The short answer: if your house was built before about 1930 with solid walls, it needs lime plaster; if it is a modern cavity-wall or plasterboard building, gypsum is fine and cheaper. The deciding factor is breathability — lime lets moisture pass through and evaporate, gypsum does not — and in an old solid wall that single difference determines whether the building stays dry or slowly fails.
What each material actually is
Lime plaster is made from burnt and slaked limestone mixed with well-graded sand, often with animal hair in older work. It is the finish found in almost every British building constructed before the 1930s, applied in successive coats over masonry or timber laths. Gypsum plaster is made from calcium sulphate, usually supplied as a pink or grey bagged powder, mixed with water and applied as a fast-setting skim or backing coat. It arrived in general use in the early twentieth century and became the default finish in post-war construction because it is quick, cheap and predictable.
Neither material is 'better' in the abstract. Each was developed for a particular kind of construction, and problems almost always start when one is used on the wall type designed for the other.
Breathability: the difference that decides everything
Solid-wall buildings — stone, brick, cob — were built without cavities or damp-proof courses. They stay dry by managing moisture rather than excluding it: rain soaks in a little, moisture rises a little, and the whole lot evaporates back out through vapour-open surfaces. Lime plaster is part of that system. It absorbs moisture from the wall and releases it into the room air, where ventilation carries it away.
Gypsum is far less vapour-open, and it is also moisture-sensitive: it softens and loses strength when it stays damp. Put it on a solid wall and two things happen. First, moisture that would previously have evaporated through the surface is held back in the masonry, so the wall gets progressively wetter. Second, the damp eventually reaches the gypsum itself, which decays, blows and falls away. On a modern cavity wall none of this applies — the cavity keeps the inner leaf dry, so the plaster never has to handle moisture in the first place.
Setting vs carbonation
Gypsum sets by a fast chemical reaction with water — a skim coat is hard in hours and paintable in days. Lime hardens mainly by carbonation: it slowly reabsorbs carbon dioxide from the air and turns back into limestone. That process takes weeks, and each coat must be allowed to firm up before the next goes on. It is the main reason lime work takes longer and costs more, but it is also why lime remains slightly flexible and self-healing: micro-cracks can re-carbonate and close up rather than propagating.
Flexibility, cracking and repair
Old buildings move. Shallow foundations, timber lintels and seasonal ground moisture mean a Victorian or Georgian wall flexes in ways a modern one does not. Lime plaster is relatively soft and accommodates that movement; gypsum is harder and more brittle, so it tends to crack and part company with a moving background. Lime is also genuinely repairable — a failed patch can be cut out and reinstated invisibly, keeping the surrounding original material. Gypsum repairs are possible but the material itself is essentially disposable: once it has been wet, it is replaced, not repaired. In a building expected to stand for another century, that repairability is worth more than it first appears.
Cost and speed: the honest comparison
Gypsum wins on both, and it is not close. A gypsum skim over sound plasterboard is done in a day and painted within the week. Full three-coat lime plastering costs meaningfully more per square metre, and the carbonation time between coats stretches the programme to weeks rather than days. Every wall is different — preparation often costs more than the plastering itself — so a real price needs an inspection. The point is that choosing lime is never about saving money in the short term. It is about not spending far more later putting right the damage the wrong material causes.
Pros and cons at a glance
Lime plaster
- Pros: vapour-open, so solid walls can dry; flexes with building movement; repairable in place; resists mould because surfaces stay drier and lime is alkaline; correct and often required for listed buildings; lasts generations
- Cons: costs more per square metre; slower to apply and cure; needs breathable paint rather than standard emulsion; requires a plasterer who genuinely knows the material
Gypsum plaster
- Pros: cheap, fast and widely available; sets hard in hours; gives a crisp, flat modern finish; any competent plasterer can apply it; entirely appropriate on plasterboard and modern masonry
- Cons: low vapour permeability; softens and fails if it gets damp; brittle on moving backgrounds; not repairable once moisture-damaged; the wrong material for pre-1930 solid walls
When gypsum is perfectly fine
- Cavity-wall houses built from roughly the 1930s onwards, where the inner leaf stays dry
- Plasterboard on stud walls and ceilings, in buildings of any age, provided the board itself is isolated from damp masonry
- Modern extensions to old houses, where the new walls are cavity or timber-frame construction
- Internal partitions that never see moisture from the external envelope
When lime is essential
- Any solid-wall property built before about 1930 — the overwhelming majority of Victorian, Georgian and older housing stock
- Listed buildings, where like-for-like lime repair is usually a condition of consent
- Stone, cob and timber-frame buildings, which depend entirely on staying vapour-open
- Rooms with a history of damp, especially where gypsum or cement has already failed once
What happens when gypsum goes onto an old solid wall
This is the failure we are asked to put right most often. The gypsum goes on, looks immaculate, and for a year or two everything seems fine. Meanwhile moisture accumulates in the masonry behind it, carrying dissolved ground salts upwards and into the plaster. The salts crystallise near the surface as the wall tries to dry, pushing the finish off from behind; the damp softens the gypsum itself; and the owner sees spreading stains, bubbling paint and hollow-sounding 'blown' patches, usually along the bottom metre of the wall. The instinctive response — another skim, a waterproof paint, an injected damp course — treats the symptom and traps more moisture. The lasting fix is to remove the gypsum from the affected walls, let the masonry dry, and reinstate a lime finish that lets the wall behave as it was designed to.
How to decide for your building
Ask one question first: are the walls solid or cavity? A house built before about 1930, walls around 230 mm (9 in) or more thick, with no visible cavity at window reveals, is almost certainly solid-walled and should be finished in lime. If you are unsure, or the building is listed, get advice before anyone opens a bag of pink plaster — the wrong call is cheap on the day and expensive for years. We are based near Newport and assess solid-wall buildings across South Wales and the West; we will tell you honestly if gypsum is the right answer for your walls, because sometimes it is.