Stone walls have been used in construction for centuries. They’re valued not just for their durability but also for their aesthetic appeal, providing timeless character and a sense of history. However, behind their solid appearance hides some insulation challenges.
Poorly insulated stone walls are responsible for heat loss, which, in terms of energy efficiency, is equivalent to flushing money down the drain. Energy bills aside, poor insulation can lead to dampness, chilly draughts, and even pest infestations. The latter is likely to happen if there are gaps in your stone walls that pests can burrow into and call their home.
On that account, this in-depth guide will cover everything you need to know about how to insulate stone walls. We’ll look at the benefits of proper stone wall insulation, the challenges associated, and the installation process.
Key Benefits of Insulating Stone Walls
Thermally inefficient in most climates, stone walls are considered prime candidates for insulation. Here are some of the many ways in which insulating stone walls can make a huge difference in improving comfort and efficiency:
Insulation acts as a barrier to heat flow, meaning it slows down the transfer of heat out of a building in cold weather. And in warmer months, insulation keeps the hot exterior from heating up the interior.
So, basically, insulation enables stone walls to retain interior heat in the winter and remain cool in the summer. This reduction in respective heat loss and heat gain reduces energy costs for heating and cooling.
Installing insulation helps keep interior wall surfaces warm, which reduces the risk of condensation and associated mould growth on cold surfaces.
Insulating your stone walls will also aid in controlling the movement of moisture through the stonework. This in turn prevents moisture-related damage, like mortar joint erosion.
Did you know that cycles of freezing and thawing can deteriorate stone and mortar over time? You see, water can seep into porous stone and mortar joints, and expand as it turns into ice. This expansion stresses stone walls and causes cracks and flakes to develop.
Insulation minimises this freeze-thaw damage in two ways:
- Professional insulation keeps your stone walls warmer, which in turn reduces the frequency and duration of freezing temperatures penetrating your walls. Less freezing means a lower risk of ice expansion damage.
- Insulation also reduces moisture penetration. And with less water infiltrating the stone and mortar that make up your walls, the risk of freezing and ice expansion is reduced.
Warm, dry walls eliminate cold draughts and dampness problems that make a living space uncomfortable. Besides, insulation minimises noise transmission through walls, so it creates a quieter interior living space.
And if that’s not enough, insulating your stone-walled property increases its market value, making it more desirable and easier to sell.
Common Insulation Materials for Stone Walls
Some of the most common materials used to insulate stone walls include:
Rigid foam insulation boards, like polystyrene and polyurethane, boast a high R-value per inch, which simply means they provide reliable thermal resistance. They’re also valued for their water resistance and moisture control.
The thing about using rigid foam boards is that it has to be coupled with a highly protective finish since foam boards are vulnerable to damage. They must also be treated with a fire retardant, as they can be combustible.
Note that while foam boards are lightweight and easy to install, some contain blowing agents that deplete the ozone layer. If possible, you should avoid such varieties.
Mineral or stone wool insulation encompasses both rock wool and glass wool. They’re made from natural materials, so they’re more eco-friendly than foam boards.
Mineral wool also provides excellent fire resistance with high melting points. In fact, it can resist temperatures up to 1,000 °C! What’s more, it can maintain its R-value when damp, meaning it’s great for minimising moisture risks.
Having said that, we should point out that stone wool doesn’t have the highest R-value to begin with. It also has the tendency to settle, which leads to reduced thermal performance over time.
This type of insulation is made from recycled paper products. It’s very effective as far as moisture and noise control, and it’s eco-friendly considering its production relies on recycling.
One of the great things about cellulose is that it usually uses boric acid as a fire retardant. And if you know anything about boric acid, you know that it’s used to control and keep out insects and other pests.
No insulation material is perfect, though.
Like mineral wool, cellulose settles and loses its thermal performance over time. This is why we recommend buying a variety that has a high initial density to ensure long-lasting insulation.
If your stone walls have a lot of cracks and gaps, polyurethane spray foam may just be the most suitable insulation material for your needs.
