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Cavity Wall Insulation vs. External Wall Insulation

Last modified: 29.09.2022
Gotowe – oto gotowy do użycia opis ALT w formacie przyjaznym SEO i przeznaczony na stronę internetową: external wall construction around front door showing insulation layer between blockwork and existing façade, illustrating a typical installation by a company specialising in external wall insulation and render systems

If you’ve ever dealt with cold rooms, damp patches or eye-watering energy bills, there’s a good chance your walls are part of the problem.

Back in early 20th-century Britain, homeowners and builders were fighting a constant battle against damp walls and soggy carpets. Solid brick walls looked sturdy but moisture had an annoying habit of finding its way straight through them.The solution to this debacle was cavity wall, a simple but game-changing construction method that transformed how homes were built across the UK.

Today, cavity walls are incredibly common and understanding whether your home has one can make a huge difference to comfort, efficiency and running costs.

Let’s break it down.

A cavity wall is made up of two layers of brickwork, known as “skins”, with a gap between them, usually around 20–100mm wide.

That gap acts as a barrier. It stops moisture from travelling from the outside wall to the inside of your home, which dramatically reduces damp problems compared to older solid walls.

Because of this design, cavity walls are thicker than solid brick walls and that thickness is one of the easiest ways to identify them.

Cavity walls became popular in the early 1900s as builders looked for better ways to:

  • Prevent damp penetration

  • Improve durability

  • Increase comfort inside homes

They quickly replaced solid brick construction as the go-to method and today the majority of UK homes built after the 1920s have cavity walls.

There are a few tell-tale signs you can check yourself.

  • Stretcher bond usually indicates a cavity wall

  • A mix of short “header” bricks and long “stretcher” bricks often points to a solid wall

  • External walls over 260mm thick are likely cavity walls

  • Thinner walls are often solid brick

These checks are a good guide but they’re not foolproof. A professional survey is always the safest way to confirm your wall type.

The two brick skins in a cavity wall are held together using metal wall ties.

Older properties used iron, mild steel or copper ties, which were later found to corrode over time. By the 1960s, wall tie corrosion became a known issue, leading to cracking, bulging and even structural instability.

Modern cavity walls use stainless steel wall ties, which are far more resistant to corrosion and help keep the structure sound for decades.

Cavity wall insulation involves filling the gap between the two brick skins with insulating material to reduce heat loss.

It’s one of the most cost-effective energy efficiency upgrades available:

  • Installation usually takes just a few hours

  • Costs are typically in the hundreds, not thousands

  • It can significantly reduce heat loss and energy bills

For homes with suitable cavities, it’s often the first insulation upgrade recommended.

Not all cavity wall insulation is the same. The right choice depends on your property and budget.

  • Most common retrofit option

  • Cost-effective

  • Good thermal performance

  • Higher insulating capability

  • More expensive than fibreglass

  • Often used where better performance is needed

  • Installed during construction

  • Already present in most modern homes

The amount of insulation that can be installed depends entirely on the width and condition of the cavity.

Solid walls are exactly what they sound like: brickwork with no cavity at all.

Typically, a solid wall is around 9 inches (225mm) thick, with bricks interlocking across the full width of the wall. These are most common in properties built before the 1930s.

Because there’s no cavity to fill, solid walls need a different insulation approach.

Solid walls can be insulated in two main ways:

  • Insulation added to the inside of the property

  • Generally cheaper

  • Reduces internal room space

  • Insulation fixed to the outside of the building

  • More expensive upfront

  • Delivers better thermal performance

External wall insulation involves fixing insulation boards (typically EPS, mineral wool or phenolic boards) to the outside of your property. These boards are then secured with fixings and finished with a decorative, weather-proof render.

It’s a bigger investment than cavity wall insulation, with costs usually in the thousands rather than hundreds but the performance benefits are significant.

In many cases, yes.

Unlike cavity insulation, external wall insulation isn’t restricted by cavity width or internal space. Insulation thicknesses can reach up to 200mm, delivering excellent thermal performance.

EWI can also:

  • Reduce thermal bridging

  • Improve airtightness

  • Refresh the appearance of the property

If you already have cavity wall insulation and want to push your home’s energy efficiency even further, external wall insulation can be a powerful upgrade.

Not every cavity is suitable. Issues like existing damp, debris in the cavity or exposure to severe weather can affect performance.

That’s why a professional assessment is essential before installation.

Yes and it’s becoming increasingly popular for homeowners looking to maximise energy savings.

If you already have cavity wall insulation and want to take things further, we’ve covered this in detail in our dedicated guide:
“Can I Install EWI on a Cavity Wall?”

  • Cavity walls are common in UK homes built after the 1920s

  • Cavity wall insulation is affordable, quick to install and highly effective

  • Solid walls require internal or external insulation

  • External wall insulation offers the highest performance where budgets allow

The right solution depends on your wall type, property condition and energy goals.

If you’re unsure what type of walls your property has  or which insulation option is right for you a professional assessment is the best place to start.

Get in touch with our team today to find out how cavity wall insulation or external wall insulation could make your home warmer, more comfortable and cheaper to run.

No. Most UK homes built after the 1920s have cavity walls, but older properties, particularly those built before the 1930s, are more likely to have solid walls. Some homes built during transitional periods may have a mix of wall types.

If you’re unsure, a professional survey is the most reliable way to confirm.

When installed correctly and in a suitable cavity, cavity wall insulation should not cause damp.

Problems usually arise when insulation is installed in:

  • Cavities that are already damp

  • Cavities filled with rubble or debris

  • Properties exposed to severe weather

This is why a proper pre-installation assessment is essential.

Most cavity wall insulation materials are designed to last at least 25–30 years, and often much longer when installed correctly. Modern materials do not settle or degrade in the same way older insulation sometimes did.

 

For suitable properties, yes it’s widely considered one of the most cost-effective energy efficiency upgrades available.

Benefits include:

  • Reduced heat loss

  • Lower energy bills

  • Improved comfort

  • Quick installation with minimal disruption

Yes. If insulation has failed or is causing issues, it can be professionally removed and replaced with a more suitable material or the cavity can be left unfilled if necessary.

  • Cavity wall insulation fills the gap between brick skins

  • External wall insulation adds insulation to the outside of the property

External wall insulation typically delivers higher thermal performance, but costs more and involves more work.

Yes. Many homeowners choose to install external wall insulation on top of an already insulated cavity to further improve energy efficiency, reduce thermal bridging, and modernise the appearance of their home.

In most cases, no but planning permission may be required if:

  • The property is listed

  • The building is in a conservation area

  • The external appearance will significantly change

Always check before starting work.

The best option depends on:

  • Your wall type

  • The condition of your property

  • Your budget

  • Your long-term energy goals

A professional assessment will identify the most effective and appropriate solution.