Description
Two structural jobs on an insulated wall, one bag. Ceresit ZU bonds the boards to the masonry, then comes back a few days later to carry the reinforcing mesh. One code on the order, one product in the certification chain, and one set of site conditions to plan around.
Ceresit ZU on a Ceretherm Wall — What It Does at Each Stage
Ceresit ZU is a cementitious and lime mortar with mineral fillers and modifiers, written by Henkel for expanded and extruded polystyrene inside the Ceresit Ceretherm Popular system, on new build and on thermal renovation alike. It fixes the boards at an assumed 5.0 kg/m² and forms the mesh-reinforced layer at 4.0 kg/m², which is why one 25 kg bag does two different amounts of wall depending on which stage it is going into. Renders World lists Ceresit ZU in the EPS adhesive and basecoat range as the Ceresit-brand route through both.
The declared scope of Ceresit ZU is EPS and XPS. Henkel recommends white or graphite boards meeting EN 13163 for external wall insulation, so a project mixing white and graphite EPS insulation boards keeps a single adhesive, and the manufacturer states it can also be used for energy-efficient construction.
Before any of that, the wall has to earn it. Ceresit ZU develops its bond on sound, load-bearing substrates — masonry, render and cement backgrounds free of grease, bitumen and dust — and the data sheet is specific about the three cases that need work first:
- Existing renders and paint coats are assessed before work starts. Hollow render comes off, and vapour-tight or poorly adhering coatings are removed completely.
- Contaminated facades are washed under pressure with a cleaning agent added, and walls carrying fungi, moss or algae are cleaned and then saturated with a fungicidal solution to the maker's instruction.
- Highly absorbent walls such as aerated concrete blocks are primed with Ceresit CT 17 Profi and left to dry for at least two hours — the sequence the substrate preparation guide works through wall by wall.
Voids and hollows are filled with cement render before bonding begins, not levelled out in adhesive afterwards.
Why Installers Specify This Mortar for EPS Facades
- One product across both structural stages. Board fixing and the reinforced coat come out of the same bag, so the paperwork behind the wall stays with one product rather than two.
- Bond declared to ETAG 004, in the operators the sheet prints. Adhesion to concrete is given as > 0.25 MPa and to EPS boards as > 0.08 MPa, while adhesion between layers after ageing is given as ≥ 0.08 MPa. The change of sign between the fresh bond and the aged bond is the manufacturer's own, and the aged figure is the one that matters on a wall that has been through a few winters.
- Flexural resistance ≥ 5.5 N/mm² to EN 1015-11:2001+A1:2007 — the property that carries the reinforced coat through thermal movement once the render is on.
- A 90-minute pot life, long enough to bond a full lift from one batch. The declared data sits at +23 °C and 50 % relative humidity, so a warm bright day shortens it and a cool one stretches it.
- Machine application is provided for. Henkel names Wagner PC 15 and SPG Baumaschinen PG 20, with a nozzle of diameter 6 — the sheet prints the number without a unit, so specify the nozzle with the machine supplier rather than off the data sheet.
- Assessed for natural radiation against ITB Instruction No. 234/2003, which is a question that comes up on dwellings more often than the trade expects.
Declared Technical Data
Values below are given in the form Henkel prints them for Ceresit ZU, operators included, because ≥ 20 N/mm² and 20 N/mm² are not the same declaration.
| Property | Declared value |
|---|---|
| Base composition | Cement and lime with mineral fillers and modifiers |
| Bulk density | approx. 1.3 kg/dm³ |
| Mixing ratio | 4.5–5.0 l water per 25 kg |
| Temperature of application | +5 °C to +25 °C, substrate and ambient |
| Pot life | up to 90 minutes |
| Compression resistance | ≥ 20 N/mm², class CS IV, EN 1015-11:2001+A1:2007 |
| Flexural resistance | ≥ 5.5 N/mm², EN 1015-11:2001+A1:2007 |
| Adhesion to concrete | > 0.25 MPa, ETAG 004 |
| Adhesion to EPS boards | > 0.08 MPa, ETAG 004 |
| Adhesion between layers after ageing | ≥ 0.08 MPa, ETAG 004 |
| Water absorption after 24 h | < 0.5 kg/m², ETAG 004 |
| Assumed consumption — fixing boards | approx. 5.0 kg/m² |
| Assumed consumption — reinforced layer | approx. 4.0 kg/m² |
| Assumed consumption — putty layer | approx. 1.0 kg/m² |
| Natural radiation | meets ITB Instruction No. 234/2003, p. 6.2.1 |
| Shelf life | up to 12 months from production, dry and cool |
| Packaging | 25 kg bags |
| Reference conditions | +23 °C, 50 % relative humidity |
Two quantities follow straight from the table. A bag covers 25 kg ÷ 5.0 kg/m² = 5.0 m² of board fixing, or 25 kg ÷ 4.0 kg/m² = 6.25 m² of reinforced layer. The third consumption line, the 1.0 kg/m² putty layer over a reinforced surface, is the one most quantity estimates leave out — on a 100 m² elevation that is another four bags nobody ordered.
