Airtight Paint vs Parge Coat: Cost, Speed and Test Results

On blockwork, the two realistic routes to an airtight wall are a sand and cement parge coat or a liquid applied airtight membrane. Both can work. The honest answer on which to choose comes down to three things: how the wall's junctions and penetrations will be sealed, how much programme time you can give to a wet trade, and what airtightness number you actually need to hit. Here is a straight comparison, including where a parge coat is perfectly fine and where it is not.

What a parge coat does

A parge coat is a wet-applied sand and cement (or proprietary) basecoat trowelled or sprayed across the inner face of blockwork before plasterboard or finishing plaster goes on. Its airtightness job is simple: bare blockwork is surprisingly leaky through the blocks and every imperfect mortar joint, and a continuous parge layer closes that background leakage across the field of the wall. Done well, over the flat, uninterrupted middle of a wall, it works. Parging is a familiar trade with familiar materials, and on sites already carrying wet trades it slots into the sequence without new skills or kit.

Where parge coats fail

Airtightness tests are rarely lost in the middle of a wall — they are lost at the edges and the holes. This is exactly where a rigid cementitious coat struggles:

• Shrinkage cracking. Sand and cement shrinks as it cures. Hairline cracks in the field of the wall may matter little, but cracks tracking along junctions become continuous leak paths.
• Wall-to-floor and wall-to-ceiling junctions. A rigid coat cannot bridge the movement joint between different materials. Where blockwork meets slab, joist zone or ceiling, the parge typically stops — leaving the most important line on the wall unsealed unless someone details it separately with sealant or tape.
• Service penetrations. Most holes are drilled after the parge is on. Every socket box chase, waste pipe and cable run cut through the coat reopens the air barrier, and a cement coat cannot self-seal around a pipe the way a flexible membrane detail can.
• Drying time. A parge coat is a wet trade: it needs days to dry before boarding, longer in cold or damp weather, and adds real water into the building at exactly the stage you are trying to dry it out.

How a liquid airtight membrane compares

A liquid applied membrane such as Passive Purple approaches the same wall differently: a two-coat system, sprayed or brushed, that cures to a flexible, seamless airtight and vapour control layer. The differences that matter on site:

• Junctions and penetrations are part of the system. The membrane carries on across the wall-floor junction, into reveals and around penetrations, staying continuous where a rigid coat stops. It remains flexible, so minor movement does not crack the air barrier, and pipe and cable details are sealed with compatible airtight sealant and tapes.
• Speed. Spray applied, one installer can cover up to 300m² a day, and the thin-film coats dry far faster than a cementitious layer, without loading the building with water. On a tight programme, that difference compounds across every plot.
• Tested performance. Passive Purple has a tested air leakage of 0.02 L/s·m² at 75Pa (ASTM E2178) and is Passivhaus certified, BBA approved and ICC-ES listed (ESR-4715). A parge coat's real-world result depends almost entirely on the plasterer's consistency on the day and how the junctions were handled — there is no certificate that travels with it.

On cost, be wary of comparing materials prices alone. Sand and cement is cheap per bag; the true comparison is labour hours, drying-time delays to following trades, and the cost of remedial sealing when the test finds the junctions. Our heat loss calculator also puts a number on what ongoing leakage costs the occupant — worth having in front of you when the budget conversation happens.

When a parge coat is fine

Honestly: if your target is a modest air permeability, your walls are large and simple, your programme has slack for drying, and someone competent owns the junction detailing with sealant and tape, a well-applied parge coat will get you through. Plenty of houses pass this way. It is also a reasonable belt-and-braces base layer where wet plastering is happening anyway.

When a liquid membrane wins

Choose the membrane when any of these apply: you are chasing a strong test result (design targets of 3 m³/(h·m²) or better, or Passivhaus at 0.6 ACH @50Pa); the job has lots of junctions, joist zones and penetrations relative to plain wall; the programme cannot absorb wet-trade drying time; or you want certified, repeatable performance rather than workmanship-dependent results. On blockwork specifically, this is the approach we call the masonry revolution — treating the airtight layer as an engineered product rather than a by-product of plastering. You can see the full airtight paint range for spray and brush options to suit the job size.

Frequently asked questions

Is a parge coat airtight?

Across the field of the wall, a well-applied continuous parge coat significantly reduces air leakage through blockwork. Its weaknesses are shrinkage cracks, junctions with floors and ceilings, and every penetration cut after it is applied — which is where most test failures happen.

Is airtight paint really airtight, or just marketing?

A proper airtight membrane paint is a tested air barrier, not ordinary paint. Passive Purple, for example, has tested air leakage of 0.02 L/s·m² at 75Pa (ASTM E2178) and is Passivhaus certified and BBA approved. Always check for that kind of third-party test data rather than taking "airtight" on trust.

Can I use both a parge coat and an airtight membrane?

Yes — where wet plastering is happening anyway, a parge base with a liquid membrane over the top (and over the junctions the parge cannot bridge) combines background sealing with certified continuity. On most jobs, though, the membrane alone does the airtightness job in fewer steps.

How many coats of airtight membrane do I need?

Passive Purple is a two-coat system — always. Two coats build the film thickness the tested performance is based on, so plan labour and materials for both coats rather than treating the second as optional.