The Masonry Revolution: Stop Parge Coating. Start Spraying

Why does a blockwork inner leaf leak?

An inner leaf is not a surface. It is several hundred units per storey with a joint under every course and between every block — hundreds of metres of joint line, laid by hand, mostly from one side, invisible once the wall is up. Where perps are dry-jointed or left unfilled on the cavity face, air moves straight through. Dense aggregate blocks add a second route: the block body is permeable enough that even well-laid work leaks through the units, not only around them.

The assumption is that the plasterboard lining deals with it. It does not. Board on dabs sits clear of the blockwork, and that void is continuous — behind sockets, past skirtings into the floor zone, past ceiling perimeters into the loft. The lining looks sealed and behaves as a plenum.

Johnston and Lowe, in Building Services Engineering Research and Technology 27(1), 2006, tested dry-lined masonry dwellings at 24 to 26 m³/h·m² at 50 Pa. Sealing the void behind the board cut leakage “by almost 55%, to a mean of just over 11 m³/h per m² at 50 Pa”. Nothing else changed. The void was the leakage path.

What does Approved Document L actually say?

Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, at paragraph 4.21(e), states:

Liquid membranes are named as an accepted way of reducing air permeability, alongside plaster and parge coat — and plasterboard linings are stated not to be appropriate as the air barrier.

Be precise about what that means. Approved Document L is statutory guidance, not the regulation; it sets out one way of showing compliance and prohibits nothing. The legal test on a new dwelling is the pressure test under regulation 43, and Volume 1 paragraph 7.4 confirms every new dwelling is now tested rather than a sample of each type. The limiting value is 8.0 m³/(h·m²) at 50 Pa; the notional dwelling sits at 5.

Scope it properly. Paragraph 4.21(e) is in Volume 1, which covers dwellings, and applies to new dwellings in England — not Volume 2, which covers buildings other than dwellings. Wales, Scotland and Northern Ireland each have their own regulations and guidance, and the wording differs.

The Passivhaus Trust reaches the same conclusion. Its Good Practice Guide to Airtightness, June 2020, section 3.1.1.4, lists “dot & dab plasterboard”, “silicon sealant” and “decorators caulk” among products that “do not form robust and reliable air barriers”. Its permitted list includes “Wet plaster > 5mm depth”.

Parge coat or liquid membrane — how do they compare?

Both are named in the same sentence, and both work. They differ in how they are delivered, and on a live site that is what decides it.

A parge coat taken tight to every junction and left to dry is a good air barrier. The difficulty is rarely the material; it is getting a wet trade back onto a floor plate other trades have started, for a layer nobody will see again.

What about dense aggregate blocks and other porous substrates?

Paragraph 4.21(e) singles out dense aggregate blocks because that is the substrate where the leaf itself, not only the joints, lets air through. They are also thirsty: a porous or dusty face pulls a water-based membrane into the block instead of letting it form a film. Allow the right quantity — Passive Purple External covers 1.0–1.2 kg/m² on blockwork and brickwork, against 0.8–1.0 kg/m² on OSB and plasterboard — and prime where the background needs it rather than solving absorbency with extra membrane.

Three primers cover the usual backgrounds: Supergrip Primer at 0.2–0.3 kg/m² where adhesion needs improving, Lime Prime at 0.15–0.25 kg/m² on lime-based backgrounds, and IM Primer at 0.15–0.25 kg/m² for absorbent and mixed backgrounds.

Adhesion is measured to BS EN ISO 4624. On concrete, pull-off adhesion is 1.02 N/mm² — the highest of any mineral substrate in the published set, and the closest published figure to a prepared masonry face.

Which details actually fail?

On a masonry dwelling that misses its target, the failure is rarely a large hole. It is the same few junctions, repeated on every storey.

The sequence is the same on every masonry project: detail first in brush grade, then coat the field.

Which products go on a masonry wall?

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Has it been proved on masonry buildings?

At Dudley College, a 1950s masonry building went through a £7.5m deep retrofit and tested at 0.91 ACH — an existing masonry structure brought to a figure many new dwellings do not reach. At Oxford Brookes University, on a £60m scheme, Passive Purple was used on blockwork behind external wall insulation. At The Dairy Passivhaus, it formed the airtight and vapour control layer on a certified Passivhaus project.

Elsewhere: Ferndale Rise, a 1930s semi stripped back to brick, 0.54 ACH; a London retrofit, 0.3 ACH; Everton Heath, 0.3 ACH. 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.

Dudley College — 0.91 ACH Oxford Brookes University The Dairy Passivhaus

Can we see it on blockwork, and where are the drawings?

The 360° tours let you walk a coated inner leaf and look at reveals, joist zones and penetrations as they were left; the video channel shows the detailing and spray sequence on site. Masonry junction details — perimeters, joist ends, reveals, penetrations and cavity closers — sit with the rest of our airtightness and vapour control details.

360° site tours Application videos Technical details and drawings

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