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Baby it’s cold inside: a guide to cavity wall insulation

Baby it’s cold inside: a guide to cavity wall insulation

Insulating a home is the very first step a household can make on a mission to improve the home’s energy efficiency. Specifically, cavity wall insulation (CWI) will be the suitable choice for many.

Considering its huge ecological importance, the government announced grand targets in 2017 to improve the energy efficiency of as many homes as possible by 2035. Three years on, little effort to lift the vagueness of these plans has been made.

Whats more, national household insulation programmes of recent years have been totally flawed and achieved nothing.

Clearly, government action is not enough if the entire country is to go net carbon-zero by 2050, let alone cleaning up the housing sector by 2035.

Alternatively, homeowners and landlords can determine to improve their energy efficiency themselves. When installed correctly and appropriately, cavity wall insulation can be the single most important decision a household makes to improve their home’s energy efficiency.

How to ensure a successful installation? Continuing reading for answers.

Better insulate than never: what is insulation?

Insulation ensures that a home stays warm.

This is essentially a means to wrap the building up in a blanket: a material that is a poor conductor of heat surrounds the home to act as a barrier between the warm interior and the cool exterior.

This keeps the home warm in Winter meaning that central heating systems need not be switched on for as long, as the house can better retain the warmth they provide. The result is a reduction in energy usage.

Oft-overlooked is the ability of home insulation to keep a house cool in Summer too.

This time, the insulation acts as a barrier between the hot exterior and the cool interior. As in winter, energy usage could decrease from less need for electric fans, refrigeration and air conditioning.

CWIck fix: what is cavity wall insulation?

Currently, ⅓ of all heat loss takes place across a home’s walls. 

Many houses are made from two walls separated by an air gap called a cavity. When the central heating warms the air in the cavity, it rises. Cold air subsequently flows in to replace it, which again warms up and rises.

This cycle depends on a continuous source of heat, i.e.: constant central heating.

Alternatively, the cavity could be filled with an insulating material which traps warm air and prevents heat conduction. This is cavity wall insulation (CWI). The result is that home retains its heat for far longer.

Not all homes have cavity walls. The way to identify them is by inspecting the brick structure:

Image showing a wall that can have cavity wall insulation installed.
Cavity walls have a regular brick structure.

Other red-brick buildings have solid walls. Such buildings have subtly different brick structures, where the long and short sides of each brick align per an alternating pattern:

Image showing a wall that cannot have cavity wall insulation installed.
Notice the difference?

Homes built between 1920 and 1990 are likely to have uninsulated cavity walls. 

Cost of cavity wall insulation

The cost of installing cavity wall insulation originates from a number of factors.

Labour costs are around £100-£150, depending on the scale of the work. Insulating material supply costs can be anywhere between £13 and £30 per square metre, depending on the type of CWI desired. A survey cost is usually around £75. 

Energy Savings Trust puts the average total cost of installing cavity wall insulation at £725. Other sources stipulate that the total cost of CWI can be as low as £370 for a terraced house.

Type of insulating material

The material used for cavity wall insulation varies, each type accompanied by a different price tag and amount of embedded carbon (i.e.: the emissions caused by the material’s production). The following table summarises the take-home points of each:

Mineral fibre£13-£18/m2Outstanding insulating material and resistance to rotThermal conduction increases if damp
Sheep wool fibre£25-£30/m2Easy installation; very good insulation; most environmentally friendly Most expensive option
Expanding polystyrene (EPS) beads£18-£22/m2Outstanding insulating material – 20% less thickness required than mineral and sheep’s wool fibre; damp resistant; moderately eco friendly.Poor sound insulation; if work is done to the walls, beads can gush out.
Polyurethane (PUR) foam£22-£26/m2Most effective insulating material as it can spread to all cracks; easy installation. Fairly expensive and not environmentally friendly. 
Cavity wall boards£15-£17/m2Low cost and very effective.Most often designed for specific houses.

