External Wall Insulation

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External Wall Insulation

Before the 1970s, the main purpose of a wall was to hold the roof up, but things soon began to change, and walls became an integral accessory of the home for keeping heat in.

Knowing the importance of wall insulation, in 2014, the government started to give incentives to homes that adopt the strategies, coming in the form of the Green Deal. 

However, just like other insulations, external wall insulation comes with its challenges, including the hassle of dealing with moisture.

Here in this article, we look at what to know about external wall insulation. 

Moisture Penetration

One common challenge faced with external wall insulation is the issue of moisture penetration.

External wall insulation gives room for water to enter the house, especially when it’s raining. 

Before the popularity of the cavity wall, there were options for preventing the wall from absorbing water from the outside. These options included solid brick walls or stone walls. 

While a solid brick wall is an impermeable wall that prevents moisture into the wall, a solid stone wall lets moisture penetrate the wall at a certain degree and later evaporate. 

The cavity wall tends not to follow any of these techniques. The wall rather immediately evaporates any rainwater that enters the wall. This is achieved by air movement in the cavity.

Many insulation materials used today are usually non-permeable as they don’t encourage rainwater penetration. 

However, these materials are not ideal for preventing moisture from the inside, as moisture from people in the house easily penetrates the wall. 

With this defect, more ventilation is needed to prevent the moist air from getting to the wall inside.

Dew Point

One important factor to consider during external wall insulation is the dew point. 

The dew point is the point where the air meets the required temperature for air-to-water condensation to occur. 

This temperature changes towards the wall when the air moves to the internal temperature from the outside ambient temperature.

The dew point always takes place often slightly inside the wall’s external surface, where the moisture is likely to evaporate. 

The dew point occurs in the ventilated cavity in walls with a cavity. 

The place where the dew point occurs changes with the introduction of wall installation. 

Adding external insulation to the wall warms the wall, moving the dew point towards the colder external air. This reduces the risk of moisture condensing on the internal surface.

However, this comes with another concern that the dew point can occur between the wall and the insulation, or most often in the insulation. 

One common technique to combat this is using a vapor barrier between the wall and the insulation but requires that you check with your supplier.

External Insulation with Solid Stone Walls

While the name would suggest that they’re ‘solid’, solid stone walls are not really solid.

They’re two stone skins with a rubble-filled cavity between them.

These walls have their dew point between the cavity and the external surface, where moisture can evaporate or fall out of the wall. 

Introducing external insulation has little to no significant impact on these walls. 

The only changes here are the dew point moving just a little towards the external surface, and the tendency of moisture entering the wall from inside the house is combated with the wall.

However, stone walls have one unique trait that stands them out – breathability. This quality ensures that the wall functions as designed. 

The ideal form of insulation here is breathable insulation, such as cork or wood fiber, coupled with a lime external fiber.

One common technique is to mechanically fix a 90mm laurel of wood fiberboard to the wall. 

These materials, including Pacatex and Diffutherm, are great to accept render. 

Two coats of 10mm-thick hemp-lime render is applied. The hemp-lime is then painted, usually with a lime wash.

Another option here is a 70mm rigid foam insulation. 

The insulation is also fixed mechanically to the wall and clad with a good weatherproof layer, such as timber cladding. 

While this is a cheaper option, it renders the wall breathless, which might not be a concern. 

Walls with no signs of penetration and are in good condition are ideal for this method, as rendering them into a non-breathable wall has no impact on them.

External Insulation with Solid Brick Walls

Brick walls vary in permeability and quality. These walls are prone to spilling, a condition where the surface flakes off, indicating freeze-thaw. 

This gives room for moisture penetrating the wall.

While adding external insulation to stone walls changes little, such as a change in the thermal performance, brick walls feel no impact of external wall insulation since they’re not breathable walls. 

This means that any form of external insulation added to brick walls is never a deal-breaker. 

The insulation here is mechanically fixed to the wall with excellent cladding, such as timber or render.

External Insulation with Cavity Walls

The cavity wall is usually ventilated during construction, which is how it gets its job done as a cavity. 

However, this doesn’t mean that the ventilation exhausts the heat from the house that manages to penetrate the internal skin to the cavity. 

It only means that there is no call for external insulation as little heat reaches the insulation when installed. 

There’ve been several complaints about cavity-fill insulation failing because the insulation allows rainwater into the cavity. 

This necessitates the installer to finish the external insulation with a weatherproof render. 

This prevents rainwater from penetrating the wall, making cavity-fill insulation function as an excellent thermal barrier.

The common idea is to split the insulation thickness between the external and the cavity-fill insulation. 

While 50mm of thickness goes to the cavity, the external insulation is made 20mm thick, the same idea for solid walls. 

Reveals, Cills, and Eaves

One major evident change that external insulation brings to the wall is the change in the wall thickness, especially at the reveals and eaves. 

The common challenge faced is with eave width. 

Short eaves widths require that the eaves be expanded, adding to the cost of installing the external insulation. 

