A Dry Wall Is a Warmer Wall

Sam Salthouse

6 hours ago

5 min read

When we talk about making an old stone building warmer, the conversation usually turns immediately to insulation.

But there's another important factor that is often overlooked:

Moisture.

The amount of moisture held within a traditional stone wall can affect how that wall performs thermally. Put simply, wet masonry generally transfers heat more readily than dry masonry.

This is one of the reasons we believe managing moisture correctly is so important in historic buildings.

And it's also one of the reasons lime mortar matters.

Why Does Wet Stone Feel Colder?

Natural stone contains a network of pores.

Depending on the type of stone, these pores can absorb and hold varying amounts of moisture. Traditional masonry walls will naturally experience moisture from rainfall, condensation, ground moisture and the everyday occupation of a building.

The important thing is giving that moisture somewhere to go.

Water conducts heat considerably better than still air. When the pore structure of a material contains more water, rather than air, its thermal conductivity can increase.

In simple terms:

A damp wall can lose heat more readily than the same wall when it is dry.

The exact change varies enormously depending on the stone, moisture content, wall construction and exposure, so there isn't one universal U-value that can be applied to every stone wall.

But the principle is important.

Keeping traditional masonry able to dry can contribute to better thermal performance as well as protecting the building fabric itself.

This Is Where Lime Mortar Comes In

Traditional stone buildings were generally constructed very differently from modern cavity-wall houses.

They weren't designed around creating completely impermeable barriers.

Instead, traditional construction used materials capable of absorbing moisture and, crucially, releasing it again when conditions allowed.

Lime mortar plays an important role in that process.

An appropriate lime mortar is generally more vapour permeable and more accommodating of moisture movement than a dense cement-rich mortar.

Rather than trying to completely seal water out of a traditional wall, the aim is to create a construction that can manage periods of wetting and then effectively dry again.

That's a very different philosophy.

The Mortar Joint Should Do the Work

In traditional stone masonry, we generally want the mortar joint to be the more sacrificial and moisture-accommodating part of the wall.

That's important.

If a very hard, dense and relatively impermeable cement mortar is introduced around softer or more porous stone, moisture movement through the joints can be restricted.

The moisture doesn't simply disappear.

It may instead remain within the wall for longer or be encouraged to evaporate through the stone itself.

Over repeated wetting, drying and freezing cycles, this can contribute to deterioration of vulnerable masonry.

We've all seen old stone buildings where the cement pointing remains standing proud while the edges and faces of the surrounding stone have deteriorated.

The mortar has survived.

The stone hasn't.

That's the opposite of what we want.

Lime Helps the Building Dry

One of the major advantages of using an appropriate lime mortar is its ability to support moisture movement through traditional masonry.

When conditions become drier, moisture held within the wall can migrate towards the surface and evaporate.

This doesn't mean lime magically removes every source of damp.

If you have a leaking gutter, defective roof, high external ground level, failed drainage or another significant source of water ingress, that problem still needs addressing.

Lime mortar is not a cure for a water problem.

What it does is give a traditionally constructed wall a better opportunity to manage moisture and dry in the way it was originally intended to.

And the drier we can keep that masonry, the better its thermal performance is likely to be.

Why U-Values Don't Tell the Whole Story

A U-value measures the rate of heat transfer through a building element.

The lower the U-value, the better its insulating performance.

But assigning a simple U-value to an old stone wall isn't straightforward.

Traditional walls vary enormously.

Different stones have different densities and pore structures. Walls vary in thickness. Some contain rubble cores and voids. Mortar composition varies. Exposure varies. And importantly, moisture content varies.

A laboratory value for a dry piece of stone therefore doesn't necessarily describe how a 600 mm thick, 200-year-old external wall performs during a wet northern winter.

This is why we think historic buildings need to be considered as complete structures rather than simply applying modern numbers and modern materials to them.

Cement Can Create the Wrong Conditions

Cement has an important place in modern construction.

But that doesn't mean it belongs in every historic stone building.

We regularly encounter traditional properties where hard cement pointing has been introduced during previous repairs.

Often the intention was understandable: create a strong joint and stop water getting in.

Unfortunately, traditional walls aren't always that simple.

If moisture gets into the masonry through cracks, defective detailing, the stone itself or from elsewhere within the building, a dense external joint can restrict the wall's ability to release that moisture through the mortar.

The wall can then remain damp for longer.

And if wetter masonry conducts more heat, that isn't only a conservation issue.

It can become a comfort and energy issue too.

Breathability Isn't About Having Draughty Walls

The word "breathable" gets used constantly in conservation.

It can be slightly misleading.

We aren't talking about air blowing through your walls.

We're primarily talking about the ability of materials to manage and release moisture, including water vapour.

A properly maintained lime-pointed stone wall can still protect a building from the weather.

The difference is that we're not trying to create an impermeable shell around a structure that was never designed to function that way.

Start by Understanding the Building

When somebody wants to make an older property warmer, adding modern insulation isn't necessarily the first thing that should happen.

First, look at the building.

Are the walls damp?

Are the gutters working?

Are external ground levels too high?

Has the building been cement pointed?

Have impermeable renders or coatings been applied?

Is water being directed away from the building properly?

Are the original lime materials still present?

Sometimes improving the way a historic building manages moisture can make the internal environment noticeably more comfortable before more invasive interventions are considered.

Traditional Materials Aren't Outdated

There is a reason lime mortar was used successfully for centuries.

Traditional builders understood their materials, even if they didn't describe them using modern terminology such as thermal conductivity, vapour permeability and U-values.

They created buildings capable of getting wet and, importantly, capable of drying again.

That's something we should be very careful about changing.

At Salthouse Stonemasonry, we use traditional hot-mixed non-hydraulic lime mortar for our pointing and conservation work because we want the mortar to work sympathetically with the stone around it.

We aren't trying to seal an old building.

We're trying to help it function as it was intended.

Because when it comes to traditional stone buildings, a wall that can properly dry is generally a healthier wall – and a drier wall can be a warmer wall.

Salthouse Stonemasonry Ltd