External Insulation on Cavity Walls

Nothing can be as uncomfortable as extreme temperatures in a home. Already you may be spending thousands of dollars on heating and cooling alone without seeing the desired results.

But there is a cheaper way of keeping your house always cool both in winter and summer. Home insulation is an effective method of increasing energy efficiency and cutting costs.

Whether you are living in a modern home or an old inherited property, there is a way of artificially regulating the ambient temperature. In this article, we focus on cavity wall insulation discussing the types you can install and their benefits.

Cavity Wall Insulation Explained

Was your home built in the late 20s or after? Then most likely it has cavity walls. This was a building technique adapted in early 1920 as a way of preventing damping.

The walls were made of two parallel bricklayers leaving a space of approximately 20-100 mm in between. Consequently, cavity walls are broader than the traditional solid walls made of a single stone or bricklayer. 

You can tell whether your home has cavity walls or not by measuring the wall breadth at the door area. A brick wall measuring more than 260 mm wide is highly likely to be having a cavity but this isn’t true for a stone house.

Exposed brickwork pattern also differentiates a cavity and a solid wall. For the former, the pattern is regular with the bricks laid lengthwise while for the latter, bricks are placed in an alternating pattern.

Though initially meant for damp prevention, it was not late before professionals discovered it could serve a second purpose: insulation! And now we have the famous cavity wall insulation (CWI).

Cavity wall insulation is achieved by filling the space with a suitable insulation material either during the construction or later as a retrofit. Notwithstanding, it will significantly enhance your home’s energy efficiency and reduce heating or cooling costs. The outcome largely depends on the cavity size.

Despite CWI being the default type of wall insulation for the recently constructed homes, it comes with some setbacks. If poorly installed, damping that was the primary reason for cavity wall practice adoption can occur through cold bridging.

Fortunately, a cavity wall does not limit you to CWI as the only method of insulating your home. You can still add external insulation and eliminate the possibility of condensation or damping without forgetting other undermining other associated benefits.

“What types of insulation materials can I use on cavity walls?” I know you ask. And we are leaving no CWI stone unturned, so let’s get to it.

Types of External Cavity Wall Insulation


Injectable Insulation

Retrofit installation of cavity wall insulation involves inoculating the appropriate type and amount of insulant into the space using a specific approved technique and equipment. Insulation materials that can be injected into the wall cavity include urea formaldehyde and polyurethane foams.

Previously, urea formaldehyde foam was the preferred cavity wall insulation as it required drilling much smaller holes through the brickwork. Today, PU foam is the darling of many homeowners who are focused on reducing energy costs.

 Therefore, we consider it as the injectable insulation for this discussion.

Polyurethane Cavity Foam

PU foam is manufactured by chemically modifying polyurethane making it impervious to air and consequently boosting its U-value. Production in the UK is in strict adherence to the BBA Certificate quality control protocol guaranteeing premium quality.

With proper installation, PU foam insulation will remain effective for a lifetime without any maintenance works required.

If you are having defective ties in your cavity wall that are resistant to other treatments, polyurethane foam will not only insulate your home but also restore the ties. It is the most cost-effective insulation option in circumstances where external or internal insulation is non-viable.

In constructions with a 65 mm wide cavity, PU foam can bring the U-value to 0.33W/m2K while for a standard 100mm cavity, it gives a U-value of 0.22W/m2K.

How do you install PU foam insulation?

Before you begin the actual process, a pre-installation assessment should be conducted by a professional.

For the installation, you drill injection holes in the mortar in between the bricks as specified by the relevant manual. Elevations such as gable peaks and any other cavities that may occur in the party wall line are filled.

PU foam is then injected into the wall cavity via the drilled holes using an injection gun. This is done in a particular sequence as directed by the operation manual to ensure the cavity is filled.

The injection holes are then sealed and given a render that matches or nearly matches the original wall appearance.

  What are the benefits of using PU foam injectable insulation?

  1. Superior U-value
  2. Effective treatment for corroded wall ties
  3. Water-resistant preventing home damping

External Wall Insulation

The size of a cavity determines how much insulation one can apply which is directly proportional to the outcome in terms of energy efficiency. For a 20mm cavity, the results with CWI are quite slim too and additional insulation is necessary. This is where external insulation comes in hand.

External wall insulation uses insulation boards that are either made from wood fiber, mineral wool, EPS, or phenolic.

EWI boards are fixed to the outer wall surface using adhesive and screws then a weather-resistant, decorative render is applied on top of the boards.

Though expensive, EWI is not restricted by space limitations and can be up to 200mm thick bringing the U-value to around 0.3. This ensures that maximum heat is preserved in your home thereby cutting energy expenditure by a huge chunk.

Benefits of EWI on cavity walls

  1. ·Further minimizes chances of damping or interstitial condensation.
  2. Greatly reduces thermal bridging
  3. EWI increases the aesthetic value of old homes due to the decorative render
  4. Significantly boosts energy efficiency thereby reducing heating costs


Generally, it is assumed that homes with cavity wall insulation are adequately insulated and cavity walls are not EWI compatible. But the opposite is true! A combination of CWI and EWI is likely to eliminate any potential damping issue and also grant optimal energy output in your home. The long-term benefits of having the two outweigh the initial cost of installation. So be wise and play your cards right!