Lime Plaster vs Gypsum Plaster in Period Homes: Breathability, Damp and Repair

The idea in one minute

The choice between lime plaster vs gypsum plaster in period homes is mostly a choice about moisture. Old solid walls take in water and give it back; the SPAB says buildings from before the mid-19th century rely on letting absorbed moisture evaporate from the surface, while modern buildings rely on impervious barriers to keep water out.^[1] Put a dense, waterproof plaster on an old wall and the moisture has to go somewhere else.

Our companion guide lime plaster vs gypsum plaster explains what each material is and how it sets. This guide is for the decision you face when a wall actually fails: what the conservation bodies say, how to read a damp problem before you plaster over it, how to repair, and what to paint on top.

This is general information, not a survey. We make ceiling roses and wall lights, not plaster for walls, so we cite what conservation bodies publish, and we label our own rules of thumb as ours.

What breathable really means

‘Breathable’ is a trade word, and the SPAB warns that it is often claimed for products that are only slightly vapour permeable.^[1] What it means in practice is that the wall can dry. Historic England says moisture is normal in all buildings and becomes a problem only when there is too much of it.^[2]

The SPAB note says dense plasters block evaporation, so moisture rises and accumulates, and salt crystals can then push the plaster off the wall or make it bulge. It adds that dense plaster over rising damp may only push the damp higher, reaching plaster that was sound and timber in contact with the wetted wall.^[1] It also says plasters should be relatively permeable, and that traditional plasters were usually lime-based.^[1]

Historic England makes a related point about retrofit: removing or covering breathable finishes reduces a building’s ability to manage humidity.^[3] Lime plaster is part of how an old wall manages itself.

Why gypsum is the usual problem

The New Forest National Park Authority says gypsum, in the form of plaster of Paris and modern bagged plaster, is generally unsuitable for historic repairs. It calls it stiff and waterproof, says it can stop a wall breathing, and notes that it may break down in damp. It adds that bagged types contain a water repellent and are incompatible with lime.^[4] Tendring District Council says the same and adds that the incompatibility applies even to small patches.^[5]

Tim Ratcliffe, a lime plasterer writing in The Building Conservation Directory, goes further. He says that on walls meant to breathe, gypsum plasters encourage condensation and mould growth.^[6] That is one practitioner’s view rather than a test result, so treat it as informed opinion. Ratcliffe also notes that plaster of Paris may be added to a finishing coat to get a blemish-free surface, but that this has to be weighed against technical compatibility with the old plaster.^[6]

Two caveats. First, gypsum is not wicked everywhere: it is the right material for a new partition or a dry modern extension, and cast ornament has long been made in it. Second, none of this makes every old house a lime house. The Building Conservation Directory piece is about walls meant to breathe, and our earlier guide explains how to test what you actually have.

Read the damp before you plaster

Plaster is often blamed for damp it did not cause. Historic England warns that time and money can be wasted if the cause of damp is not properly diagnosed, and that remedial work based on a wrong diagnosis can make the problem worse.^[2] The New Forest leaflet lists common sources: defective roofs and flashings, blocked or faulty gutters and downpipes, cement render that traps water behind it, eroded or cement-based pointing, ground levels that sit too high against the wall, and condensation from poor ventilation and heating.^[7]

Cotswold District Council agrees that the most common cause is poor maintenance and inappropriate materials, and says to identify the cause before acting. If it is unclear, it recommends a suitably qualified surveyor who follows BS 7913, BS 5250 and BS 6576.^[8] The New Forest leaflet adds that older buildings were built without damp-proof courses, and that a surveyor without experience of traditional buildings may misread the higher moisture readings found in old walls as a defect.^[7]

Both councils are sceptical of the quick fixes. Cotswold advises against injected damp-proof courses, cement render, tanking and waterproof paints, saying they are unlikely to solve the problem.^[8] The New Forest leaflet says chemical treatment alone cannot provide a long-term solution, that injection works poorly in flint and rubble-filled walls, and that contractors may ask for plaster to be stripped up to about a metre.^[7] Its gentler route: let the plaster dry, brush off salts, and repair with lime plaster.^[7]

The SPAB warns that saturated solid walls can take many months or even years to dry, and that salts deposited during drying can cause severe breakdown.^[1] Patience is part of the repair.

How lime plaster is built up

Traditional lime plaster goes on in coats. Ratcliffe says three coats were typical in most buildings from the second half of the 17th century, which allowed a flat finish, while one or two coats were common in agricultural buildings where an undulating surface was acceptable.^[6] He gives a scratch coat of about 1 part lime putty to 2.5 parts sand, no thicker than 15 mm; a floating coat of about 1:3, also no thicker than 15 mm and usually without hair; and a setting coat of 2 to 5 mm applied in two or three thin layers.^[6]

Hair is added to the scratch coat for tensile strength. It is not strictly necessary on stone or brick, but it becomes essential on laths, where it helps the plaster hold.^[6] The New Forest leaflet specifies goat or horse body hair, says mane, tail and human hair are unsuitable, and says hair should be added at mixing because wet lime degrades it over time.^[4] Traditional laths were riven oak or chestnut, and it recommends brass screws for replacement laths rather than nails.^[4]

One practical consequence: Ratcliffe says old plaster that sounds hollow is often still sound if it contains hair.^[6] Do not condemn a ceiling on sound alone. Get it looked at by someone who works in lime before it comes down.