Not only does it expand and harden to insulate cracks and gaps; it also makes for airtight insulation that minimises drafts and energy leaks. Besides, most varieties have a high R-value.
The downside of spray foam is that it’s expensive compared to other insulation materials. That’s not to mention the potent off-gassing as the foam is curing—proper ventilation is crucial!
Preparing Stone Walls for Insulation
Inspecting stone walls and making the necessary repairs is important before insulation is installed. Otherwise, you run the risk of compromised insulation and having to redo the entire project all over again.
You should look for:
- Cracks or damaged areas of stone or mortar that require repointing.
- Signs of moisture damage, like eroded joints, efflorescence, and stains.
- Loose pieces, missing mortar, or gaps that require filling.
- Debris, dirt, or salts that should be brushed or washed off pre-insulation.
Taking the time to identify and address these issues not only prolongs the efficacy of insulation but also prevents further structural deterioration.
Techniques for Installing Stone Wall Insulation
There are many different techniques for installing insulation in existing stone walls. Which technique you choose boils down to whether you’re treating the exterior, interior, or cavity of your stone walls.
Exterior insulation is applied to the outer surface of the stone wall. In most cases, the goal behind this insulation technique is to minimise heat loss by fully encasing the wall in insulation.
Here are the general steps:
- Start by adhering rigid foam boards directly to the cleaned exterior of your stone wall using an appropriate insulation adhesive that’s designed for masonry applications. A good example is the Loctite PL 300. Make sure the boards are cut to fit tightly and properly joined at the seams to prevent thermal bridging.
- Mechanically fasten the insulation boards with masonry anchors and washers, sealing joints with a strong weatherproofing tape for protection against wind loads.
- Finish your exterior insulation with a weatherproof coating like cement rendering or cladding to protect the wall from weathering and bulk water intrusion.
Insulating a stone wall from the inside involves installing non-structural wall framing against the interior wall to hold the insulation material and provide space for a new finish.
The general steps are as follows:
- Erect new stud walls—could be made from wood or metal—with enough air gap space to the stonework.
- Fill the stud cavities with a high-density insulation material like glass or mineral wool.
- Stagger the joints and tightly fit the insulation material into the cavities without any gaps.
For interior insulation, make sure there’s proper ventilation and vapour control (via vapour retarders) in the air gap between the insulation and the stone to minimise condensation risks.
For cavity walls, injecting loose-fill insulation—as in small particles of fibre, foam, or another material—or inserting rigid boards can add insulation while maintaining historic finishes.
The process involves injecting mineral wool, cellulose, or a loose-fill material of your choosing through holes drilled into the external mortar joints. Then, fill tubes are used to pack the insulation into the cavities to prevent settling.
Alternatively, you can cut rigid boards and fit them into the cavities, securing them with masonry anchors if needed. Just bear in mind that going this route may require you to remove the interior finish to access the cavity, which may not be convenient if you’re trying to preserve the finish.
Climate and Performance Considerations
When choosing an insulation material for stone walls, it’s important to consider your local climate so you can figure out the appropriate thermal resistance (R-value) and moisture control properties for your project.
In temperate or hot regions, insulation with lower R-values like R-3 will suffice, with moisture control and ventilation being the main priority.
In colder regions, high R-values are recommended to provide sufficient thermal resistance. We’re talking R-values between R-5 and R-10, depending on climate severity.
Thermal resistance aside, choosing an insulation material that has water-resistant properties (like foam boards) is crucial in damp areas where the risk of condensation is high.
You also want to consider your performance goals:
- If thermal efficiency is the main goal, a high R-value insulation material is the way to go.
- For improved acoustics, noise-dampening insulations like mineral wool are perfect.
- For moisture control, vapour permeable insulation like cellulose will allow the walls to breathe.
Insulating stone walls may not be easy, especially historic ones, but it certainly pays dividends in improved comfort and energy savings while preserving character and structural integrity.
With careful planning and selection of insulation techniques and materials, you can meet modern-day performance goals without compromising the timelessness of traditional stone construction.