Lime plaster and why older walls need to breathe

Lime plaster is one of the oldest wall finishes still in use, and many older American houses were finished with it. Unlike gypsum board or cement-based plaster, lime is porous, slightly flexible and able to move moisture in and out of a wall, which suits the way both solid masonry and timber-framed walls behave. Understanding why explains why lime is often the right finish for an older wall, and why modern patches and paints can cause damage that is not visible at first.

A traditional wall is designed to dry out, and its finish should not stop that from happening. Rain, ground moisture and the moisture from everyday living, such as cooking and bathing, all find their way into the wall. A finish that lets vapor escape protects the wall. A finish that traps water moves the damage into the wood, masonry or framing behind it, where it is harder to see and harder to repair.

How lime sets and hardens

Lime plaster is made by mixing lime, produced by burning limestone or chalk, with sand and water. Traditional plasters were often reinforced with animal hair such as cattle or horse hair, which held the plaster together while it cured. In air lime, the most common form, the plaster hardens through carbonation, a slow reaction in which the lime takes carbon dioxide from the air and returns to calcium carbonate. This takes weeks to months, and the plaster gains strength gradually.

Hydraulic lime sets partly through reaction with water and gains strength faster. Both types are softer and more flexible than cement, so they accommodate small movements in a wall without cracking as harshly. That softness is part of how the material protects the older structure around it.

How lime plaster grips wood lath

Many older American houses are wood-frame, with lime plaster applied over wood lath, which is thin strips nailed across the studs with narrow gaps between them. The plaster is built up in coats. The scratch coat is pressed hard against the lath so that it squeezes through the gaps and forms keys, small bulges that hook behind the strips. While it is still soft, the scratch coat is scored with horizontal grooves that give the next layer something to grip. The brown coat builds the body of the wall and brings it flat, and a thin finish coat makes the smooth surface.

The keys do most of the holding. When the lath rots, or the keys break after years of movement or a poorly placed fixing, the plaster can sag, bulge or pull away even though the surface looks intact. Such a wall should be checked before anything heavy is hung on it or patched over.

Why older walls need to move moisture

Solid masonry walls, common in older brick and stone houses, have no air cavity between the outside and the inside. They absorb water from rain and the ground and release it by evaporation over time. Lime is vapor-permeable and allows that movement, while impermeable paints and cement-based coatings slow it and force water to escape elsewhere.

Ground moisture is a frequent problem. Water can rise through masonry by capillary action, and many older American foundations were built with no damp-proof course, the impervious layer that blocks this rise in newer construction. Clogged gutters, downspouts that discharge against the wall and grade that has built up around the house all add water to the masonry. A plaster or paint that stops the wall from breathing hides these problems until the damage appears in a different form.

Patches and paints that trap moisture indoors

Inside an older house, lime is most often damaged by repairs that do not match it. Gypsum plaster, joint compound and cement-based patches are harder and less permeable than lime, and they set at a different rate. A patch in lime plaster tends to crack at its edges, and moisture that cannot pass through the patch is pushed out through the lime beside it. On wood lath, moisture that cannot escape the plaster can reach the lath and framing behind it, and decayed lath lets the plaster fall.

Paint is the other common problem. Vinyl, oil-based and high-gloss paints form a film that resists vapor, and moisture trapped under that film can blister and peel, taking the surface with it. Mineral paints and limewash allow the wall to breathe and can be renewed without building up a film. Test any new finish on a small area first.

If you are not sure whether a wall is lime or gypsum, look at its history first. Gypsum board came into common use in the twentieth century, but gypsum plaster was also used in older work, so the test that settles it is chemical. Lime plaster, once carbonated, is largely calcium carbonate and fizzes when dilute acid is applied, while gypsum does not. A conservator can run that test on a loose fragment. Do not try it on the wall yourself, because acid is hazardous to skin, eyes and finishes.

Caring for lime plaster in daily use

Day-to-day care is mostly about keeping the wall able to breathe. Keep foundation and crawl-space vents clear, and ventilate kitchens and bathrooms so that steam does not accumulate in the wall. Small cracks can often be filled with a compatible lime-based material, and fine hairline cracking is a normal part of how the material ages.

Lime is alkaline, so wear gloves and eye protection when mixing or cutting it out. Paint on plaster in homes built before 1978 may contain lead, so test before sanding or scraping. Harsh chemical strippers and aggressive mechanical methods can damage lime plaster, so work slowly and test any method on a small area first.

When to bring in a specialist

Damp diagnosis, structural cracking and large areas of failed plaster call for an assessment by a preservation contractor, a building conservator or a structural engineer, who can identify the moisture source and the cause of the damage. An experienced lime plasterer can also match the mix and technique to the original wall.

Houses in historic districts, or individually landmarked buildings, may require permission before finishes, pointing or repairs are changed, and those rules vary by city and county. Check with the local historic preservation commission or building department before starting significant work, and keep a record of what was done. Future owners and professionals will benefit from knowing what the wall was built from and how it has been repaired.