Retrofit Airtightness for Social Housing & Existing Buildings

Why do cold, damp homes come back?

Damp and mould in existing stock is a recognised risk in social housing, and it is the complaint that comes back year after year in the same properties. The pattern is familiar to every asset team: a resident reports mould in a bedroom corner or behind a wardrobe, the surface is cleaned and treated, and it returns the following winter.

It returns because surface mould is a symptom of two conditions occurring together — a surface cold enough to sit near dew point, and moisture in the air with nowhere to go. Uncontrolled air leakage drives both. Cold air entering at floor perimeters, joist ends, service penetrations and around window reveals keeps those surfaces cold and creates the draughts residents respond to by closing trickle vents and turning off extract fans. Warm, moist air leaving through ceiling perimeters and the void behind dry lining carries humidity into cold construction, where it condenses out of sight.

Older stock is leaky by construction, not by neglect. Solid brick walls, suspended timber floors, unsealed joist ends, chimney breasts, service runs cut in over decades, and dry lining added in a previous refurbishment that left a continuous void behind the board. None of it was detailed as an air barrier, because nothing at the time required one.

Why must airtightness and moisture control be solved together?

This is the part that gets retrofit programmes into trouble. Sealing a home without addressing vapour and ventilation does not fix the moisture problem — it moves it. Reduce the air leakage that was accidentally ventilating a property, add insulation that changes where the dew point sits inside the construction, and leave no vapour control layer and no adequate ventilation, and the moisture that used to leave through the fabric now has to go somewhere else. It goes into the coldest surface it can reach.

Three things have to be designed as one package.

Interstitial condensation risk in an existing wall depends on the construction, the insulation position and the exposure. It is assessed, not assumed. We do not carry out condensation risk analysis ourselves — on a retrofit it is normally produced by the insulation manufacturer, and several of them provide it free with their system. We have listed those free services on our condensation risk analysis page.

Can it be done in an occupied home?

This is the first question a housing provider asks, and it deserves a careful answer rather than a marketing one.

Passive Purple is water-based, and its emissions are classified M1 — the lowest-emission class under the Finnish building material emissions classification — with the classification valid to 01.09.2027 under Rakennustieto. That is a tested emissions figure, not a general claim about how a product smells or how soon a room can be reoccupied.

What can be said without qualification is that applying the membrane externally removes the question entirely. On overcladding, external wall insulation and re-render programmes the airtight and vapour control layer goes on the outer face from the same scaffold as the cladding works. No decant, no appointments inside flats, no internal works package, no making good, no restricted hours in occupied dwellings. On a block of flats, the cost and programme risk of getting inside every unit is frequently larger than the cost of the membrane.

Internal or external — how do you choose?

Most programmes end up using both — external on the blocks being overclad, internal on the voids and the houses being refurbished room by room.

What do the numbers actually say?

Only measured results are quoted here. Every figure below is a recorded air test on a real building.

For scale: the current limiting value for a new dwelling under Approved Document L is 8.0 m³/(h·m²) at 50 Pa, with the notional dwelling at 5, and every new dwelling is now pressure tested. Dudley College is a 1950s masonry building brought to 0.91 ACH. On projects we install, we guarantee 0.6 ACH or lower. 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.

Which products go on a retrofit?

View all products

Has it been proved on existing housing?

Ferndale Rise was a 1930s semi stripped back to brick and rebuilt as a deep retrofit; it tested at 0.54 ACH, inside the figure we guarantee. A London retrofit reached 0.3 ACH. Dudley College took a 1950s masonry building through a £7.5m deep retrofit to 0.91 ACH.

On brickwork specifically, the membrane has been applied directly over the existing wall as a continuous seal rather than as a lining behind a batten and sheet build-up.

Ferndale Rise — 0.54 ACH Retrofit sealing on brickwork Warm roof retrofit, Tottenham

Can we see it before committing a programme to it?

The 360° tours let a client-side team walk completed applications and look at joist ends, reveals and penetrations as they were left. The video channel shows the detailing and spray sequence on site. Retrofit junction details sit with the rest of our airtightness and vapour control drawings.

360° site tours Application videos Technical details and drawings

Frequently asked questions