Wall insulation in Iwerne Minster, cob included

Working out what you have. Cob is usually rendered so you cannot see it, but the wall thickness gives it away, often 500mm or more, along with deep reveals and slightly uneven faces. Flint and rubble walls look similar and behave similarly. Later brick extensions and post-war houses may have a proper cavity, and that gets the borescope treatment: width, whether it is clear, and whether an old fill has slumped or the cavity is wet.

Cob, and why sealing it is fatal. Earth walls hold and release moisture constantly. Put an impermeable lining or a cement render on one and the water that gets in through the top, the base or a failed gutter has nowhere to go. It collects at the back of the barrier, the earth loses its strength and the timbers built into the wall rot. This is not a slow inconvenience, it is how cob walls fail.

What we line cob with. Wood fibre board bedded in lime and finished in lime plaster, or a lime based insulating plaster where the wall is too uneven to take a board. It gives less thermal performance per millimetre than a rigid foam, so it goes on thicker, and that is the correct trade. The dew point moves into a build up that can dry rather than one that cannot.

The hat and the boots first. On any earth or rubble wall the roof overhang and the base do more for it than insulation ever will. Gutters running, downpipes discharging clear, ground level dropped away from the wall, cement render and cement pointing removed where it is trapping water. We will not line a wall that is still getting wet, and we will say so.

Cavity fill where there is a cavity. On the later houses, bonded bead for a narrow, obstructed or exposed brick and block wall and blown mineral wool for a wide, clean, sheltered one. Drilled from outside, filled, holes pointed to match. One day, and Part L sets what the finished wall should reach.

Where a rigid board is right. On a sound modern wall or a later extension, insulated plasterboard or PIR such as Celotex or Kingspan on a batten or stud frame with taped joints and a continuous vapour control layer is the correct build up. Different walls want different answers, and the mistake is to use one specification on a whole house.

External systems. EWI with a mineral or lime render carrier can be done breathably and it keeps the whole wall warm and dry, but on a cob cottage in this village the appearance is controlled and the answer is usually no. It is worth asking about on a rear elevation or a later extension.

Junctions and air. Reveals, thresholds, the floor junction, the eaves and the joist ends built into the wall are the cold bridges and the rot risks together. Shallow reveals take an aerogel laminate. And an old cottage that has been tightened up needs trickle vents, working extract fans and air bricks kept clear, to Approved Document F.

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.

Iwerne Minster has its own conservation area and forty listed entries, so anything altering the outside face of a wall is an application, and on a listed cottage consent covers internal work as well under section 7 of the 1990 Act; Dorset Council holds the designation records. Internal work elsewhere is not development, per the Planning Portal's insulation guidance, though 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. On an old wall, tell us what is on the outside of it as well, because cement render trapping water is behind a great many of these calls. The phone is answered 24 hours for emergencies; scheduled work runs Monday to Friday, 8am to 6pm.

Call now: 07435 839745

This is one of the oldest villages we work in, with something approaching four in ten homes predating 1919, and cob under lime render is a common way of building here. A cob wall is not weak, it is moisture dependent. Dry, it will outlast most of the brick around it. Wet, it turns back into the field it came from.

That gives a clear order of work, and insulation is not first on it. Keep water off the top and away from the bottom: roof overhang, gutters, downpipes, ground level. Take off cement render or pointing that is holding water in. Only then think about warmth, and use lime and wood fibre when you do.

The honest advice is often that the walls are not where the money should go at all. Loft insulation to full depth, draughts sealed round doors, floors and unused chimneys, and secondary glazing will cut more of the heat loss you actually feel than lining a cob wall, and none of them risk the fabric. If somebody quotes you for wall insulation on a cob cottage without asking what is above your head or what the gutters are doing, be careful.

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