External wall insulation
External wall insulation in Hull
External wall insulation (EWI) wraps the outside of the house in insulation boards, then covers them with a basecoat reinforced with glass-fibre mesh and a finish such as silicone render, dash or brick slips. It is mostly used on solid walls with no cavity. The wall gets thicker, so everything attached to it, from downpipes to sills, has to be moved out and re-detailed.
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Preparation decides most of the result
JUB's installation overview for its Jubizol system is a useful checklist of what happens before a single board goes on. The walls are surveyed and remedial work noted; every fixture and fitting is removed and its refitting planned; loose material is taken off and the wall cleaned and treated for algae; flues and pipes are extended by the appropriate engineers, and telephone lines that need moving are notified to Openreach. Visible damp or salt problems are to be put right before installation starts.
That last point matters in a low-lying city. Hull City Council's flood strategy notes that over 90% of Hull is below the high tide level and that surface water pools in low spots. Insulation will not cure a wet wall, so leaking gutters, bridged DPCs and raised ground against the wall are dealt with first.
- Existing wall Prepared; fittings removed and the base track fixed above ground level.
- Insulation boards Bonded with adhesive and, in most systems, mechanically fixed to the wall.
- Basecoat with mesh Reinforcement mesh embedded across the board joints and around openings.
- Primer Where the system requires it.
- Finish render A thin-coat finish that weatherproofs the insulation.
Wall on the left, finish on the right. Not to scale: coats, thicknesses and primers follow the chosen manufacturer’s system and the condition of the wall.
Starting at the base
The first row of boards sits on a base rail. JUB says the rail should be just above DPC level, or approximately 150mm above the ground, fixed level at no more than 300mm centres, with packers behind it on uneven walls. Where insulation continues below the DPC as a plinth, it uses a tougher board and a waterproof coating, and JUB's detail includes a 150mm gravel trap around the foot of the plinth rather than soil or paving hard against it.
Above that, boards are bonded with adhesive, butted tight with any gaps filled, staggered so no vertical joints line up, interlocked at corners and cut in an L shape around openings. Mechanical fixings go through each board into the wall; their length and number depend on the wall construction and pull-out tests.
From board to finished surface
On expanded polystyrene boards, JUB's figures give an overall basecoat thickness of 4–6mm, with drying time of about a day per millimetre applied.
Rasp the fixed boards flat so the basecoat goes on evenly.
Bed diagonal mesh patches at every window and door corner to resist stress cracking.
Fit frame seals, then fix corner and edge beads.
Apply about 4mm of basecoat, trowel the mesh in with 100–200mm overlaps, and add a further 1–2mm over it.
Let the basecoat dry, then prime it if the finish needs a primer.
Apply the finish to its grain size, keeping a wet edge, or apply a dash receiver and throw the aggregate.
Refit fittings on brackets made for insulated walls.
Sills, reveals and the roof edge
JUB asks for all sills and trims to overhang the finished face by at least 30mm, and for no flat rendered surfaces; where one is unavoidable, it should slope at 15 degrees or more. In window reveals, a 20mm board can be returned to the frame if there is room. If there is not, the reveal is rendered only, with double mesh, and stays a cold strip around the window.
At the top, the existing roof overhang may not cover the thicker wall. Extending it is roofing work, and in Garden Village, Hull City Council's Article 4 directions require planning permission for any alteration to the roof of a house. In any conservation area, the permitted development guidance treats cladding a house as needing a planning application.
Weather, regulations and upkeep
JUB's limits for its system are no application below 3°C on a rising thermometer or 5°C on a falling one, no surface hotter than 30°C, and protection from rain, frost and snow; humidity above 85% slows drying. Boards should only be bonded in dry conditions.
A full re-render is a practical time to assess EWI because access and preparation are already part of the job. It can also cross Approved Document L's renovation thresholds for dwellings: more than half of a thermal element or more than a quarter of the external building envelope, with the upgrade subject to feasibility. Ask the designer to show the calculation and proposed wall build-up. After installation, retain the system details, use fixings designed for EWI for anything added later, and repair damage with compatible basecoat and finish rather than an unrelated filler.
Common questions
Will external insulation fix condensation or damp?
It can make walls warmer, but it does not cure water getting in. Leaks, bridged DPCs and salt damp should be put right before boards go on.
Do the windows have to change?
Not always. Existing windows can stay if the reveal and sill details work with the new wall thickness, which is checked at survey.
See housing and planning information for Hull and nearby places, including the source and geography behind each figure.