It is worth mentioning that the standard insulating material for cavity walls used to be urea-formaldehyde foam. Over time, this foam shrinks causing its effectiveness to reduce. It also releases the toxic formaldehyde gas in the process.

Households with urea-formaldehyde foam in their cavity walls should have it removed immediately.

Environmental concerns of different insulating materials

Mineral fibre, whilst theoretically recyclable, relies on vast quarries and other intensive polluting processes during its production, stimulating the emission of toxic gases such as carbon monoxide.

Image showing the possible harm from minerals used in cavity wall insulation.
Quarrying minerals depends on lots of heavy duty machinery.

Figures suggest mineral fibre has embodied energy of 4-7 kWh/kg. This, however, is a fraction of the expected energy that insulation will save.

On the other hand, mineral fibre can include up to 23% secondary industrial waste.

Sheep’s wool fibre is the most natural form of insulating material and thus the most environmentally friendly by far.

It is hard to predict, however, how dramatic increases in sheep grazing would impact the environment should this form of cavity wall insulation become mainstream.

Whilst PUR foam saves the most energy, it is incredibly unenvironmentally friendly due to the extreme heat required to produce the material. As with mineral fibre, though, the embedded carbon is only a fraction of that saved through insulation.

EPS beads are made from plastic which comes from crude oil. Don’t be fooled, however, in thinking that this material is a no-go.

EPS beads are, in fact, far more eco than mineral fibre and PUR foam as their production requires relatively low heat. Furthermore, the water used in production is recycled. 

EPS beads are 100% recyclable. Unfortunately, the material ends up in landfill far too often due to its low profitability, and the toxic styrene subsequently causes harm to wildlife.

There is hope, though, that the popularisation of EPS beads for CWI could foresee mainstream recycling of the material.

Overall, EPS beads are a good trade-off. The affordability and effectiveness of the material couples well to the moderately low environmental impact of its production.

Factors affecting cost

Clearly, the size of the area that requires insulation affects the cost.

For this reason, many good installers offer heat loss surveys as part of their service to determine exactly where cavity wall insulation is most needed. For £200, a heat loss survey could end up reducing the overall cost of insulation, if the survey identifies areas that do not need it.

Other costs can be incurred if the cavities are harder to treat. This could be due to unconventional external walls, or the need to remove old insulation materials and/or urea formaldehyde foam. These costs can add anything from £50-£1,500 to the total bill.

Saving the way to success: savings from cavity wall insulation

Unless wanting to insulate a listed building, which requires planning permission, there are few checks and balances associated with the legality of cavity wall insulation. It is the homeowner’s responsibility to ensure the council receive a building notice, but good installers will take care of this.

Most reputable sources, including Energy Savings Trust and MoneySupermarket, suggest CWI can save from £70 for a small flat to £250 for a detached house on annual energy bills. The savings for a mid-terraced house are around £90 per year.

During the covid-19 lockdown it is estimated that the average household’s energy bills have increased by £32 per month. If, in the aftermath of the pandemic, working from home becomes more common, savings from home insulation are likely to be even greater.

The generally-accepted payback time of cavity wall insulation is around 5 years. Meanwhile, at an average gas price of 3.8 p/kWh, these savings correspond to 1,842-6,579 kWh less gas used each year, a colossal 340-1,200 kg of CO2 emissions saved. Energy Savings Trust put the average amount of CO2 saved by installing CWI at 560 kg. That has the same effect on the environment as planting 24 large trees every year. Yikes!

Whilst cavity wall insulation will not become a highly valuable asset on the property, since most new-built homes will no doubt come with insulation already installed, it will likely prevent the value of property from decreasing.

Waiving goodbye to heat loss

Since 2013, the government has been attempting to financially assist people in insulating their homes by running the Energy Company Obligation (ECO) scheme. 

