² each. Allow the brush grade separately for junctions at 6–8 m² per tub, because it goes on heavier. Masonry drinks more than board.

Full coverage rates by product and substrate

What if I miss the number?

You get one test on one house, and by then the money is spent, and the walls are closed. If you buy the materials and apply them yourself, we will talk you through the detailing and the sequence, but the workmanship is yours. If we supply and install, the result is ours: we guarantee 0.6 ACH or lower, the Passivhaus figure. Our best result on a school is 0.078 ACH at Wester Hailes High School, Edinburgh, a whole-building pre-test after our works, measured by EDP Building Sciences to ATTMA TSL4 in November 2025 against a 0.6 design target.

The test is no longer sampled. Approved Document L, Volume 1 sets a limiting air permeability of 8.0 m³/(h·m²) at 50 Pa, with the notional dwelling at 5, and states at paragraph 7.4 that "an air pressure test should be carried out on every dwelling". Under the 2013 edition, a proportion of dwellings on a site were tested; now every self-build is.

What does Passivhaus actually require?

For airtightness, Passivhaus asks for 0.6 air changes per hour at 50 Pa. In practice, that means the airtight line has to be continuous everywhere, not merely good on the flat areas; a house can be excellent across every wall and still miss on a joist zone and a run of downlights.

It also means the material has to be measured, not assumed. Passive Purple is certified by the Passivhaus Institute in Darmstadt at 0.02 (±0.002) m³/(hm²), certification class A. It is tested at 17 N/50mm tensile with 219% elongation, so it moves with a timber frame as the building dries and settles, and it carries M1 emissions classification, valid to 01.09.2027, for what it gives off indoors once you are living there.

More on Passivhaus and certification

Which products go on a self-build?

Pull-off adhesion to BS EN ISO 4624: 1.23 N/mm² on CLT, 1.02 on concrete, 0.75 on plywood, 0.42 on OSB.

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Has it worked on real homes?

Measured results on completed buildings, not modelled figures.

Ferndale Rise matters if you are refurbishing rather than building new: a 1930s semi, with all the awkwardness that implies, came in at 0.54 ACH.

See it before you commit

The 360° tours show coated walls, ceilings, reveals and junctions on completed projects. The videos show the brush and spray sequence on site, the fastest way to judge whether this is work you want to take on yourself.

360° site tours Application videos

Frequently asked questions