Insulation
What External Wall Insulation Changes on a Whole House
External wall insulation (EWI) ends with a rendered face, so it is easy to think of it as render with a layer added. On a whole house it is a different build: boards fixed to every wall, a reinforced base coat, a thin finish, and a wall face that moves outwards by the thickness of the insulation. This guide explains the sequence and everything that has to change around it.
On this page
How the system is built up on the wall
The Insulated Render and Cladding Association (INCA) publishes a general guide to EWI installation that sets out the usual sequence. The existing wall is checked, repaired where needed, cleaned and left dry. A U-shaped starter track matched to the board thickness is fixed above the damp-proof course to give a level base and protect the bottom edge. Boards are bonded with adhesive round each board's perimeter plus dabs, laid in a staggered bond and interlocked at corners, with no gaps over 2 mm, then mechanically fixed into the wall to the manufacturer's pattern.
Corner and movement beads go on next, then a reinforcing base coat of about 6 mm with glass-fibre mesh embedded just below its surface, lapped by about 100 mm, with extra patches at the corners of openings. Once the base coat is dry, a primer prepares it for the finish, which on many houses is a thin silicone finish. K Rend's guide, for example, describes its HP14 base as designed for EWI boards and as the background for its Silicone TC, which is applied at the thickness of its largest grain, 1.5 or 3 mm.
Each manufacturer's system differs in detail, and INCA's guide says the manufacturer's own installation guide must be followed. For a homeowner, the point is that boards, adhesive, fixings, mesh, primer and finish are specified together as one system, not assembled from different ranges.
Everything on the wall has to move out
Insulation moves the face of the wall outwards, so anything that sat flush or projected slightly ends up in a recess. Window sills that used to throw water clear will finish inside the new face unless they are extended or replaced with deeper ones. Reveals get deeper; insulating them, even with thinner board, reduces the cold edge around each frame but can foul opening casements and trickle vents, so every opening needs its own detail.
At the top, the eaves and verges may no longer overhang the thicker wall enough to protect its upper edge, so roofline trims or extensions are often part of the job. Downpipes, soil stacks, water pipes, meter boxes, satellite dishes, lights and alarm boxes all come off and go back on longer fixings or stand-off brackets, and gas meters and flues need the appropriate trade. On a semi or terrace the insulation has to stop at the party line with a neat closed edge, and Table C1 of Approved Document L asks for the impact of the extra wall thickness on adjoining buildings to be assessed.
When Part L brings insulation into a re-render
On many houses EWI enters the discussion because of Building Regulations rather than choice. Approved Document L counts rendering the external surface of a wall as renovating it. If more than 50% of an individual wall element, or more than 25% of the whole external envelope, is renovated, the guidance expects the whole element to be improved to 0.30 W/m²K. Table C1 lists an external insulation system with a rendered finish as one typical way of reaching that figure when an elevation's render is renewed.
The expectation has limits. Paragraph 4.13 lowers it where the full upgrade is not technically or functionally feasible or would not achieve a simple payback of 15 years or less, and Table C1 asks for condensation and moisture risk to be assessed. Homes in conservation areas need not comply fully where insulation would unacceptably alter their character, and traditional stone or lime-rendered walls should only be improved where that will not cause long-term deterioration. Building control decides, taking the conservation officer's advice into account.
Planning can decide it too. EWI with a rendered face is cladding the outside of the house, which is not permitted development on article 2(3) land such as conservation areas and National Parks. Sheffield City Council notes that the High and Low Bradfield, Upper Midhope and Langsett conservation areas are Peak Park areas planned by the National Park Authority, so check which authority covers the address.
Is the wall ready to be insulated?
Insulation is not a cure for a wall that lets water in. INCA's sequence starts from a substrate that is repaired, clean and dry, so the same survey a re-render needs comes first: sounding the old render for hollow areas, tracing cracks to their cause, and checking sills, copings and gutters. Loose render is a weak base for adhesive and fixings, so hollow areas are generally stripped before boards go on.
Older solid stone walls need particular care. SPAB explains how impervious cement coatings restrict evaporation from old walls, and Approved Document L's own provision for traditional walls exists for a related reason. As a hypothetical example, on a nineteenth-century stone cottage on Sheffield's western edge, a breathable lime finish without insulation may turn out to be the right answer, and building control can accept a lesser standard where the evidence supports it.
Keep a record of what went on: board type and thickness, fixing pattern, system name and finish colour. Anyone mounting a satellite dish or a hanging basket later needs to know there is insulation, not brick, behind the render, and that a fixing must reach the masonry. For how insulation fits with the other options for a whole house, see the Sheffield house rendering overview.
Common questions
How much thicker does EWI make the wall?
That depends on the board and the U-value it has to reach, which the system designer calculates for the wall. Add the base coat, which INCA's guide puts at about 6 mm, and the finish. Ask for the total build-up dimension early, because it decides the sill, eaves and reveal details.
Can EWI go on some walls and not others?
Yes. Some houses insulate the rear and side walls and keep a stone or brick front. The edges where the insulation stops need closing details, and the front may still come under Part L if its render is renewed at the same time.
Does EWI need a different finish from ordinary render?
It needs a finish that belongs to the same system as the boards and base coat. Thin silicone finishes over a mesh-reinforced base coat are common, and INCA's guide also lists thicker renders, dash textures and brick-slip finishes.
See housing and planning information for Sheffield and nearby places, including the source and geography behind each figure.