Essentially, this scheme holds energy companies to account over the efficiency of homes they supply, by administering targets. In order to meet the targets, energy companies provide funds to eligible households for measures to improve insulation, including cavity wall insulation. 

Eligibility is most often means tested, where low-income households and those receiving benefits/universal credit are entitled to free insulation. 

Other eligibility criteria include those in social housing whose energy performance certificate has a rating of ‘E’ or lower. Some long-term customers may even receive free insulation as a reward for their loyalty. More information on the ECO scheme can be found at Ofgem’s website.

Properly-installed CWI should last the lifetime of the house. A 25 year guarantee usually accompanies CWI installation, certified by the industry-funded regulator Cavity Wall Insulation Guarantee Association (CIGA).

For installers to be registered with the ECO scheme, it is mandatory to be certified by either CIGA, SWIGA (for solid wall insulation), Kinell or GDGC. All are private limited companies.

Sadly, CIGA has been known to grossly misevaluate the credibility of certain installers. The consequences have been abominable.

ECOck-up: failings and scandals of national schemes

Registered with ECO, Miller Pattison installed cavity wall insulation in over 800,000 homes that subsequently had to be removed.

Inappropriately installed CWI can lead to damp and mould in the home (see ‘caution’), the extent of which has in the past been deemed uninhabitable. Prior to mass removal of their installations, Miller Pattison received over 40 complaints of the sort each month.

The company has since gone bust. Not a penny of compensation has been paid nor were repair costs covered.

In court, CIGA admitted fault and that they should never have certified Miller Pattison’s CWI. Nonetheless, they were not willing to fork out the £60k for repairs on each home.

Guess who had a seat on the management board of CIGA as the scandal unfolded? The then-chair of Miller Pattison. 

See you later regulator

CIGA has staggered on since the botched Miller Pattison job and has learned nothing.

Miller Pattison’s ex-chair still occupies a seat on their management board. MPs and housing campaigners have since declared the certification body not-fit-for-purpose.

Aware that the board was almost exclusively composed of senior directors with close ties to industry giants, CIGA appointed a ‘consumer champion’ to their board in 2015, whose role was to represent consumer views.

Even she, in her 2018 annual report, revealed astonishing incompetence at the heart of CIGA, stating that the company does not live up to its publicised vision. 

In the scathing report, she revealed that some unresolved complaints dated back up to three years. Carillion’s 2018 collapse also left many complainants in the dark.

It is one thing to dishonour a 25 year guarantee and receive complaints of damp and mould, but for cases over a year old to remain unresolved raises serious questions about the effectiveness of CIGA’s complaints handling procedure.

The report states that “CIGA still has a long way to go before all individual complainants experience a consistently speedy and high quality response.” Established in 1995, one might expect CIGA to have reached maturity by 2018. Evidently not. 

Alarmingly, the report raises the concerning point that the cause of CIGA’s flawed service is the rooted in the proliferation of certification and independent surveillance of assessment bodies.

Competition between such bodies has resulted in a reduction in quality assurance. Without a public, independent regulator for the insulation sector, the situation is unlikely to improve.

Of course, there is nothing better for a growing business than a healthy pinch of constructive criticism. So, what action did CIGA take in response to their outgoing consumer champion’s 2018 report? They eliminated her role, replacing it with a ‘customer services committee’.

Green-dosh: the Green Homes Grant Scheme

Following the ECO scheme, the government has pledged to improve the energy efficiency of UK homes in the wake of post-covid public spending. 

The Green Homes Grant Scheme (GHGS) will provide homeowners and landlords with a means-tested voucher to install full insulation. For median-income households, the grant will cover ⅔ of the costs, up to a maximum of £5,000. 

Low-income households, meanwhile, will receive closer to the full costs of insulation, up to a maximum of £10,000. 

Whilst the figures sound excessive compared to the relatively low cost of cavity wall insulation, the cost of fully insulating a house is likely to be greater than £5,000 due to the expected costliness of double glazing.

