Liquid Airtight Membranes vs Parge Coating

If you are trying to make a blockwork or masonry building airtight, the traditional answer has been a parge coat — a thin coat of sand-and-cement render skimmed over the inner wall. It works, up to a point. But modern liquid-applied airtight membranes now do the same job faster, more reliably and to a certified standard. Here is an honest comparison of parge coating vs liquid airtight membranes, and why more builders are making the switch.

What is a parge coat?

A parge coat is a thin (typically 5–10mm) layer of sand:cement or lime render applied to the internal face of masonry. On airtightness projects it is used to seal the porous surface of blockwork, because un-rendered blockwork leaks air through the blocks themselves and through the mortar joints. The parge coat closes that surface so the wall can pass an air test.

Why parge coating is used for airtightness

Blockwork alone was never designed to be an airtight barrier. Small gaps, mortar inconsistencies and the natural porosity of the blocks all let air through. A parge coat gives the wall a continuous skim that reduces this uncontrolled air leakage — which is why it became the default method on masonry builds. (We cover this in more detail in The Airtightness Challenge: Why Blockwork Alone Is No Longer Enough.)

The drawbacks of parge coating

Parge coating does the basic job, but it carries real limitations on a modern, high-performance build:

• It cracks where it matters. Rigid sand-and-cement render is prone to shrinkage cracking and cracks at junctions and material changes (block-to-concrete, block-to-steel, around openings) — exactly the points where air leakage happens.
• It struggles at penetrations. Sealing neatly around pipes, cables, ducts and joist ends with render is slow and inconsistent.
• It is a wet, heavy trade. Mixing, applying and drying a cement parge adds water into the build, takes time to cure, and needs skilled plastering labour.
• Quality is variable. Airtightness depends on the consistency of the application — thin spots, missed areas and shrinkage can all undermine the air test.
• No movement accommodation. Buildings move; a brittle parge layer does not move with them, so hairline cracks open over time.

The liquid-applied alternative

Liquid-applied airtight membranes are sprayed, rolled or brushed onto the same surfaces to form a continuous, flexible, seamless airtight layer. Two products cover the range:

• Passive Purple — Passivhaus-certified liquid airtight vapour control membrane for whole-wall coverage, spray or roller applied (up to ~300m² per day per operative).
• Airtight White — a VOC-free, BBA-certified white airtight membrane that can be left on show or painted over, ideal where the layer is visible.

Because they are liquid, they flow into cracks, junctions and penetrations and cure to a tough, elastic film that flexes with the building instead of cracking. On difficult or dusty substrates, a quick coat of primer first ensures a reliable bond.

Parge coat vs liquid airtight membrane

	Parge coat (sand & cement)	Liquid airtight membrane
Airtight continuity	Good on flat areas, weak at junctions	Continuous and seamless throughout
Cracking / movement	Rigid — prone to shrinkage & movement cracks	Flexible & crack-bridging
Penetrations & detailing	Slow, inconsistent	Easily seals pipes, junctions, joist ends
Speed	Wet trade, slow to apply & cure	Up to ~300m²/day, fast curing
Water added to build	Significant	Minimal
Certification	None inherent	Passivhaus / BBA certified
Finish	Render, must be covered	Can be left on show (Airtight White) or covered

When might a parge coat still make sense?

Parge coating is a long-established method and on simple, low-spec projects it can be perfectly adequate — particularly where plastering is already part of the programme. But for buildings targeting strict air-test results, Passivhaus or low-energy standards, or any project with lots of junctions and penetrations, a liquid-applied membrane gives a more reliable, faster and certifiable result.

Make the switch

If you are specifying or building to an airtightness target, our team can help you choose the right system and achieve a confident air-test result. Get in touch for technical advice, or browse the full airtight paint range.