External wall insulation (EWI) is a system that fixes insulation to the outside face of a building’s walls and covers it with a reinforced render. It is sometimes called wrapping, external cladding insulation, or simply “the wrap”. Because the insulation sits outside the structure, it is the only wall insulation method that treats the whole envelope as one continuous layer, which is why it delivers the largest improvement in wall performance and attracts the largest SEAI grant.
This guide covers what EWI is, how the system is built up layer by layer, which walls suit it, what it costs, and where projects most often go wrong.
What is external wall insulation?
Insulation boards are bonded and mechanically fixed to the outside of the existing wall. A reinforcing basecoat with an embedded glass fibre mesh is applied over the boards, then a primer, then a decorative topcoat render. The result is a continuous insulated skin with a weatherproof finish.
The important word is system. Adhesive, board, fixings, basecoat, mesh, beads, primer and topcoat are tested and certified together. Mixing components from different manufacturers voids that certification and is the most common cause of long-term failure.
EWI can be applied to solid masonry, cavity walls, ICF and most modern construction types, provided the substrate is sound and the system is specified for it.
How external wall insulation works: the build-up
A typical EWI installation goes on in this order:
- Substrate preparation. The wall is cleaned, organic growth treated, loose or hollow render removed, and cracks made good. Any existing damp or rain penetration is identified and fixed first, never covered over.
- Starter track. A base rail is levelled and fixed at the bottom of the wall. It sets the horizontal line, carries the first course of boards and protects the exposed board edge.
- Adhesive. Applied to the back of each board, either as a perimeter bead with centre dabs or as a full bed, depending on how flat the substrate is.
- Insulation boards. Laid tight-butted and breaking joint, with joints staggered at corners. Board joints are never aligned with the corners of window or door openings.
- Mechanical fixings. Once the adhesive has cured, the boards are anchored. The number and pattern of fixings comes from a wind load calculation for that building’s height and exposure, not from a rule of thumb.
- Beads and profiles. Corner beads, stop beads, movement joints and drip beads at sills are set before the basecoat.
- Reinforcing basecoat and mesh. The mesh is embedded within the basecoat, not laid on top of it, with overlaps at every join and extra diagonal patches at the corners of every opening.
- Primer. Evens out suction and colour so the topcoat cures consistently.
- Decorative topcoat. The render finish that determines appearance, texture and weathering behaviour.
The details that decide whether it lasts
Most EWI problems are detailing problems rather than product problems. On a typical Irish house the following all need designing in before work starts:
- Window and door reveals insulated with a thinner board, otherwise the jambs stay cold and attract condensation
- Cills extended or replaced so water throws clear of the new face
- Eaves and verge checked for enough overhang to cover the added thickness
- Rainwater goods removed and refixed on extended brackets
- Flues, vents and boiler terminals extended through the new build-up
- Meter boxes, external taps, alarm boxes and lights packed out and refixed
- The plinth and ground line, where a water-resistant board is used in the splash zone and the system stops cleanly above finished ground level
- Boundary and party lines, which can physically restrict the build-up on terraces and semis
Which walls suit external wall insulation?
Well suited:
- Solid masonry walls, where there is no cavity to fill and the performance gain is largest
- Cavity walls where the cavity cannot be filled, or has been filled and more is needed
- Buildings with tired or failing render that needs replacing anyway, so the facade work is happening regardless
- ICF and modern methods of construction, using a render system designed for ICF
- Mixed-fabric buildings where extensions and original structure perform differently
Needs careful assessment, or another approach:
- Protected structures and buildings in Architectural Conservation Areas, where the facade must not change
- Walls hard on a footpath or boundary with no room for the build-up
- Roofs with insufficient overhang, unless the eaves are being modified
- Any wall with existing damp, rain penetration or defective pointing, all of which must be resolved first
- Timber frame, which requires a system specified for that construction
Where the facade must stay as built, internal insulation is usually the answer instead. See external vs internal vs cavity fill compared.
Choosing an insulation type
Four insulation materials cover almost every Irish project:
- EPS: the default for most retrofits. Best balance of cost and performance.
- Mineral wool: non-combustible and vapour open. Specified where fire performance or breathability governs, and commonly required on taller buildings.
- Phenolic: the best thermal performance per millimetre, for where overhang at eaves and reveals is tight.
- Wood fibre: vapour open and hygroscopic, for older solid masonry that needs to keep breathing.
Full specifications and system components are on the Baumit EWI systems pages.
How thick does external wall insulation need to be?
Thickness is driven by the U-value you are targeting, the existing wall construction and the insulation material chosen. Phenolic reaches a given U-value in less thickness than EPS, which matters where eaves overhang or reveal depth is limited. See EWI thickness and U-values for Irish homes for the numbers.
What it costs, and what grants are available
EWI is the most expensive of the three wall insulation methods, largely because of scaffolding and access, and it carries by far the largest SEAI grant to offset that. Current grant values run from €3,000 for an apartment to €8,000 for a detached house.
- SEAI grants for external wall insulation: current rates, who qualifies and how to apply
- EWI costs, savings and payback: what a project costs and how long it takes to pay back
What a typical project involves
A survey and moisture assessment comes first, establishing wall type, exposure and condition. Design and specification follow, covering board type and thickness, fixing pattern, and every junction detail listed above. Scaffolding then goes up, repairs are carried out, and the system is installed in the sequence described earlier, with curing time allowed between the basecoat and the topcoat. Weather governs the programme throughout: render is not applied in frost, driving rain or strong direct sun.
Occupants can usually remain in the property for the duration, which is one of EWI’s practical advantages over internal insulation.
Common EWI installation mistakes
- Insulating over existing damp. EWI conceals rain penetration, it does not cure it. Fix the cause first.
- Too few mechanical fixings, or the wrong pattern. Adhesive alone does not resist wind load. The fixing schedule comes from a calculation.
- Mesh laid on top of the basecoat instead of embedded in it. This is the single most common cause of cracking, and it is invisible once the topcoat is on.
- No diagonal reinforcement at opening corners. Stress concentrates there and cracks radiate from window and door corners.
- Reveals left uninsulated. Creates a cold bridge at exactly the point where condensation and mould show up first.
- Cills not extended. Water tracks back behind the system.
- Board joints aligned with opening corners. Guarantees a crack line.
- Rendering in the wrong weather, or not allowing the basecoat to cure. Causes colour variation, shrinkage cracking and poor adhesion.
- Mixing components from different systems. Voids the certification and the warranty.
Render and finish options
The topcoat determines appearance, self-cleaning behaviour and long-term maintenance. Silicone, silicate, acrylic, mineral and lime finishes each weather differently and differ in vapour permeability. Baumit’s range includes SilikonTop, GranoporTop, StarTop and PuraTop.
See all Baumit facade finishes, or read more on choosing a facade finish.
Training and technical support
EWI performance depends far more on installation quality than on product selection. Baumit Ireland runs practical installation training through the Baumit Training Academy, and structured CPD training for specifiers and consultants. System details, specifications and installation guides are in Downloads.
Next steps
Every building needs assessing on its own terms: wall construction, exposure, condition, and what the facade is allowed to look like afterwards. Contact Baumit Ireland for system literature, specification support, or to be put in touch with a registered contractor for a survey.
More background on the benefits of external wall insulation and Baumit’s approach to sustainability.