Wall insulation in Ashmore, front and back apart

What the wall is. A flint and rubble wall, a nine inch brick wall and a post-war brick and block cavity look less alike than they sound. The reveal depth is the first test and the borescope is the last one: cavity or not, how wide, clean or bridged, dry or a wet cavity, empty or already filled and slumped.

Cavity fill where the camera says yes. Bonded bead into a narrow, obstructed or exposed cavity; blown mineral wool only where the gap is wide, clean and sheltered. Drilled from outside on a grid, filled, holes pointed to match. One day, and the shortest payback of anything on this page, with Part L setting what the wall should reach.

Solid walls, which is most of the older village. Nothing to fill, so the choice is internal wall insulation or external wall insulation. IWI is cheaper and invisible from the road. EWI gives the better U value and deals with the eaves and the joist ends, but it changes the appearance and needs consent here.

Doing the back and leaving the front. That is a legitimate way to run this job and we do it often. External insulation on a rear elevation that nothing overlooks, internal work on the frontage, or nothing at all on the frontage where the reveals are too shallow to take anything sensible.

The join. Wherever an insulated wall meets one that is not, you have made a thermal bridge. The lining is returned along the uninsulated wall for half a metre or so, carried down past the floor junction and taken tight into the reveal, so the cold strip has nowhere to form. That returning detail is the single thing most quotes leave out, and it is where the black mould turns up when it is missed.

Build ups. Insulated plasterboard or PIR such as Celotex or Kingspan on a batten or stud frame with taped joints and a continuous vapour control layer where the wall is sound and modern. Wood fibre and lime where it is old and needs to stay breathable, so the dew point sits somewhere that can dry. Shallow reveals take an aerogel laminate; an external system takes a mesh basecoat and a render carrier with a finish chosen to suit the street.

Ventilation. Trickle vents, extract fans that shift air, air bricks kept clear rather than covered over, to the rates in Approved Document F. Airtightness without ventilation swaps a cold house for a damp one.

Putting failures right. Fill that has slumped, a wet cavity, a lining that is cracking and black behind. Extraction is drilled from outside, the wall is dried and checked, and we say honestly whether it should be done again.

Same team from first visit to last, one person to ring, and no sub-contracting your work out to somebody you have never met. The quote is itemised, not a figure on the back of a card: the survey and any borescope check, the system and its thickness, making good around windows and doors, ventilation works, and disposal on separate lines, so you can compare it properly against anyone else's.

Nothing is left open or unusable overnight. The site is swept at the end of each day rather than the end of the job, and what comes out leaves with us. If something turns up once we start that nobody could have seen, you get a photograph and a price before we carry on.

Ashmore has its own conservation area, so anything that changes the outside face of a wall is an application, and consent is needed inside as well as out on a listed building; Dorset Council holds the designation records. Internal insulation on an unlisted house is not development, per the Planning Portal's insulation guidance, and regulation 23 brings building control in once more than half a wall is renovated.

Ring rather than use the form. Damp patches or black mould appearing on an outside wall after cavity fill or internal lining usually means water is tracking across the cavity, or warm air is reaching a cold surface at a junction. We look before we take anything out, because the fix depends on which of those it is. Say whether the mould sits in a corner, along the skirting or round a window, because each of those points at a different cold bridge. The phone is answered 24 hours for emergencies; scheduled work runs Monday to Friday, 8am to 6pm.

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The whole village sits inside a conservation area and inside the Cranborne Chase National Landscape, and both of those are about how things look. For wall insulation that has a very specific effect. Nobody is objecting to the insulation. What gets scrutinised is a 100mm thicker wall, windows that end up sunk into deep reveals, an eaves overhang that no longer throws water clear of the wall, and a modern looking finish on a street of flint, brick and lime. Dorset Council publishes what is designated and where to apply.

So we survey elevation by elevation rather than pricing a house. A rear wall behind a garden, screened by outbuildings and not visible from any road, is often a straightforward external job. The frontage on the same house may be internal work or nothing at all. Splitting it that way is usually the difference between a scheme that gets consent and one that does not.

The trade off is honest and worth knowing before you start. Doing half a house properly gives you less on paper than doing all of it, and it puts a real premium on the junction detailing where the two halves meet. Done well you get a warm dry house. Done carelessly you get a warm house with one cold damp corner, and that corner is worse than the wall you started with.

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