Wall Insulation: The Snugg 2024 Guide

Wall insulation helps keep your home warm and comfortable. In this guide, we look at how it works, how effective it is, what it costs, and more.

Wall insulation

Wall Insulation: The Snugg 2024 Guide (updated January 2024)

Insulation helps keep your home warm. Without it, a third of your home’s energy could be lost through its walls. So insulating your walls could make your home much more energy efficient, and - perhaps even more importantly - keep you warm through a cold winter.

However, certain types of wall insulation can be costly and disruptive. So speak with an installer for the best advice for your home.

In this guide, we cover:

  1. What is wall insulation?
  2. How does wall insulation work?
  3. Does wall insulation make a difference?
  4. What cavity wall insulation is best?
  5. How much does wall insulation cost?
  6. Wall insulation FAQs

What is wall insulation?

If you’re looking to make your home more energy efficient, wall insulation could be an effective way of keeping the heat in and your bills low.

There are three main types of wall insulation:

  • Internal wall insulation
  • External wall insulation
  • Cavity wall insulation

Internal wall insulation

A wall can be insulated on the inside of your home in one of two ways:

  • By attaching a metal or wooden frame to it and filling the gap with mineral wool fibre. The new inner wall is then plastered so that you can decorate it like a regular wall.
  • By attaching insulated plasterboards directly to it. 

In both cases, the existing plaster will need to be removed first so that the frame or boards can be attached directly to the brickwork.

The main benefits of insulating the inside of your walls are that it’s usually cheaper than other methods, and you’ll be able to retain the original look and design of your external walls.

The downside is that you’ll lose some internal space since the insulation will bring your walls a few inches inward. Insulated plasterboards take up less space than insulating frames.

External wall insulation

As the name suggests, external wall insulation is fitted to the outside of your home. This is usually done by attaching insulation boards to the outside of the existing wall, applying an extra layer of render and a decorative finish.

The benefits of external wall insulation are that it doesn’t eat into internal space, and you can continue to live in your home without disruption while it’s being installed.

The downsides are that it’s more expensive than either internal wall insulation or cavity wall insulation, you may need planning permission, and you’ll probably need to put up scaffolding which can cause disruption.

Cavity wall insulation

A more modern approach for keeping the heat in is to fill a gap between an inner and outer wall with insulation (usually mineral wool or polystyrene beads).

Up until the early 1900s, most homes were built with solid walls. But since then, most homes have been built with a gap (cavity) between two external walls. You can tell if your home has a cavity wall by looking at the external brickwork. If the pattern is long-short-long, then it probably doesn’t have a cavity.

The cavity was originally intended to prevent moisture from seeping into the inside of the home and add some thermal protection against the cold. But more recently, homeowners and property developers have been filling those empty cavities with insulation to improve its effectiveness.

Modern homes are required to be fitted with cavity wall insulation when they’re built, and this is usually done by installing mineral wool between the outer and inner brick layers.

The benefits of cavity wall insulation are that it’s an unintrusive way of making your home more energy efficient and it’s often cheaper than other wall insulation methods.

The downsides are that the insulation may interfere with moisture management and cause damp problems if not installed correctly, and the thickness of the existing cavity will limit the amount of insulation that can be installed.

How does wall insulation work?

An insulating layer acts as a thermal barrier, slowing down the rate that heat works its way from the inside to the outside of the home. This helps keep the inside of the home at a more consistent temperature.

Since heat naturally flows from warmer objects to cooler objects, insulation helps warm air stay inside the home in the winter, and it helps to keep the cool air inside in the summer.

Wall insulation not only makes the space more comfortable, but it can also help to reduce energy consumption and costs associated with heating and cooling the building.


When you’re choosing the type of insulation to install, you’ll want to consider its R-value.

R-value is a measurement of how resistant a material is to transferring heat. The higher the R-value, the more effective the insulation. Different types of insulation have different R-values per inch, which will determine how good it is at insulating a space.

If you’re looking to reduce your energy bills, you’ll want to consider higher R-value materials.

Does wall insulation make a difference?

A third of your home’s energy could be lost through its walls. So wall insulation can make a big difference to your home’s energy efficiency and thermal comfort.

By slowing down the speed at which heat escapes through the walls, wall insulation can help reduce the amount of energy needed to heat or cool your home, resulting in cost savings on energy bills.

But research from the University of Cambridge suggests that while wall insulation can make a home more energy efficient, any savings in gas usage only last a few years. The researchers say this can be due to a change in usage habits (with more efficient homes, owners are more likely to turn up the thermostat and live in a more comfortable environment) or due to wall insulation typically being installed at the same time as an extension, which requires further energy to heat.

Wall insulation can help maintain a more consistent temperature inside the home, making it more comfortable to live in. But you’ll need to keep the thermostat at the same temperature to see efficiency savings.

Wall insulation can also make the home quieter by providing additional soundproofing. But if reducing noise is your main goal, you might be better off installing specialised soundproofing panels or double-glazed windows.

