An old solid wall was built to get wet and then dry out again. Almost every retrofit that fails does so by stopping the second half of that sentence — usually with materials that would have been perfectly fine in a new build.
Doing these in the wrong sequence is how a warm house becomes a damp one. Nothing here starts with buying insulation.
Before anything else. Failed gutters, a cracked downpipe, ground raised above the floor level, a cement render trapping damp, a blocked airbrick. Almost everything blamed on “rising damp” is one of these, and they are all cheaper to fix than to insulate around.
Weeks, and mostly freeA wet wall dries slowly — a season for a thick one, longer if it has been wet for years. Insulating before it is dry seals the water in, and hemp gets blamed for what the gutter did.
One season, sometimes twoCement render, cement pointing, plastic paint, bitumen. These were added to keep water out and mostly keep it in. Removing them is disruptive, unglamorous and the highest-value work in the whole project.
The messy partHemp batt on a studded-out frame, or lime-hemp render for a thinner build-up that keeps the wall in one piece. Vapour-variable membrane, never polythene. This is the step everyone wanted to start at.
Where the material choice finally mattersA tightened house needs a way for moisture to leave, and trickle vents and an extractor that actually runs are the minimum. A sealed old house with no ventilation is worse than a draughty one, because the draught was doing a job.
Do not leave this to lastSize the system to the house you have made, not the one you started with. Specifying the boiler or the heat pump first is how people end up with an oversized system running badly for twenty years.
Last, and cheaper for waitingBefore you insulate, the wall is roughly as warm as the room and any moisture in it moves towards the inside and evaporates. Put insulation on the inside face and the masonry is now outside the warm layer — colder, wetter for longer, and cycling through freezing more often than it used to.
This is not a reason to avoid it. It is the reason to use a vapour-open build-up rather than a sealed one: the wall has to be able to dry outwards, because it can no longer dry inwards. Hemp and lime are chosen here precisely because they let that happen, and because they buffer the moisture that does arrive instead of holding it against the brick.
It is also the reason to be careful with embedded timbers. Joist ends buried in an external wall sit in the newly colder zone, and a beam that has been dry for a century can start to rot after a well-meant retrofit. Look for them before you board over.
Not everywhere, and the honest version is useful. On an old building hemp earns its place in three roles: lime-hemp render where you want to insulate a little without losing the wall's own character or taking 100 mm off the room; batt on a studded frame where you have the depth and want a real improvement; and loose shiv or hempcrete on the floor, where an old suspended timber floor is being taken up anyway.
Where it does not fit is any detail that needs to be sealed — below ground, behind a tanked basement wall, or anywhere standing water is expected. Those want materials designed for water, and asking hemp to do that job is asking it to be the thing it is not.
It is the visible, satisfying, grant-eligible step, so it goes first and the water is still there behind it. Every other failure on this list is downstream of this one.
Harder than the brick around it, so the brick face spalls instead of the mortar, and water runs through the joint it was meant to close. Repointing in lime is slow and is the correct answer on almost every old wall.
Draughts were doing unglamorous, essential work. Remove them without replacing the function and the moisture from cooking, washing and breathing has nowhere to go but the coldest surface in the room.
They sit in the part of the wall your new insulation has just made colder and damper. Find them, check them, and give them air — before they disappear behind the finish.
A cheap pin meter tells you about the surface and lies about the depth. Better is a humidity logger left in the room for a fortnight, plus the old test: tape a square of plastic sheet tightly to the wall and leave it a few days. Water on the wall side means the wall; water on the room side means condensation. Those two have opposite fixes.
Check before you specify anything, not after. Consent can rule out external insulation and external render entirely, which makes an internal breathable build-up the only route and changes the whole plan. It is a free phone call and it has reshaped a great many projects.
Yes, and it is usually the sane way to afford it — but do a whole wall including its reveals and its junctions, rather than stopping at a convenient corner. A part-insulated wall concentrates the cold at the edge of the work, and that edge is where the condensation appears.
Be careful whose survey. A report from a firm that also sells chemical injection and tanking tends to recommend chemical injection and tanking. An independent building surveyor with old buildings on their books is a different profession and worth the fee.