Other measures included in the scheme are solid wall insulation, loft insulation, underfloor insulation and room-in-room insulation.

The scheme will also allow households to upgrade to heat pumps and/or solar thermal systems.

Before applying for GHGS vouchers, homeowners and landlords will need to obtain a quote from a government-accredited installer. They can then apply for the voucher at from September.

This exposes the first flaw in the Chancellor’s grand green plan.

A surge in traffic on day one of applications could crash the server making the first few weeks hideously messy. (For this reason, we are urging people to not rush to the application page to avoid growing disheartened with the scheme).

Frozen ice is good. Frozen computers are not.

Other reasons to be cautious of the scheme presented themselves in a recent announcement of the scheme’s accreditation.

Before analysing these cautions, though, it helps to look closely at the government’s track record with similar schemes over the past decade.

Backtrack record

Launched in the same year as ECO, the Green Deal allocated loans to homeowners which would fund installation of energy efficiency measures including cavity wall insulation. Households would then repay the loan with the subsequent savings on their energy bills.

The scheme was a total disaster and funding was pulled in 2015.

A 2016 National Audit Office report concluded that the Green Deal achieved virtually nothing, citing that out of 300,259 assessments carried out, only an ignominious 1,815 of these turned into live plans.

The problem was not that it was a loan-based system. In Germany, for instance, a very similar scheme ran successfully from 2002 which has dramatically improved the energy efficiency of 200,000 homes per year

The equivalent British scheme failed because of high interest rates on the loans at around 7-10% – completely unaffordable – compared to the German 1-4%. Critics of the scheme have speculated that the origin of high interest rates was pressure from investors looking for quick returns.

Ministers were actually informed of the dangers of setting interest rates too high at the time. Did they listen to the experts’ advice? Nope!

The Green Deal was not the last time national and local government ministers ignored expert and household concerns.

The ultimate insulation calamity was the Grenfell Tower fire, the tragic scandal that continues to rage in unsafe tower blocks today.

A preventable tragedy.

The flammable cladding on the tower was installed as insulation in a bid to improve the efficiency of social housing. One would hope that ahead of the Green Homes Grant Scheme, national and local government administrations listen to safety and quality concerns.

Green Homes Grant Scheme: reasons to be fearful

Not to mention the government’s speckled track record with national insulation schemes, the GHGS raises some serious concerns of its own.

When announced in parliament, Rishi Sunak promised colossal chunks of money for homeowners and landlords all at once to quickly stimulate the post-covid economy, guaranteeing big returns for cash-strapped investors and industry alike. The grants are currently due to run out in March 2021.

Jules Birch from Inside Housing, however, says that rushing the scheme risks achieving nothing but a greenwashed housing sector.

A quick splash of cash, they say, doesn’t allow for skills to be developed at a natural pace.

Image portraying the attitude of throwing money at the problem.
Will a spending spree fix the planet?

To that end, releasing the funds all at once encourages installation companies to rush their training and development, running the risk that cheap-and-cheerful insulation is installed by low-wage, untrained labour.

Of course, this is not to discourage people from applying. It is to merely remind households that the government does not necessarily guarantee quality and that it is their own responsibility to select a trustworthy installer.

Green Homes Grant Scheme: unanswered questions

Clearly, the initial announcement of the green homes grant scheme creates more questions than it does solutions. 

Which installers will be carrying out the work?

The installer could make or break the quality of insulation installed on the house. Recently, the government announced that installers must have accreditation with either Trustmark or MCS – two government-‘endorsed’ quality guarantees (without going into the details, neither company is necessarily trustworthy – we recommend households follow our own advice.)

What does the scheme guarantee?

Quality and safety concerns including fire risk and damp prevention must be addressed before the government releases funds if the insulation is to be long-term.

Watch this space.

Under the weather: cautions of cavity wall insulation

Households in unsheltered areas with cracks in the home’s walls should be cautious about cavity wall insulation.

