Failed Render on an Atlantic Headland

1. Random cracking of the render. This is not structural. Sand-cement render is rigid and shrinks as it cures; on dense blockwork with strong suction and rapid drying it develops tensile stress that the render cannot accommodate. Add daily and seasonal thermal movement on a dark, south-west-facing elevation, and the absence of movement joints at floor level and at corners, and fine random cracking is the predictable result. Cracks of this width (typically 0.2–0.5 mm) do not admit water on their own but they do let driven rain into the render/block interface.

2. Drummy render up to 900 mm above ground. Hollow, drummy render means loss of adhesion. The likely combination is: rain splash-back wetting the base of the wall, a render that extends down to or below the damp-proof course so that it bridges it, salts migrating from the ground and from the masonry itself crystallising behind the render, and freeze-thaw of the saturated render skin. All of this is a base-of-wall problem, not a general render problem.

3. The damp patch at first-floor slab level. Note that this is localised, not general, and appears after storms. That correlation points first to rain penetration via the cracked render and the floor-band junction (a construction joint, a change of background, and often a concrete ring beam that does not bond to the blockwork), rather than to interstitial condensation, which would tend to be widespread on the cold north elevations and worse in cold weather than in wind and rain. The first-floor slab/ring beam is also a thermal bridge, so the internal surface there sits colder than the surrounding wall and will show any moisture first; the efflorescence confirms water is moving through the masonry and evaporating at the internal face.

What to verify before specifying: a moisture meter and a cover-meter survey to locate wall ties, a close inspection of the head and cill details of the windows above the patch, and confirmation that the cavity is clear of mortar snots, since bridging is common on this type of build.

Option A: elastomeric masonry paint over everything. Cheapest, fastest, and the worst choice. It is a flexible, relatively vapour-tight film applied over render that is already cracked and partly detached. It will bridge the cracks briefly, then blister and peel as moisture behind it pushes out - exactly the blistering already seen on the roof covering in similar coastal jobs. It does not restore adhesion of the drummy base, does not deal with the salts, and traps them so that spalling follows. It cannot be described as a 25-year fix.

Option B: strip and re-render. Remove all drummy render, cut back to sound masonry, and re-render with a breathable, lower-cement or lime-based system on a suitable background, with movement joints at floor level and at all corners, a bell-mouth drip at the base finished at least 150 mm above ground, and the DPC left clear with no render bridging it. This addresses symptoms 1 and 2 directly and lets the wall dry. It does not improve the U-value or the thermal bridge at the floor band, which is where the internal damp shows.

Option C: external wall insulation with a drained, rendered carrier board. 100 mm of insulation would take the wall from roughly 0.55 to about 0.20 W/m²K, keep the masonry warm and dry, push the dew point out into the insulation, and cover the cracked elevation. On a coast the finish must be a mechanically fixed carrier board on a drained cavity, not a directly applied thin-coat render, with a silicone or acrylic finish, impact resistance at ground level, and proprietry movement joints.

Cost check. Roughly 200 m² of elevation: full EWI at around €170–200/m² would consume or exceed the entire €38,000 before any repairs.

Recommendation. Do not paint. Strip and replace the failed render with a breathable, jointed system and correct the base detail, which is essential whichever route is chosen; then apply EWI to the exposed south-west and north-west elevations within the budget as a second phase, with the remaining elevations dealt with later. Advise the owner in writing that the render alone will not change the thermal bridge at the floor band.

Base of wall. Damp-proof course at least 150 mm above external ground level and visible, with the insulation or render system stopping on a stainless steel or PVC starter track above the DPC, not bridging it. A ventilated or drained cavity behind the carrier board, a drip bead forming a clear water-shedding profile, and a paved margin or gravel strip at a fall away from the wall to reduce splash-back. This is the detail that caused the drummy render in the first place.

First-floor band. Where the floor slab or ring beam meets the wall, the thermal bridge must be broken: a proprietary insulated cavity closer or a continuous run of insulation across the floor zone. On the outside, a movement joint in the render or insulation system aligned with the band, with a backing rod and sealant, and reinforcement mesh stopped and restarted either side. On the inside, the plasterboard should be carried on a continuous bead, and a vapour-check plaster should be used locally so any residual moisture dries inwards rather than being trapped.

Openings. Proper sills with a drip and a minimum 40 mm projection, sealed and lapped so water discharges clear of the wall face; head details with a drip; cavity trays with stop ends at every opening; and insulated reveals to reduce the cold edge where mould grows. Wall ties to be stainless steel and fitted with clip-on insulation retainers so the board is mechanically fixed back to the structure, not relying on adhesive.

Reasoning. Each detail is chosen to do one of three things: shed water away from the wall, keep the masonry warm and able to dry, or allow movement without cracking. That is the basis of a durable coastal finish.