Blockwork is not an air barrier: what a sealed masonry flat actually looks like
Most photographs of airtightness work are close-ups. A hand, a brush, a joint. They are useful for showing a technique and useless for answering the question a specifier actually has, which is: does the layer go everywhere it needs to go?
Continuity is a property of a whole room, not of a detail. So here is a whole room.
Why the wall, and not the lining
A blockwork inner leaf is not one 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, and invisible the moment the wall is up. Where perp joints are left unfilled on the cavity face, air goes straight through. Dense aggregate blocks add a second route, because the body of the block is permeable enough that even good workmanship leaks through the units rather than only around them.
The usual assumption is that the plasterboard lining handles it. Approved Document L is explicit that it does not: “internal plasterboard linings are not appropriate for use as an air barrier solution”, and it names plaster, parge coat or liquid membranes as the internal treatments for dense aggregate blockwork (Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, paragraph 4.21(e)).
Board on dabs sits clear of the blockwork, and that gap is continuous: behind sockets, past skirtings into the floor zone, past the ceiling perimeter into the roof space. It looks sealed and behaves like a plenum. The full regulatory picture is on The Masonry Revolution.
Four things to look for in the tour above
1. The wall-to-ceiling junction
Look up. This is a concrete plank floor, and the line where the planks meet the blockwork is the junction that decides whether the room is sealed or merely painted. On a leaky masonry flat, this is usually where the air is going — into the lining void, along the plank soffit, and out.
2. The plank joints themselves
Precast planks butt against each other, and the joint between them is a route, not a surface. In the tour the airtightness layer is carried along those joints rather than stopping at the wall head. It is a detail that takes a brush and ten minutes a room, and it is the one most commonly left out.
3. Reveals and openings
Every opening is four reveals, a cill and a head, plus the cavity closer and the frame-to-structure joint. Pan around a window in the tour: the coating runs into the reveal rather than stopping at the face of the wall. Detail grade goes on first, the field coat goes over it, and the two are the same material rather than two systems bonded together.
4. What is not there
No laps. No tape lines. No staples. On a liquid layer the field and the detail are continuous by definition, which is why complexity in the substrate does not multiply into hours on site.
The sequence, in order
- Prepare and, where the background is absorbent or dusty, prime it. Supergrip Primer where adhesion needs improving, Lime Prime on lime-based backgrounds, IM Primer on absorbent and mixed backgrounds.
- Detail first in brush grade — Passive Purple Brush — at plank joints, wall-to-ceiling junctions, reveals, joist zones and penetrations.
- Coat the field by spray or roller with Passive Purple.
- Check wet film thickness, then let the follow-on trades back in.
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 — masonry is thirstier, and the answer to a porous background is a primer, not extra membrane.
The performance figures behind it
Passive Purple is Passivhaus certified at class A, with air permeability of 0.02 (±0.002) m³/(hm²) to EN 12114 — one fifth of the class A limit. The full test data sits on Tested & Certified.
On masonry buildings specifically: Dudley College Animal Studies Centre, a 1950s masonry deep retrofit, returned 0.91 ACH; Ferndale Rise, a 1930s semi stripped back to brick, 0.54 ACH.
See more of them
There are more masonry tours — blockwork flats, concrete plank ceilings, block and beam floors and window reveals — in the full 360° site tours collection.
Working on a masonry scheme and want the build-up checked? Send us the drawings and the block type.