Major Works That Made the Block Wetter: Five Defect Patterns and the Checks That Would Have Stopped Each One

The five patterns below cover the ways insulation work on an older block goes wrong, from internal wall insulation on a solid wall to the floor underneath it, written from the installer's side of the job. Each comes with a check you can ask for at specification stage, before a single board goes on. None of the checks needs a surveyor on your payroll.

1. The sealed wall

Picture foil-backed insulated plasterboard fixed tight to a solid brick or stone wall. For a hundred years that wall took rain on its outer face and dried out in both directions. With foil on the room side it can only dry outwards, and on an exposed elevation the weather arrives faster than the wall can shed it. Two winters on, the signs appear: staining at the skirtings, a musty smell in the ground-floor flats, and joist ends sitting damp inside the masonry, out of sight.

The check: ask for the moisture design before you approve the specification. Solid walls vary. Rendered solid brick and low-absorption brick can take a vapour-closed build-up, with a smart membrane such as Pro Clima Intello Plus controlling vapour from the room side. Sandstone needs a vapour-open build-up of lime, hemp and wood wool board, so the wall keeps drying through the new layers. If you want the physics behind that split, this explainer on whether stone walls need to breathe sets it out. A contractor should tell you which camp your wall falls into and show the reasoning. A product datasheet in reply means the reasoning does not exist yet.

2. The suffocated floor

External works render over the original air bricks, and insulation offcuts block half the remaining sub-floor vents. A suspended timber floor depends on air moving underneath it, and it needs that air more once the floor above has insulation in it. The joist ends start to rot, and the first anyone hears of it is a ground-floor resident reporting a floor that bounces.

A well-run underfloor insulation job works around the ventilation. Support netting holds the insulation between the joists, and vent extenders carry the airflow past the new layer where it would otherwise cover a vent.

The check: a ventilation schedule in the specification, listing each existing vent and its condition after the works. It runs to one page. A contractor who cannot produce it has not counted the vents.

3. One thickness for the whole block

A single build-up goes onto every elevation. The sheltered rear wall copes, while the gable, taking wind-driven rain all winter, starts to struggle, because an exposed wall soaks up more water and has more of it to dry out.

The check: ask why this build-up suits this wall. A competent contractor can show a hygrothermal check for the wall type, run in software such as WUFI. Our own build-ups come from a library we have validated in WUFI for the wall types we work on, and a library like that helps only if someone matches each elevation to the right entry. Ask for that match in writing, elevation by elevation.

4. The cold frame around a warm wall

Window reveals and lintels stay bare while the crew insulates the walls around them. The cold surfaces move to the frames, condensation follows, and within one winter each window wears a ring of mould. For a managing agent that means a run of resident complaints and a possible disrepair claim.

The check: reveal and lintel details drawn into the specification. A thinner insulated board returned into the opening is the normal answer. If the drawings say nothing about the reveals, send them back and ask for the detail before anyone signs.

5. The contractor you cannot find

The works finish, the damp arrives, and there is no one left to pursue. The block has no design file and no named coordinator, and the installer's certification falls apart the first time someone checks it. A redress route needs a paper trail before it can do anything for residents.

The check: before contract, get the PAS 2035 retrofit coordinator named in writing and ask for the installer's PAS 2030 certificate. Look the contractor up on the TrustMark register yourself instead of trusting the letterhead, and list the handover file as a deliverable next to the works.

Before the scaffold goes up

Funded work comes with deadlines. ECO4 runs until 31 December 2026, and a closing scheme gives some contractors a reason to start before the design questions have answers. Put all five checks into one email at tender stage and lay the replies side by side. A contractor who sends back a moisture design and a vent schedule within the week has done this before.

One planning point for the programme: lime plaster takes longer than gypsum because of the preparation it needs, so a vapour-open job on a block runs longer than a foil-board one. Put that in the letter to residents before anyone books the scaffold.

Calum Farquhar is director of Insulation Installers Scotland, a PAS 2030 accredited, TrustMark registered installer in Glasgow. On funded work the company uses qualified external PAS 2035 retrofit coordinators. Photographs of stripped-back walls and finished installs are available on request.