Water tightness on an insulated facade is not settled by a code of practice for render on insulation, because there is not one. The regulatory floor sits in Approved Document C, and the detailing dimensions sit in the system's own third-party certificate. Three figures do most of the work on site: a damp-proof course at least 150 mm above the level of the adjoining ground, a cavity taken at least 225 mm below the lowest DPC or replaced by a tray, and weep holes every 900 mm on that tray. The exposure the wall actually faces is read from BS 8104 before any build-up is chosen, and the Approved Document is blunt about what an insulation system does not do: it is not the air barrier, and it does not deliver water tightness on its own.
The document that applies is not the one usually quoted
Render on insulation lives with its certificate
An EWI render system is assessed as a kit, not as a set of separate products, and the assessment travels with it. In France the equivalent arrangement is explicit: no NF DTU covers render on insulation, and the reference is the system's Technical Application Document plus the CSTB technical specification 3035_V3, as set out in our piece on the DTU that does not apply to render on insulation. The lesson travels: a build-up assembled from three suppliers has no reference document at all, which is worse than a non-compliant one.
The cladding standard is a different technique
This is the most frequent specification error on both sides of the Channel. NF DTU 45.4, published in March 2023, covers external insulation with rainscreen cladding over a ventilated cavity, split by cladding material into parts P1-1-1 to P1-1-6. Nothing in it addresses render on insulation. If your specification calls for a rainscreen, the water logic changes completely: the breather membrane and the ventilated cavity become the second line of defence, as described in our piece on ventilated rainscreen insulation.
What the system does not do
Two points that clients routinely hear the wrong way round. An insulation system is not an air barrier: airtightness remains a property of the substrate wall and of its junctions with the rest of the envelope. And water tightness is delivered by the substrate and the system together, not by the render skin. If the substrate is cracked, if the joints are open or if a window leaks, insulating over it changes nothing except visibility. On a retrofit the rule is absolute: leave no entry point at a junction where water can accumulate behind the system.
The water-shedding dimensions to record before ordering
The Approved Document C minimums
| Detail | Minimum | Reference |
|---|---|---|
| DPC in an external wall, above adjoining ground | 150 mm | Paragraph 5.5(b), Diagram 8 |
| Cavity taken below the lowest DPC | 225 mm | Paragraph 5.5(c), Diagram 9a |
| Weep holes on a cavity tray | one every 900 mm | Paragraph 5.5(c), Diagram 9b |
| Tray that does not run the full wall, e.g. over an opening | stop ends plus at least two weep holes | Paragraph 5.5(c) |
| Solid wall, severe exposure, brick or stone | 328 mm thick | Paragraph 5.9(a) |
| Solid wall, severe exposure, dense aggregate blockwork | 250 mm thick | Paragraph 5.9(a) |
| Solid wall, very severe exposure | impervious external cladding | Paragraph 5.9 |
Those are the lines to record elevation by elevation before ordering accessories, because they set the take-off for base profiles and flashings as much as they set the defect risk. The reveals are settled in insulating the window reveal, and the rest of the junctions in detailing the singular points.
Drips and flashings are under-specified more often than not
A flashing too thin to hold its fall stops shedding and starts feeding the insulation behind it. Approved Document C puts it as a design rule rather than a gauge: protection should be provided wherever the top of a wall would otherwise be unprotected, and unless the protection and its joints form a complete barrier to moisture, a damp-proof course goes in as well. Damp-proof courses, cavity trays and closers all have to be designed so that water drains outwards. Every drip has to project past the finished face of the system, which means the accessory schedule is set by the finished thickness, not by the substrate.
Sealants are not interchangeable
Sealants in contact with render or insulation have to be chemically compatible with the insulant, which rules out part of the merchant's shelf on expanded polystyrene. Classify the joint before choosing the product, and record the reference: a joint filled with an incompatible sealant fails silently for a year and then opens along its whole length, usually at the head of a window where it is least visible from the ground.
Choose the build-up on exposure, not on the catalogue
Exposure zones decide what the wall may be
Approved Document C maps wind-driven rain into four exposure zones and cites BS 8104:1992 for the underlying assessment, with two modifiers on the map value.
| Adjustment to the map exposure zone | When it applies |
|---|---|
| Add one | Local conditions accentuate wind effects, open hillsides or valleys funnelling wind onto the wall |
| Subtract one | The wall does not face into the prevailing wind |
Table 4 of the Approved Document then gives, for each insulated wall construction, the maximum recommended exposure zone. A rendered finish over a built-in full-fill 50 mm cavity is limited to zone 3, while the same construction with a 150 mm cavity reaches zone 4. Read the modified zone first, then pick the build-up.
Colour is not a free choice either
Dark finishes absorb more solar radiation, raise the thermal stress in the render and increase the risk of cracking. System certificates cap the solar absorption coefficient for that reason, typically excluding values above 0.7 except on elevations shielded from direct and reflected sun, with a tighter cap at altitude. They also require a movement joint between adjacent colours whose absorption coefficients differ significantly. A palette signed off with the client without that check ends in cracking, and the remedial work is not covered.
Where the system's scope of use stops
Render systems are assessed for vertical planes, and for horizontal or inclined surfaces only where those are not exposed to rain, such as soffits and reveals under a canopy. A flat coping, a parapet top or an exposed slab edge falls outside that scope and calls for a coping or a flashing, not another pass of render. Once the scope is settled, the insulant and finish are measured from the elevation survey, against up-to-date catalogues.
What gets recorded, and what a scheme will ask for
The evidence that stands up to a loss adjuster
Without a single code of practice, proof of compliance is a bundle: the system certificate, the product data sheets, delivery notes with batch numbers, the fixing pattern chosen for the wind exposure, and dated photographs of every junction before it is closed. Add the weather record for the days of application, since neither adhesives nor renders may be applied in rain or in freezing conditions. That bundle, not a standard number, is what answers a loss adjuster after a water ingress claim.
The handover check, after rain
Do the visual inspection after rain or a light hose test, and look at where the water ran and where it stood. Check that every flashing sheds past the finished face, that the drips are not buried in render, that base profiles are not bearing directly on the ground and that no fixing has been made good without a route for water back to the outside. Run-off staining shows up long before the defect does.
What a retrofit scheme adds
Where the work is delivered under a government retrofit scheme, the installation sits inside the PAS 2035 retrofit process and the installation itself is carried out to PAS 2030, with a certificated system and a certificated installer. That adds a retrofit assessment, a designed set of details and a documented handover to the evidence above. Plan the sequencing on that basis: the assessment and the design come before the order, not after the scaffold goes up.