Repairing instead of replacing

Both leaflets prefer patching. Tendring calls simple patch repair the traditional approach to maintenance and the preferred option: remove the minimum, and keep earlier patches, which add to the building’s character.^[5] The New Forest leaflet says to identify and fix the cause of failure before treating symptoms, match new materials to the existing in composition, proportions, texture and consistency, and use the same techniques as the original.^[4]

Details that matter on site, from the same sources: apply in coats, usually three, with scratched keys and dampening between coats; avoid frost and hot, drying conditions; and do not use metal beads or guides to straighten edges artificially.^[4][5] Ratcliffe says the scratch coat usually takes about two weeks to dry, sometimes four, and is ready for the next coat when a thumb can no longer indent it; the floating coat is left about a week before the finishing coat.^[6]

He also says hydraulic limes are generally avoided internally because they are less flexible and breathable than white or fat limes, and that lime putty matured for about three months forms finer particles with better adhesion.^[6] These are the kinds of specification points to check with your plasterer. Our rule of thumb: if a quote just says ‘skim’ with no mention of lime, hair or coats, ask more questions.

Paint: the other half of breathability

The right plaster can be defeated by the wrong paint. The New Forest leaflet describes modern emulsion paints as poorly suited to lime: they reduce vapour permeability, can crack and trap water, and cause peeling. It recommends limewash or distemper instead.^[4] Tendring suggests finishing internal lime plaster with limewash, roughly 1 part lime to 2 parts water, in multiple coats, or distemper if preferred.^[5] The SPAB says the same for exteriors: paints tend to prevent evaporation, and limewash is the porous option when repainting.^[1]

If you love the matt, slightly cloudy look of limewash, our guide to wabi-sabi and limewash shows how designers use it. If you want colour on an old wall, our heritage palettes guide is the place to start.

Listed buildings and consent

In a listed building, changing the plaster can need consent. The New Forest leaflet says limited patch repairs using identical materials typically do not need listed building consent, but replacing whole walls or ceilings, or changing materials or techniques, usually does; the local planning authority decides.^[4] Replacing lime plaster with an impervious, salt-resistant plaster may also need consent.^[7] Historic England says to check with the local conservation officer first, that applying costs nothing, and that unauthorised work to a listed building is a criminal offence.^[10] If you are not sure whether your house is listed, see how to find out if your house is listed.

Plaster and insulation

If you plan to insulate internally, the plaster question gets sharper. Historic England says the wall must be free from excess moisture before wall insulation is installed, and that unsuitable insulation can trap moisture and hinder natural drying.^[11] It also notes that traditional lime and earth plasters can achieve significant airtightness.^[11] Our guide to insulating a period home without ruining it covers the wider picture.

Common mistakes

• Plastering before finding the damp source. Historic England warns that a wrong diagnosis can make things worse.^[2]
• Using bagged gypsum for a lime patch. Both leaflets say it is incompatible with lime.^[4][5]
• Hacking off sound plaster. Hollow-sounding hair-reinforced plaster may still be sound.^[6]
• Sealing with cement or waterproof paint. Cotswold says they are unlikely to solve damp.^[8]
• Painting lime with vinyl emulsion. The leaflets say it traps water and peels.^[4]
• Rushing the drying. Coats need weeks, not days.^[6]

Quick-reference table

Glossary

What to do next

Walk the outside first: gutters, downpipes, ground levels, pointing. Then tap the walls inside and photograph anything hollow or powdery. Read repairing cracked plaster cornice if the problem is on a ceiling, and how to read brick bonds and pointing if the outside is the likely cause. For ornament, a hand-cast plaster rose or wall light goes on after the wall is sound, never as a repair to it.

Keep reading

• Lime plaster vs gypsum plaster
• A short history of plaster
• Repairing cracked plaster cornice
• How to read brick bonds and pointing
• How to find out if your house is listed
• Wabi-sabi and limewash

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Frequently asked questions

What is the difference between lime plaster and gypsum plaster in a period home?

Lime plaster is the traditional, breathable material; the New Forest National Park Authority recommends it for historic buildings. Gypsum is stiff and waterproof, can stop a wall breathing and is generally unsuitable for historic repairs, according to the same leaflet.

Can I use gypsum plaster on an old solid wall?

Conservation guidance advises against it for historic repairs. Tendring District Council says bagged gypsum is incompatible with lime even for small patches. Ask a plasterer experienced in lime to specify the repair.

Why does breathability matter?

The SPAB says old buildings rely on absorbed moisture evaporating from the surface. Dense plasters block that, so moisture accumulates, and salts can push plaster off the wall.

Is plaster the cause of my damp?

Not necessarily. Historic England warns that a wrong diagnosis can waste money and make things worse. Check roofs, gutters, ground levels and pointing first, and use a surveyor experienced with traditional buildings.

How long does a lime plaster repair take to dry?

Ratcliffe says a scratch coat usually takes about two weeks and sometimes four, with the floating coat left about a week before the finishing coat. Weather and thickness change this.

What paint should I use on lime plaster?

The New Forest and Tendring leaflets recommend limewash or distemper, and describe modern emulsion as poorly suited because it reduces vapour permeability and can peel.

Do I need consent to repair plaster in a listed house?

The New Forest leaflet says limited patch repairs with identical materials typically do not need consent, but replacing whole walls or ceilings, or changing materials, usually does. Ask your conservation officer first.

Is gypsum ever the right choice?

Yes, for new partitions, extensions and dry modern construction, and for casting ornament. This guide is about old solid-walled houses, where conservation bodies prefer lime.