Traditional buildings were designed very differently from modern homes. Older stone and brick walls were built using breathable materials like lime mortar and lime plaster that allow moisture to move naturally through the building fabric.
This is what people mean when they say a wall needs to “breathe.”
What Happens in a Breathable Wall?
Moisture from rain, humidity, and everyday living can enter the wall in small amounts — but breathable materials allow that moisture to evaporate back out safely.
Lime mortar acts almost like a sacrificial material:
It absorbs and releases moisture
Allows evaporation
Helps reduce trapped damp
Protects softer stone and brick
The Problem with Modern Cement
Modern cement mortars and waterproof coatings are much harder and less breathable than traditional lime materials.
When used on older buildings they can:
✘ Trap moisture inside the wall
✘ Cause damp patches internally
✘ Lead to crumbling stone or brick
✘ Increase frost damage during winter
✘ Force moisture through surrounding masonry
Often the masonry itself starts failing before the cement does.
Traditional Buildings Need Traditional
Materials
Older buildings were never designed to be completely sealed. They manage moisture by allowing it to move and evaporate naturally.
Using compatible lime mortars and renders helps maintain that balance and protects the long-term health of the building.
Sometimes the best repair isn’t making a wall harder — it’s allowing it to function properly again.
What’s the Difference Between Hydraulic & Non-Hydraulic Lime Mortars?
Not all lime mortars work the same way. One of the biggest differences is whether the lime is hydraulic or non-hydraulic.
Non-Hydraulic Lime (Lime Putty)
Non-hydraulic lime sets by absorbing carbon dioxide from the air — a slow process called carbonation.
It is:
Softer and more flexible Very breathable Slower setting Traditionally used in many historic buildingsBest suited for:• internal plastering• sheltered masonry
• conservation work on softer historic materials
Because it cures slowly, it needs protection from frost and heavy rain while setting.
Hydraulic Lime (NHL)
Hydraulic lime contains natural clay and minerals that allow it to set with water as well as air.
It is:
Faster setting More weather resistant Stronger than lime putty mortars Suitable for more exposed locationsBest suited for:• external pointing
• chimneys
• retaining walls
• damp or exposed environmentsHydraulic lime comes in different strengths such as NHL 2, NHL 3.5, and NHL 5 depending on the job and exposure.
Why It Matters
Choosing the correct lime mortar is important for the health of a building.
Using a mortar that is too hard can trap moisture and damage historic stone or brick over time. A properly chosen lime mortar allows the building to move and breathe naturally.
Traditional buildings perform best when repaired with compatible materials — not just the strongest materials.