Wall insulation in Three Legged Cross, and where the moisture goes

What the original builder chose. Late eighties and nineties walls were often put up part filled, with boards already fixed against the inner leaf and a residual gap in front of them. Down a borescope that is unmistakable. It is also a wall to leave alone. Pump a partial fill and the bead ends up in the gap that was designed to stay clear, so the borescope earns its keep by stopping a job as often as starting one.

A cavity somebody had pumped. Common enough out here, and the question is only whether it is still doing anything. Slumped wool leaves a cold band under the eaves and a matching line of black mould on the bedroom wall inside. Fill gone wet, or water crossing on a bridged tie, throws damp patches that follow the weather rather than the seasons. Failed fill comes out and gets vacuumed away before a single thing is decided.

A clear gap nobody has touched. Wide enough, dry, no snots on the ties: that one gets drilled on a pattern, pumped and made good inside a day. Bonded polystyrene bead runs round the ties and settles where it is put. Blown mineral wool remains sound in a clean, sheltered cavity. Mortar for the holes is matched to the face of the wall.

Whoever fitted the replacement windows. New frames tend to sit forward in the opening, which shortens the reveal and leaves barely 40mm to insulate at the very place heat leaves fastest. A cold bridge, or thermal bridge, at a reveal will collect condensation whatever the u value of the wall beside it, and in a tight house it does so quickly. Aerogel or a thin high performance board is the answer there.

The extension, the conservatory, the converted garage. Rooms brought inside the heated envelope years after the fact usually have a wall the rest of the house does not: solid brick and block, or a cavity too narrow to fill. Those get lined internally, IWI, with insulated plasterboard bonded on or a batten and stud frame packed with mineral wool. PIR laminate, Celotex or Kingspan, gets there shallowest, vapour control taped so damp air cannot slip behind the board, per BS 8000-8.

Whoever pulled the noisy fan out. Or ducted it into a loft void. Or rendered across an air brick. Every one of those is undone before the walls are touched: vents in the frames that open, extraction over the hob and in the bathroom that discharges through the wall to outside, air bricks clear. Approved Document F sets the expectation, and this is the half of the trade most often skipped, because it is never what the customer rang about.

Nobody at all touched the junctions. They rarely do. Threshold, floor junction, eaves, joist ends, and the party wall, which wants a cavity barrier rather than loose fill blowing through into next door. External wall insulation, EWI, boards under a render carrier and a render finish, is the one route that settles the joist ends and the floor junction together, at the cost of changing how the house looks from the road.

No rate per square metre, because the wall area is the easy part; what happens at every window, door and floor junction is not.

Getting to it. A 1990s detached house on one of the closes, an older cottage on the crossroads and a bungalow on a deep plot are three different jobs before a hole is drilled. Cavity fill is drilled from outside and a bungalow comes off a tower where a three storey gable wants a scaffold. Internal lining is worked room by room from inside, so it is your rooms rather than a platform that gets disrupted. The work at height rules still set the platform.

Which wall you actually have. A cavity that is clear and dry can be filled in a day. A solid wall cannot be filled at all, and has to be lined inside or insulated outside, which is a different job at a different price. A borescope through one drilled hole settles it, and no honest quote is written before that.

The junctions, not the field of the wall. Reveals at windows and doors, the floor junction, the eaves and any party wall are where heat escapes and where condensation shows up if the detailing is wrong. That work is slow, it is most of the labour, and it is the first thing a cheap quote leaves out.

Comparing quotes? Ask whether the wall has actually been checked with a borescope or just assumed, what system and what thickness is specified, how the reveals and floor junction are being detailed, whether ventilation is being addressed, and who makes good the plaster and decoration afterwards. Two quotes for the same job are usually two different jobs.

We cover Three Legged Cross, Ringwood Road, Woolsbridge and the Verwood side, broadly BH21, under Dorset Council. You are dealing with Bournemouth Roofing & Building, the trading name of Bournemouth Roofing & Building Ltd, registered in England and Wales, company number 15471719, which you can look up in a minute. Thomas Hannen's business, and he is the one who comes out to price the job, not a salesman. A verified customer's 10 out of 10 on Checkatrade. The visit and the written quote are both free.