Guide
Lime or gypsum: why it matters on an old wall
A solid wall built before about 1919 has to breathe. Gypsum on one traps moisture in the masonry, and it is the most expensive common mistake in this trade.
Find out what your wall is first
Measure the wall at a window or door reveal. Around 230 millimetres of solid brick means a solid wall. Around 250 to 300 with a gap in the middle means a cavity.
The rough date test: houses built before about 1919 are almost always solid, cavity construction spread through the 1920s and 30s, and anything post-war is cavity unless it is stone. Brickwork laid in a pattern where some bricks show their short end every few courses is a solid wall showing you its bond.
Why a solid wall needs to breathe
A cavity wall keeps water out by having a gap: rain soaks the outer leaf, the cavity stops it crossing, and the inner leaf stays dry.
A solid wall has no gap, so it manages water differently. It absorbs rain into the masonry and then lets it evaporate back out as the weather improves. The wall is meant to get damp and then dry, in both directions, and every material originally put on it was chosen to let that happen.
What gypsum does to that wall
Gypsum plaster is harder than the old masonry, it is effectively closed to water vapour, and it is dimensionally different from lime.
Put it on a solid wall and you block the inside half of the evaporation route. The water that the wall absorbs now has fewer ways out, so it travels: up, along, and out somewhere you were not expecting. It carries salts with it, which come through the decoration as patches that reappear however many times you paint over them. And because gypsum is harder than what it is stuck to, it eventually loses its key and comes off in sheets.
What lime does instead
Lime plaster is vapour open, so moisture passes through it and evaporates from the surface. It is softer than the masonry, so when the wall moves the plaster accommodates it rather than cracking away. And it is mildly alkaline, which discourages mould.
The cost is time. Lime carbonates rather than sets, taking up carbon dioxide from the air over weeks, so a three coat job runs across a month or more with the wall protected from sun, frost and draughts in between.
What it costs, and what the alternative costs
Lime plastering runs £60 to £110 per square metre against £18 to £28 for a gypsum skim. It is roughly three times the price and several times the programme.
Set against that: replastering the same wall twice, chasing damp patches around the house between, and redecorating each time. The argument for lime on a solid wall is not conservation sentiment, it is that the cheaper option frequently has to be paid for more than once.
Common questions
How do I know if my wall is solid or cavity?
Measure at a window reveal. Roughly 230mm of brick with no gap is solid; 250mm or more with a cavity is not. Pre-1919 is almost always solid, and header bricks showing their short ends in the bond are a strong sign.
Can I use gypsum on a solid wall?
You can, and on a dry, sheltered, already-cement-rendered wall it may cause no visible trouble for years. On an exposed or already damp solid wall it traps moisture in the masonry, which is the most expensive common mistake in this trade.
Is lime plaster only for listed buildings?
No. It is right for any solid wall that needs to breathe, listed or not. On a listed building it is usually also a condition of consent, which is a separate reason.
How much more does lime cost?
About three times a gypsum skim per square metre, £60 to £110 against £18 to £28, and the programme is weeks rather than days because each coat has to carbonate before the next.