Wall insulation in Wimborne Minster

Before the week starts. Behind render, a nine inch solid wall and an early brick and block cavity are the same object, and in this town the two sit within a few streets of one another. Drill, scope, look: gap or none, what width, clear or fouled, dry or damp, empty or already filled. Minutes of work, and it is the fork in the road. Alongside it goes the paperwork question, because on a listed building the answer decides what may be fixed to the wall at all.

Monday, and what the wall will accept. On a pre-1900 wall, internal wall insulation is the practical route. On ordinary fabric that means insulated plasterboard bonded on, or batten and stud filled with mineral wool or PIR, then boarded and skimmed, with Celotex and Kingspan buying more per millimetre and aerogel board earning its place in a small room. On genuinely old fabric it means something else: wood fibre or a mineral board bedded in a lime based adhesive, so the wall carries on drying inwards instead of being sealed shut.

Tuesday and Wednesday, the slow bit. Lime based work goes on slower than a modern board and cures slower still, and there is no honest way to hurry either. Reveals, threshold and floor junction are detailed as the build up goes up rather than patched afterwards. This is where the heat loss in a lined room actually lives, and where a cold bridge turns into a mould patch if the detailing is skipped.

Thursday, and the joist ends. In an old house the floor joists are built into the wall itself. Board over them without thinking and you have wrapped structural timber in a cold, damp pocket, which is a considerably worse problem than a chilly room. They get detailed so the timber stays dry and ventilated. It is the single item most worth asking any quote about.

A different week entirely, out on the newer streets. Where the cavity is clear, dry and wide enough, it is filled from outside through drilled holes made good in matching mortar. Bonded polystyrene bead reaches the corners and stays put; blown mineral wool remains fine on a sheltered wall. Wet or narrow, and we will tell you to leave it. External wall insulation, board on the outer face with a render carrier and a thin coat over it, deals with the thermal bridge at eaves and corners in one pass, but in the centre it is a planning matter and on a listed building it is very unlikely to be the answer. It belongs to the outer streets.

Every week, whatever the wall. Fill that has slumped leaves a cold band at the head of the wall; a wet cavity brings penetrating damp inside with black mould behind it. The borescope reads both and extraction comes first. Then the air: trickle vents, a working extract fan, air bricks kept clear. Because insulating shifts the dew point and removes the draughts, a taped vapour control layer on modern build ups, and breathable materials on old ones, keep interstitial condensation out of the fabric.

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 period house on the square, a purpose built flat off the centre and a post-war semi on the outer streets 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 the town centre, Leigh, Colehill's western edge and the BH21 addresses around them, with Dorset Council for planning and building control. 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.