Airtightness & Waterproofing Solutions for Schools & Education

Why air quality in schools is a health issue, not a building issue

Children spend a large share of their waking hours in a classroom. What they breathe for those hours is decided, in large part, by how the building envelope was detailed, and by whether anyone can control where the air comes in.

Air does not enter a leaky envelope through a filter. It enters through service penetrations, around window reveals, at the head and base of walls, and through the joints between boards and frame, bringing with it whatever is outside: traffic exhaust from the road most urban schools sit on, dust, pollen, and moisture that condenses inside the construction where nobody can see it. None of that air is measured, tempered or cleaned, because it never passes through anything that could clean it.

So the argument for airtightness in schools is not primarily an energy argument. It is this: uncontrolled leakage cannot be filtered; controlled ventilation can. Sealing the envelope does not stop a school breathing; it decides where the school breathes from. Once the fabric is airtight, every cubic metre entering the classroom arrives by a route the design team chose, at the designed rate, through a filter the estates team can maintain.

It is also the only way the ventilation strategy an M&E engineer designed on paper survives contact with the finished building. A system sized for a classroom cannot do its job if the room is simultaneously being ventilated by a gap around a steel column.

What does the airtightness guarantee actually mean for a school programme?

On projects we supply and install, we guarantee 0.6 ACH or lower. That is the number, in writing, before the work starts.

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.

Set that against how a school air test usually goes. On a taped or sheet-membrane build-up the result is not known until the test, the test comes near the end of the programme, and a failure lands where there is no slack left, behind finished linings, in a building due back for the start of term. For a local authority or a trust running a programme of schools, that risk is carried on every site, every year.

A guaranteed figure moves that risk off the client. It is the difference between gambling on an air test and knowing the outcome.

Which junction do schools actually fail on?

Modern school buildings, particularly framework new-builds and extensions, are very often light gauge steel frame with a cement particle board (CPB) sheathing. The joints between those hard boards and the steel columns are the critical weak point in the envelope.

The reason is movement. CPB and steel expand and contract at different rates: rigid fillers crack, and standard tapes can peel. An LGS frame is also covered in thousands of protruding screw heads, every one a detail somebody has to seal by hand if the airtight layer is a tape-and-sheet system.

We treat that junction in two steps:

1. IM Fleece Tape over the jointBefore spraying, the joint is reinforced with IM Fleece Tape, a self-adhesive tape with a fabric backing designed specifically to absorb and hold the liquid membrane, rather than sit under it.
2. Passive Purple spray-applied over the whole faceThe liquid membrane is then sprayed over the full face, encapsulating the tape, the board, the frame and the screw heads in one pass. The tape and the membrane cure together into a reinforced composite joint that is strong and still flexible enough to accommodate differential movement.

As one contractor put it: taping the cement particle board would have taken weeks; we sprayed it, screws and all, in days.

More detail on this build-up is on The Fast-Track Airtightness System and Liquid Membranes for Light Gauge Steel Frame.

Can it be done inside a six-week summer window?

Most school refurbishment happens in the holidays, and the summer window is roughly six weeks from the last day of term to the first day back. There is no float, and the handover date cannot move. A liquid airtight layer suits that constraint for three structural reasons:

• It is a single trade. One operation, one gang, one sign-off, instead of a sequence of taping, lapping and detailing operations that each have to follow one another and each have to be inspected.
• There are no laps or tapes to detail across the field. The membrane is continuous by definition, so complexity in the substrate does not multiply into hours on site; penetrations, screw heads and irregular junctions are covered as the spray passes over them.
• It is sprayed. Large areas of wall, soffit and ceiling are covered in a pass.

The practical effect is that the airtight layer moves off the critical path.

Which products go on a school envelope?

Ground gases: where radon is a consideration on a school site, Passive Purple has a measured radon diffusion coefficient of 3.3 × 10⁻¹² m²/s (ISO/TS 11665-13).

Where has this been proven on school buildings?

Between them, these cover the three envelope types an education framework keeps producing: timber frame new-build, a steel-and-board extension onto a live school, and deep retrofit of ageing masonry stock.

See it for yourself

If you want to look at the finished airtight layer before you specify it, you do not have to take our word for it.

• 360° virtual tours of completed projects - walk the sprayed envelope, junctions and penetrations in the round.
• Application videos on our YouTube channel - how the membrane is sprayed, and how the details are formed on site.

Estates teams and framework contractors are also welcome to visit a live site during application. Ask us what is running near you.

Frequently asked questions - airtightness in schools

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