How to Make a Building Airtight: The Liquid Membrane System
Air leakage is one of the biggest causes of heat loss, draughts, condensation and failed air tests. Making a building airtight means creating one continuous barrier across the whole envelope — walls, floors, roof, and every junction and service penetration — using the right product for each part of the job.
Why airtightness matters
Uncontrolled air leakage wastes energy, causes cold draughts, and lets warm, moist air into the structure where it can condense and cause mould or decay. A well-sealed envelope cuts heating demand, improves comfort, and — paired with proper ventilation such as MVHR — protects both the building and the people in it.
What the regulations require
In England, Approved Document L (2021 edition, in force June 2022) sets a maximum air permeability of 8 m³/(h·m²) at 50 Pa for new dwellings, with the notional (target) specification at 5 m³/(h·m²). Every new dwelling must now be air-pressure tested. Low-energy and Passivhaus projects go much tighter — Passivhaus requires 0.6 air changes per hour at 50 Pa — which is only achievable with a genuinely continuous airtight layer.
Where buildings leak
Air finds the weakest point. The usual culprits are service penetrations (pipes, cables, soil stacks), wall-to-floor and wall-to-ceiling junctions, joist ends built into walls, window and door reveals, and the joints between sheathing boards. Get the detailing right and the large areas look after themselves.
The fast way to seal large areas
Passive Purple is a Passivhaus-certified liquid airtight vapour control membrane. Sprayed or rolled onto the warm side of the insulation, it forms one seamless layer across walls, floors and roofs with no laps or joints to fail, and it flexes with the building rather than cracking.
Tapes and brush membrane for the details
Liquid membrane does the bulk, but tapes still earn their place. We use IM Fleece Airtight Tape to seal around window and door frames and across board joints, and Passive Purple Brush to seal junctions, penetrations and cracks. Together they form a complete, continuous system — the right tool for each part of the build.
How it goes together
- Prepare the surface: sound, dust-free, with large gaps pre-filled and porous substrates primed
- Tape window and door frames and key board joints with IM Fleece Tape
- Detail junctions, joist ends and penetrations with Passive Purple Brush
- Spray or roll Passive Purple across the main wall, floor and roof areas
- Once cured, plaster, render, dot-and-dab or paint over it
Test before you finish
Before closing up, a quick smoke test reveals any remaining leaks so you can seal them ahead of the formal air-pressure test — far cheaper than chasing a fail afterwards.
Why this system works
- Seamless and flexible — moves with the building instead of cracking
- Fast over large areas, precise on the details
- Fewer failure points than taping every surface
- Passivhaus-certified components
Frequently asked questions
Can you make an existing building airtight? Yes — retrofit grades let you seal solid walls from inside or out while staying vapour-open, so old walls can still breathe.
Do you still need ventilation? Always. “Build tight, ventilate right” — an airtight envelope should be paired with controlled ventilation such as MVHR or trickle vents.
Can you plaster over it? Yes — prime the cured membrane with Supergrip Primer so plaster and dot-and-dab bond firmly.
Want the background on targets and testing? Read What is Airtightness, or contact our team to spec the right system.