Three things the documentation deliberately does not carry: a colour, a basecoat thickness in millimetres, and a number of mechanical fixings per square metre. The last one is set by the system design rather than by the mortar, and the fixing pattern and spacing guide explains how it is arrived at.
Certification Scope — Which Assessment Covers Which System
The assessments belong to complete Ceretherm build-ups rather than to the bag of Ceresit ZU on its own, and they differ from one to the next. Take the row that matches what is actually being installed.
| Ceretherm system | Assessment | Certificate and DoP | Reaction to fire, EN 13501-1 |
|---|---|---|---|
| Popular | ETA 08/0309 | 1488-CPR-0382/Z · DoP 00426 | B–s1, d0 |
| Impactum | ETA 13/0086 | 1488-CPR-0407/Z · DoP 00436 | B–s2, d0 |
| Reno | ITB-KOT-2018/0472, edition 1 (national) | 020-UWB-0895/Z · NDoC 00444 | not stated |
Both fire classes are system classifications tested on an assembled wall, so neither is a property of the mortar and neither travels to a different build-up. Taking the stricter of the two, B–s2, d0 in Ceretherm Impactum is the figure to plan against while the specification is still open, with B–s1, d0 available where the wall is genuinely built as Ceretherm Popular. Reno carries a national assessment and no fire class at all in this documentation.
For a UK project these are European and Polish system documents rather than UK product approvals, alongside Irish Agrément Board Certificate No. 09/0340 as a document of reference. Under current Approved Document B guidance the external wall is assessed as a whole construction, so the certificate that counts is the one covering the finished build-up — the sequence set out in the EWI system build-up guide.
How to Apply Ceresit ZU — Mixing, Board Fixing and the Reinforced Layer
Mixing the bag
Pour the powder into a measured 4.5–5.0 litres of cool, clean water and stir with a paddle mixer to a lump-free mass, then leave it about five minutes and mix again. Working the batch out inside the 90-minute pot life is what keeps Ceresit ZU performing as declared.
Fixing the boards
- Trowel a continuous 3–4 cm strip around the board edge plus a few spots of approximately 8 cm diameter.
- Press the board home immediately with light blows of a long float, so the squeezed mortar covers a minimum of 40 % of the board's surface.
- On even, smooth backgrounds, comb the mortar on with a 10–12 mm toothed float instead.
- Butt the boards tightly and stagger the vertical joints in the brick-like pattern Henkel specifies.
- Where the background is doubtful, bond 100 × 100 mm polystyrene test pieces and pull them after four to seven days: torn polystyrene means the wall is carrying, while a piece that lifts with mortar attached calls for mechanical fixings.
The reinforced layer
Once Ceresit ZU has set, sand the boards, add the mechanical fixings and spread fresh mortar with a 10 or 12 mm notched trowel. Bed Ceresit CT 325 reinforcing mesh into the wet bed with 10 cm overlaps and smooth until the weave no longer shows; the mesh reinforcement guide covers the embedding technique in detail, and both stocked grades sit alongside it.
Installation Notes — Curing Interval, Conditions and Site Practice
One number decides the programme, and it is worth being exact about where it comes from. The data sheet puts sanding and mechanical fixing after the mortar has set in, at three days. Henkel's product page advertises adhesion after 24 hours, but that line sits in a list of product features beside 3D reinforcement and good adhesion — it describes how quickly the bond develops, not a licence to start the reinforced layer the next morning. Two statements, two scopes: plan the lift to the three-day interval, and treat anything faster as something to agree with the system holder for the specific job.