The reason is that the insulating material provides a bridge across which rainwater can be transported into the home. In unsheltered regions prone to wind-driven rain, this can lead to acute problems with dampness in the home.

Image showing a region where cavity wall insulation should be avoided.
A common occurrence? Avoid cavity wall insulation if so.

Such was the most common complaint in the Miller Pattison scandal (see ‘ECO failings and scandal’). 

Worse still, the rain trapped inside the cavity wall expands when it forms ice overnight, leading to further cracks and fractures in the wall structure.

Areas of concern for wind-driven rain risks are the West Coast of England, the South West of England, Wales, most of Scotland and Northern Ireland.

Areas in Western Britain are not entirely suitable for cavity wall insulation. Read below for solutions.

In the absence of a reliable industry watchdog (see ‘regulation’), households themselves are responsible for judging whether CWI is appropriate.

A staggering investigation led by Which? found many CIGA-certified installers gave their undercover agents misleading information that contradicted expert advice about damp risks. Unless the installer provides or recommends measures to prevent damp in high-risk regions, their guarantee should not necessarily be trusted.

One solution to preventing CWI-induced damp in high-risk regions is to install an exterior wall coating. Whether this solution will be included in the Green Homes Grant Scheme is unknown. 

Of course, this is a pragmatic solution to a problem that shouldn’t exist and one might expect new-builds in high-risk areas are not built with cavity walls.

Importance of ventilation

Additionally, lack of home ventilation is a risk of all forms of insulation and can lead to dampness from inside the home.

Ventilation processes maintain a healthy level of fresh air in the home. Insulating a home ultimately makes it harder for outside air to enter the home.

The result is that there is no replacement of stale interior air which leads to damp and black mould. Particular triggers include cooking, taking showers and drying clothes on the radiator.

The best form of ventilation is natural ventilation. However, this method rather defeats the point in insulation if in Winter, heat is lost through open windows and doors. It can even lead to an overall reduction in energy efficiency.

It is best to ensure the installer of the insulation takes ventilation seriously and installs extra measures to allow fresh air to circulate in the home.

In addition, measures such as extractor fans in kitchens and bathrooms can be incredibly efficient. Some modern fans are as low power as 30 W.

Whole house ventilation systems also exist.

Passive stack ventilation (PSV) is the most electrically efficient, drawing no power whatsoever. However, it involves some heat loss when ducting warm air.

Mechanical extract ventilation (MEV), on the other hand, requires some electrical power but is more effective than PSV at removing moisture and stale air without heat loss.

Finally, mechanical ventilation with heat recovery (MVHR) is by far the best whole house ventilation system but is eye-wateringly expensive: £7,000 for installation and almost £90 in running costs.

Damp duty: health risks from damp and mould

Dampness and black mould can make a home uninhabitable.

Image showing possible health risks of cavity wall insulation.
Damp homes lead to black mould.

Dampness can cause respiratory infections, allergies and asthma. It can also weaken the immune system. 

Damp homes pose severe health risks to those already medically vulnerable too.

Rather pertinently, living in damp accommodation increases the risk of death from covid-19, especially when far more time is spent in the home. The trends relating death to socio-economic status throughout the pandemic should shock no one.

Clubbing together

Since the suitability of cavity wall insulation is dependent on geography, it makes sense to cooperate with neighbours during the decision-making process.

As ever, clubbing together can also lower the price per household. For example: the price per square metre to insulate an entire row of terraces as if it’s one detached house is likely to be cheaper than individually insulating each mid-terraced house.

What would the dodo do?

The dodo, if it were alive today, would be reluctant to insulate its prehistoric house. After some persuasion, a dubious company installs PUR foam into the house’s cavity walls, despite the region’s susceptibility to Jurassic rainstorms. To those households fortunate enough to have the choice of affordable insulation options, don’t do that.


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