What cavity wall insulation is best?

If you’re considering improving the effectiveness of your home’s cavity walls and you’re wondering which cavity wall insulation is best, you’ll have a few types of insulation material to choose from:

  • Mineral wool
  • Polystyrene beads
  • Cellulose
  • Sheep’s wool
  • Hemp
  • Cork

Mineral wool

Despite its fluffy-sounding name, mineral wool is usually made from molten rock. The rock is heated to an extremely high temperature and then spun at a high speed to form a wool-like fibre. The fibre is then packaged either into slabs that can be placed within the wall’s cavity or left loose so it can be blown into the cavity.

Mineral wool is a highly effective insulator. It’s effective at minimising external noise, it’s fireproof and it’s also moisture-proof, preventing mould from growing within the cavity. In its slab form, its semi-rigid nature means it can only be installed by removing part of the inner or outer wall.

Polystyrene beads

As the name suggests, polystyrene beads are made from plastic. They’re little balls of polystyrene that can be loaded into a special injection gun and blown into the cavity of your wall. Modern polystyrene beads include an adhesive that keeps them from blowing around if the cavity is exposed or accessed at a later date.

Polystyrene beads can be a slightly more effective insulator than mineral wool. They have the advantage of being able to fill the cavity by simply drilling a small hole in the inner or outer wall. However, they have the potential to leave empty spaces within the cavity which can reduce the overall effectiveness.


Cellulose is a material made from recycled paper. It’s fibrous in form and can sold in semi-rigid slabs or as a loose material that can be blown in to a wall’s cavity with a specialised injection gun.

Cellulose is a very effective insulation material. It’s a particularly good noise insulator too. But it’s susceptible to absorbing moisture, which can lead to mould and rot. And in loose form, it can compress over time, reducing its thermal effectiveness.

Sheep’s wool

Sheep’s wool insulation is a natural and sustainable option.  It’s particularly good at regulating the moisture and temperature within a cavity wall since it’s a breathable material. This will reduce the chance of damp and mould from building up.

Sheep’s wool is also a good sound insulator and is naturally fire-resistant. It’s a more expensive option than some other materials.


Made from the fibres of the hemp plant, this is another eco-friendly option. The natural thermal properties of hemp make it an excellent insulator. And, like wool, it’s great at managing moisture and reducing noise.

Because it’s recyclable and biodegradable, it could be a good choice if you’re looking to reduce your environmental impact.


Cork insulation is made from the bark of cork oak trees, which is harvested without harming the tree. This makes it a highly sustainable choice.

As well as being an effective heat and sound insulator, cork is particularly resistant to mould, rot and fire, making it a safe material to consider.

How much does wall insulation cost?

The cost of wall insulation will vary depending on the type of wall insulation you choose, the material you choose and the amount of material you need.

Cavity wall insulation is the cheapest option. Installing the insulation is relatively non-intrusive and can be done in a matter of hours. You can expect it to cost around £2,700 for a semi-detached home.1

Internal wall insulation is more expensive. It’s a more involved process that takes longer due to needing to build a new wall. It could cost around £7,500 depending on the size of your home.2

External wall insulation is the most expensive. It involves the most work, including erecting scaffolding (unless you live in a bungalow). It could cost around £9,000 for a semi-detached home.3

Wall insulation FAQs

When was cavity wall insulation introduced?

Cavity walls became a feature of most newly built homes in the 1920s. But most new homes were built with insulated cavity walls by the 1970s.

When was cavity wall insulation made compulsory?

Building regulations were updated to require insulation to be installed in cavity walls of newly built homes in the 1990s.

How to tell if your walls need insulation?

Homes without wall insulation can be hard to heat effectively. In your home becomes cold quickly after turning off the heating, it could be a sign that it doesn’t have wall insulation.

You can usually tell whether your walls are insulated by looking at the external brickwork. If they’re in a long-short-long pattern, they’re unlikely to be insulated. But if they’re in a long-long-long brick pattern, they might well be. If your home was built in the 1990s or later, it should have cavity wall insulation.

To be sure, you can hire a registered installer to complete an inspection. You can also use Snugg’s home energy efficiency planner to see whether wall insulation is recommended, online and for free.

How much wall insulation do you need?

The amount of wall insulation you need could depend on several things:

  • The size of your home.
  • How efficient your home is at retaining heat.
  • Whether your walls were built with a cavity, and how deep that cavity is.
  • The type of insulation material you plan to use.

Does wall insulation go bad?

Good wall insulation should last a very long time indeed, perhaps for up to 100 years. But wall insulation can go bad for a number of reasons.

The most common issue is caused by dampness, which can occur if the wall insulation isn’t fitted properly.

Polyurethane foam can also shrink over time, and may need to be replaced after as little as 20 years.

Does wall insulation reduce noise?

All types of wall insulation will reduce external noise to some degree - some as much as 80%. Some of the best types of wall insulation to reduce noise include mineral wool and cellulose.

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