Insulating a Wall: Everything You Need to Know

How to insulate a wall

A significant amount of your home’s heat is lost through the wall – 35-45% of your home’s overall heat loss according to Northern Energy. Not only does this cost you more to keep your house warm, but it also requires you to burn more infamously planet-unfriendly fossil fuels to heat it. 

Wall insulation is a cost-effective way to improve your home’s thermal efficiency, and in the case of some forms of insulation, you’ll enjoy other benefits such as fire safety, a reduction in damp, and even soundproofing. 

But before you go ahead and book an installer, there are a few things to consider. Is your wall a cavity wall or a solid wall? Do you need internal or external insulation or external insulation? Find out the types of insulation to look for, how much you can expect to pay, and the advantages of insulating your walls in the article. 

Solid Wall vs Cavity Wall

A solid wall does what it says on the tin – it’s solid with no hollow interior. If your home is older, from before the 1920s for example, it’s more likely to be a solid wall. You can usually tell whether your wall is solid by the way your bricks are placed.

Solid walls have bricks of different lengths and an alternative pattern, giving your house a less consistent look than the bricks generally associated with cavity walls. Solid walls also tend to be thinner than cavity walls because the wall doesn’t need extra space for a hollow cavity. Because these walls (as the name suggests) are completely solid, there are no gaps between bricks to insulate. 

A cavity wall, on the other hand, is made up of two thin walls with a gap (cavity) in between. If your house was built after the 1920s, it most likely has cavity walls. The bricks of cavity walls usually have an even pattern, with all the bricks laid lengthways. 

Cavity walls are thicker than solid walls because of the extra space needed for the cavity. If your wall is more than 26cm thick, there’s a high chance it’s a cavity wall. If you’re unsure either way, you can get a professional to take a look. 

If you insulate a cavity wall, you’ll need to insert insulation into the gap between the two thin sides of the wall. If you’re insulating a solid wall, you’ll install the insulation either externally or internally. 

Internal Insulation vs External Insulation

If you’re insulating a solid wall, you’ll have to decide to insulate the wall either internally or externally. External insulation tends to involve adding insulation boards to the external walls of the property, before adding rendering on top of the insulation to give it a finish. 

Internal wall insulation involves attaching insulation to your property’s interior walls. You’ll only need to insulate walls that are in contact with the outside air. If your house was a mid-terrace, for example, you wouldn’t need to insulate the walls between two properties. 

Internal insulation is generally cheaper to install than external insulation, but it will slightly reduce the space in any room where it’s installed. Internal insulation will disrupt your daily routine significantly more than external insulation during the installation process, but you can have it done room by room to help minimise disruption. 

During the installation of internal insulation, skirting boards, door frames, and fittings must be removed and detached. Plus, you’ll need to fix any problems with damp and condensation before installing it. 

External wall insulation tends to be the better option, simply because it doesn’t take up any extra space within your home, only outside of the home. External wall insulation, however, tends to cost more than internal wall insulation, so bear this in mind if you’re on a strict budget. 

Here are some of the key advantages of choosing external insulation:

  • Increases the lifespan of your walls by protecting its brickwork
  • Can be installed without disrupting your home
  • Reduces condensation on internal walls which helps to prevent mould. 
  • Improves sound resistance and weather-proofing. 

External solid wall insulation does sometimes require planning permission, however, especially if your home is listed or in a conservation area. 

Solid Wall Insulation Types

Stud walls

A stud wall tends to be made from wood and is installed against your original wall, with thin columns connecting the top to the bottom. The installer will then fit an insulation material – rigid insulation boards are an option – and cover the result in plasterboard. 

Bear in mind that installing a stud wall with significantly reduce the floor space in your room. 

Rigid insulation boards

Rigid insulation boards tend to be between 2.5 and 4 inches thick. They’re made of plasterboard and backed with any type of insulation material. You can fix rigid insulation boards with adhesive or ribbons of plastic. Alternatively, you can fit the boards to the battens, especially if your walls are uneven and textured. 

Upon installation, the installer will add the necessary fittings to hold the boards firm before sealing the joints between them to keep them in place. Rigid insulation boards make your home more soundproof, but they’re flammable, so not ideal for fire safety. 

Cavity Wall Insulation Types


PUR foam (polyurethane foam) is probably the most effective type of cavity wall insulation, but you’ll pay a higher price for this effectiveness.

The foam is liquid-like, and it expands and fills any available gaps once the installer injects it into the cavity. This expansion helps the foam fill any and every gap in your cavity, so you’ll be left with seamless insulation. 

Blown mineral fibre

While all insulation benefits your carbon footprint, blown mineral fibre is the most environmentally-friendly option for your cavity walls since it can be made from recycled materials. 