In most cases, this cost outweighs the benefits of the insulation.

The same applies to cills. What determines if cills are also a deal-breaker is the width. 

The cost of expanding it also comes into consideration when it has a small width.

Additionally, it’s almost not ideal to extend the insulation to the window and door reveals. 

The reason being that most window and door frames have small width to accommodate the external insulation comfortably. 

It’s important to insulate the reveal as failure to do so might cause a significant cold bridge, reducing the insulation value. 

There’re thin insulations specifically manufactured for reveals that are a great means of avoiding this challenge. 

U Values

The U value for a 225mm solid brick wall stands at around 1.20W/m², while a 450mm stone wall has the same value. 

Building regulations demand that the value is reduced to 0.30W/m² or less. 

External Wall Insulation Costs

The cost of external wall insulation is usually higher than the cost of internal wall insulation.

While external insulation for a three-bedroom semi requires a budget of £5,000-9,000, a larger detached home might require a budget of £8,000-15,000. 

The budget for the insulation will determine which materials to use. 

While specialist installation might be a bit expensive for those on a low budget, materials from builders’ merchants are a great deal for a low budget.

Regardless of the class of materials used, there’re fixed costs involved, including the cost of erecting scaffolding and removing pipes and cables fixed to the wall that should be removed.

Does External Wall Insulation Require Planning?

Among other changes that external wall insulation brings, it causes changes in the house’s external appearance. 

This then necessitates getting planning consent before embarking on the project. 

As such, it’s important to contact your local authority before the work starts. 

There’re cases where consent is not forthcoming, which include listed buildings and houses in Conservation Areas.

Building and Regulations require that if up to 25 percent of the wall is insulated, the wall must meet the current standards. 

This makes sense in that if you’re ready to afford the cost of installing external wall insulation, you should make sure that the wall is in good condition and up to standard. 

These regulations also require that the U value for the insulated wall’s thermal performance does not exceed 0.30.

Is External Wall Insulation Worth It?

While installing external wall insulation comes with different challenges, it offers several benefits that outweigh the challenges. The benefits of external wall insulation include:

  • It reduces energy bills by reducing heat loss
  • It increases the sense of comfort and reduction in draughts
  • The installation causes no disruption to the house
  • No alteration to the internal floor area
  • The ability of the wall to add to thermal mass
  • It boasts the wall lifespan
  • It reduces the risk of condensation on internal walls
  • It promotes sound resistance and weatherproofing

How To Install External Wall Insulation

Before you embark on external wall insulation, it’s important to know the steps involved to know what to expect during installation. 

Here’re the steps taken when installing external wall insulation.

Render Test

Render test is the first step to take when planning to install this insulation. 

This is a process where the render is tested for its ability to support the insulation. 

Strong renders are a good fit for the installation. 

All that requires is to smooth the render so that the insulation can fit directly onto it. 

In the case where the render is not strong enough, it’s removed before the work starts.

Remove and Adjust Pipework

Having tested the render, your installer will remove all pipework from the building before the commencement of the installation. 

However, removing these pipes can cause water to run down the sides of the building, which therefore calls for the installation of temporary downpipes. 

Pipes that can be extended away from the building are extended so that the building maintains its functionality while the installation is ongoing.

Protect Windows

Windows and doors that are not protected are prone to damage during this installation. 

As such, it’s crucial to protect them before starting the installation. 

This is achieved by fitting protective layers of film over the doors and windows. 

Fitting a Starter Track

One crucial aspect of installing external wall insulation is knowing where the installation should cover. 

In most cases of external insulation, they don’t start from the ground level but some inches above the ground. 

The aluminum start track is then run from this level upward.

Installing Insulation Boards

Coming next to the starter track installation is the application of the insulation boards onto the wall. 

Having applied adhesive to each of the board, they’re stuck onto the wall. 

The boards are cut into pieces when it reaches the turn of the windows and other obstructions. 

To ensure a flat surface, the edges of the boards are filed off using a rasp.

Adding Beading

Adding beading is necessary for the installation, as this ensures secure and tight insulation around the doors, windows, and other obstructions. 

This also gives tight and neat edges and prevent the insulation from coming in contact with other materials. 

Adding Reinforcing Mesh and Render

What comes next is adding two layers of renders separated with a fiberglass mesh. 

These layers of render are required to strengthen the properties so that they can function properly. 

Apply Primer

Having installed the strengthening render, a primer is applied. 

It’s recommended to use a primer of the same color with the final render. 

The primer also lets the final render coat stick when applied. 

Apply the Final Render

Here is where the last coating in the installation process takes place. 

Being the last coating, it’s important to choose your exact color and texture as this is what is visible on the outside of the building. 

After this is done, reinstall the pipework.


External wall insulation is a great investment in the building. 

Though it’s expensive, it offers several benefits to the house and even the owner. 

While internal wall insulation might be a great idea for an attractive front elevation, external wall insulation will do a perfect job on less-attractive side and rear elevations.