- Work dry, between +5 °C and +25 °C for both background and air, and protect the finished reinforced layer from rain — sheeted scaffolding is recommended in the documentation itself.
- Stay off sunlit walls during the reinforced layer, so the mortar does not skin before the mesh is bedded and smoothed.
- Cover the boards within two weeks. Beyond that the board faces need assessing, and yellowed or dusting surfaces are taken back with coarse sandpaper before work continues.
- Clean tools wet. Fresh splashes wash off with water; hardened material has to come off mechanically.
- Treat the powder as alkaline. Cement and lime react with water, so gloves and eye protection during mixing are sensible, and eyes that catch a splash are rinsed thoroughly with water.
Across a whole elevation, the three-day pause is what shapes the schedule rather than the mixing or the trowelling, since it falls between board day and mesh day on every lift. The day-by-day EWI installation timeline puts that gap into a realistic UK programme.
Choosing Between Ceresit ZU and the Other Bonding Routes
| Your project | Route that fits | Why |
|---|---|---|
| Ceresit Ceretherm EPS or graphite EPS facade | Ceresit ZU 25 kg | ETA 08/0309, one product across both stages |
| A different system, or a white-cement base | the bonding and basecoat mortars Renders World stocks | Brand-matched 2-in-1 mortars for other systems |
| Fast board fixing with no mixing station | foam against cement comparison | Canister foam, board fixing only |
| Stone wool slabs on a fire-sensitive elevation | stone wool slabs and their adhesives | The declared scope here is EPS and XPS only |
Is This the Right Mortar for Your Build-Up?
- Yes, for a Ceretherm EPS or XPS facade. Ceresit ZU covers bonding and reinforcement, with ETAG 004 bond figures and an ETA against the build-up it belongs to.
- Insulating with stone wool instead? Stone wool has different fibre structure and vapour behaviour and takes a wool-rated adhesive, which keeps both the bond and the system documentation intact.
- Still weighing up brands? The adhesive selection guide for EPS sets the Ceresit and Atlas mortars side by side on board type, system and certification.
- Ordering for a whole elevation? Count both stages plus the putty coat, and order the CT 325 mesh with it — Renders World lists both in the same range, and it is needed the same week as the second pass.
FAQ — Quantities, Compatibility and Compliance
How many bags does 100 m² of external wall insulation need?
Both stages count, using the assumed consumption:
- Board fixing: 100 × 5.0 = 500 kg ÷ 25 = 20 bags.
- Reinforced layer: 100 × 4.0 = 400 kg ÷ 25 = 16 bags.
- Putty layer, where the build-up includes one: 100 × 1.0 = 100 kg ÷ 25 = 4 bags.
That is 36 bags for the two structural stages and 40 with the putty layer. Consumption is declared as assumed, so a rough or irregular background will take more.
Can it be used on mineral wool boards?
No. Henkel writes the scope of Ceresit ZU as EPS and XPS within Ceretherm and recommends white or graphite EPS to EN 13163. Wool slabs take a wool-rated adhesive, and using one keeps the documentation intact as well as the bond.
What are the weather limits on the day?
Dry conditions, background and air from +5 °C to +25 °C, no reinforced-layer work on sunlit walls, and protection from rain once the layer is on. Sheeting the scaffold keeps a marginal spring or autumn day workable and the declared figures meaningful.
Is Ceresit ZU certified for UK use?
The documented cover is European and national system assessment — ETA 08/0309 in Ceretherm Popular and ETA 13/0086 in Ceretherm Impactum, with Irish Agrément Board Certificate No. 09/0340 as a reference document. UK compliance is assessed on the finished external wall, so specify the mortar as part of a named system and keep the manufacturer's documents with the project file.
Does it also work as a levelling or skim coat?
There is a third consumption line, a putty layer at approximately 1.0 kg/m², which is the thin closing pass over a reinforced surface. It is not a substrate levelling render: hollows and uneven backgrounds are filled with cement render before bonding starts.
Which data sheet applies in the UK?
Henkel publishes the Ceresit ZU technical data sheet in two English editions under one document code, C_ZU_TDS_1_0819. They carry identical figures and differ only in vocabulary — the British edition writes render where the global edition writes plaster. Either supersedes all earlier editions of the sheet, and the British edition is the one linked below. Renders World supplies Ceresit ZU in the 25 kg bag only, stored dry and cool for up to 12 months from the production date.
Technical Documentation