Blown mineral fibre comprises fibreglass flakes and is blown into the cavity by the installer using compressed air. Blown mineral fibre can be cheaper than foam, also it’s not quite as effective at filling your entire cavity. 

Polystyrene beads or granules

Just like blown mineral fibre, beads are granules that are blown into your walls’ cavities. Beads or granules come either loose or held together in a packet with sticky resin. 

Polystyrene beads and granules tend to be cheaper than other cavity wall insulation but less efficient. If you’re more interested in cost-effectiveness and willing to pay more upfront, blown mineral fibre or PUR are probably better options. 

Pros & Cons of Insulating a Wall


Money saving 

According to the Energy Savings Trust, you can save up to £930 per year off your energy bills in a detached home and up to £240 in a flat. By keeping your home warmer in the winter and cooler in the summer, you’ll save the money you would have spent on bills heating up and cooling down your property. 

Retain heat

A seriously significant amount of heat is lost through your walls. By insulating your cavity or solid walls, you can improve its thermal efficiency. 

Lasts a lifetime

If upgrading a part of your home requires constant maintenance, it can feel like more hassle than it’s worth. The good news about wall insulation is that it lasts a lifetime and requires very little maintenance, so you won’t have to spend thousands of pounds installing it only to spend more down the line keeping it maintained.  


Potential damp problems

The decision to insulate your walls and how to do it should be supported by a professional installer, who can take a look at your solid or cavity walls and advise on whether or not they can be insulated in the first place, and which type of insulation you should opt for. 

Because of the crucial role that walls play in balancing your home’s moisture, if your insulation isn’t properly installed (or you try a DIY installation that goes wrong), this can lead to damp issues in your home. 

Common signs of damp include condensation and mould showing up in your home. If your property doesn’t experience any additional heat as a result of insulation, this is also a cause for concern. 


If you have cavity walls, getting them insulated won’t set you back too much. Solid walls, on the other hand, cost a significant amount to insulate. While you should see returns on your investment over a lifetime in the home, if you’re short on cash, consider another type of insulation, such as loft insulation.   

How Much Does It Cost to Insulate a Solid Wall?

Internal insulation is the cheaper option when it comes to solid wall insulation, but it does come with various disadvantages, so bear this in mind before making a decision on pricing alone. 

Unfortunately, solid wall insulation is significantly more costly than cavity wall insulation. Depending on the size of your property and where in the country it’s located (South East England tends to be more expensive, for example), you can expect to pay between £3000 to £15,000 to have your solid walls insulated internally. Obviously, the larger your property, the more you’ll pay. 

External insulation is more expensive again. A terraced house will set you back upwards of £5000, while you can expect to pay up to £20,000 for a large detached house. If solid wall insulation is out of your budget, consider another type of insulation, such as loft or roof insulation. 

How Much Does it Cost to Insulate a Cavity Wall?

As with solid wall insulation, the cost of insulating a cavity wall depends on the size of your property, its location, and other factors.

Generally speaking, cavity wall insulation is much cheaper than solid wall insulation. It costs around £200 per wall, and you’ll only need to insulate the walls that aren’t attached to another house. You’ll pay much less to insulate a mid-terrace house than a detached house, for example. 

For a detached house, you can expect to pay £800 upwards. You’ll pay slightly less to insulate a bungalow. For the most accurate idea of how much cavity wall insulation costs in your area, get in touch with a professional installer for a quote. 

Solid Walls and Moisture

The reason why cavity walls were created was to control the level of moisture in the home, which is partly why older homes are more prone to condensation, damp, and mould.

In buildings with solid walls, water vapour can move freely through the building and the bricks that make up the walls.

Insulating a building with solid walls changes the way that the water vapour behaves. For one, you’ll usually cut down on the draughts through the walls and around the windows. Insulating the inside of a wall, however, makes the wall colder, so any water vapour that enters may condense inside the wall. 

When fitting solid wall insulation, you’ll need to work closely with a professional installer to make sure you don’t create new damp problems in the future by developing a moisture control strategy. 

One option is to use breathable insulation materials that continue to allow the vapour to permeate through the walls. Otherwise, you could create a continuous vapour barrier to make sure no vapour enters the walls in the first place. 

How to Find a Wall Insulation Installer

Both solid wall and cavity wall insulation should be installed by a professional; neither is easy to do as a DIY project. 

If you’re looking for an insulation installer, the first port of call is to search online for installers who cover your location and check reviews and testimonials to see if they offer good value for money. 

When possible, ask friends and family for personal recommendations. Remember, before committing to a project with an installer, ask them for a quote, an idea of how long it will take to complete the work, and any relevant qualifications. Make sure any installer you choose covers your local area. 

Not sure whether your property needs solid or cavity wall insulation, and whether you’d prefer internal or external insulation? Discuss your options with a professional installer for an impartial opinion.

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