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Cavity Wall Insulation Cost: 2020 Price Comparison (per m2)

Want to know the benefits of cavity wall insulation and how much it will cost you?

About a third of the heat lost from your home disappears through the uninsulated cavity wall. That’s a significant proportion of your energy bill. So, if you insulate your walls you’ll save money and do your bit for helping the environment. 

Generally, houses built from 1920 onwards will have cavity walls. But, what are they? A cavity wall has two leaves of brickwork separated by a 50mm cavity. Simple isn’t it? Its purpose is to prevent damp from travelling from the outside surface exposed to rain, to the inner plastered and decorated surface. Having a cavity prevents the onset of the various types of damp so common in houses older than this. To a certain extent, it also provides a degree of heat insulation as air is a worse heat conductor than brick. 

However, to insulate your walls further, a specialist contractor injects into the cavity one of the following materials through small drilled holes. 

  • Polystyrene beads
  • Mineral wool
  • Glass wool
  • Polyurethane foam

Afterwards, the contractor seals the holes with cement mortar. If all goes to plan, you will hardly notice the repair.

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How Much Does Cavity Wall Insulation Cost?

The cost of cavity wall insulation will vary depending on the size of your home and how many cavities you have to fill. But, no matter how large your home, you can usually recoup the cost of insulation in about 5 years from the savings in your energy bills. Not a bad R.O.I. is it? 

To put it in figures you can understand, it will cost about £10 to £30 per m2 depending on the insulating material, glass wool being the cheapest.  So, if you own a typical 3 bedroom property in the UK, it’ll usually cost between about £500 and £600 in total. 

To look at this in more detail let’s look at some more average estimated cavity wall insulation costs. 

Type of house  Estimated cost 
Mid terraced  £300 to £400 
Semi-detached  £500 to £600 
Detached  At least £800 
Detached bungalow  About £400 

Unfortunately, if you live in a flat, it’s a bit more complicated because your neighbours must also agree to share the cost. The cavity extends from the bottom to the top and around the building and is shared by all residents, so you can’t just have your part insulated.

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House Cavity Insulation Cost Factors

There are a few factors that determine the wall insulation costs and type of insulation to install in your walls.

Wall type

Houses built before 1920 have solid walls that aren’t suitable for cavity wall insulation. Houses built from about 1920 onwards will have cavity walls. And those from the 1990s onwards will usually already have cavity insulation.

The cavity will be about 50mm across and will extend around the entire house above damp proof course level.

What type of wall do you have?

If you can see the bricks, and there is an even pattern of whole bricks all the way along (forget the corners), then you have a cavity wall. If the bricks have an alternating pattern of whole and half bricks on each course, then you have a solid wall. Many homes have the outside brickwork covered with render. If so, measure the thickness of the wall. The best places to do this are at windows or doors. If it’s a brick wall and more than 260mm thick then you probably have a cavity. However, if the thickness is less than this then it’s probably solid. Alternatively, if the wall is made from stone then it will usually be much thicker, between 300mm and 600mm. This is solid too. 

If you have a timber frame or steel frame house or one made from prefabricated concrete, ask an insulation specialist for advice. 

If you have solid walls of any type, you’ll have to install internal or external wall insulation. But that’s another story. 

Insulation material

Polystyrene beads are white, grey or black granules. Insulation values range from HR to HR++, with the latter being the best insulator. They aren’t particularly good at sound insulation or fire protection and will tend to leak out of any crack or hole either inside or outside the house. This costs anything from £18 to £22 per m2. 

Glass wool is a good insulator and quite cheap. It comes in the form of glass flakes and is a good insulator. It’s also very fireproof which is a good addition. Once again HR++ glass fibre has the best insulating properties. The average prices of this type range from £13 to £18 per m2. 

Polyurethane foam only comes in HR++ variety. It can also fill the smallest gap or crack, unlike the other insulation types. However, there are two problems with this type. You can’t recycle used foam and older types of urethane foam give off toxic fumes when it decomposes with age. Having said this, modern materials are safe and overall it’s one of the cheaper alternatives. Foam costs on average about £22 to £26 per m2.

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What is cavity wall insulation?

Motionless air is a really bad conductor of heat, that means it’s a good insulator. If we put something into the cavity that is full of little air pockets, and at the same time doesn’t allow water to travel across the cavity, then we have a good insulator to put in the cavity. 

All the different types of insulation we talked about here contain trapped gas. First, the glass fibre chips lodge air between each individual granule. Next, the polystyrene granules lodge air pockets between each particle as well as having gas pockets within each granule. Finally, after you pump the polyurethane foam into the cavity, it expands to fill the space using gas and then solidifies trapping the gas bubbles within itself.

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How Long Does The Job Take?

The entire job doesn’t take very long to do, but it’s worthwhile knowing the steps so you can appreciate what happens. 

Preparation

  1. Confirm you need insulation. Get a specialist borescope surveying company to drill a hole and insert a borescope camera. If you already have insulation you need go no further. A borescope is a small lens and light source on the end of a flexible wand. This goes through a pre-drilled hole into the cavity. The surveyor can then inspect the cavity and whatever is inside. The report will detail the condition of the cavity and whether it is suitable for injected insulation. Some wall types, those without a cavity or timberframed buildings, will only be suitable for external wall insulation. 
  2. After obtaining 2 or 3 quotes for the job, contact your chosen cavity wall insulation company to do the work. 
  3. The company will check the condition of the walls and repair any obvious cracks or holes that might allow the insulation to escape the cavity. If you intend having foam insulation, this will fill cracks anyway and will increase the structural stability of the walls. So, only the largest repairs need to be done. 

Insulation

  1. Drill 20mm holes at 1metre horizontal intervals along the wall. 
  2. The insulation blower has a nozzle on a hose that fits into the hole. Insert this into the hole and clamp in position. 
  3. Inject the insulation into each hole, in turn, keeping an eye on the pressure gauge. When the pressure reaches a specific level the operator knows that the section is full. He then moves onto the next hole. 
  4. When the wall is full of insulation, fill the holes with cement mortar and tidy up the area.  

Completion

  1. The operator issues a certificate of completion and information on the type of insulation and method of application. 
  2. If the surveyors inspection highlighted any other work such as repointing brickwork, it’s worthwhile having this done now.  
  3. Hire an external decorator to paint the filled holes and give your home’s exterior a coat of paint to freshen everything up.

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How Much Can I Save On Energy Bills?

The answer to this depends on what type of insulation you decide to use and the condition of other insulation types in your house. For example, insulated walls aren’t any use if you lose heat through other locations. To be fully effective, you need double glazed windows and doors as well as an insulated loft. 

Additionally, savings also depend on the type of property you own and its size. For example, a small bungalow will cost about £400 and you’ll potentially save about £100 per year. While a detached house will cost about £800 and you’ll save about £250 per year. On average, however, you can usually make your money back in about 5 years or so. And, considering that the insulation doesn’t usually need replacing for a very long time, this sounds like a good investment.

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Cavity Wall Insulation Cost Q&A

Q. Is there financial help towards having your walls insulated? 

If you receive certain incomerelated or disabled benefits, there is money available to help with the cost. You will receive a loan from the central government through the Green Deal finance plan. 

Q. How long does cavity wall insulation last for? 

The National Insulation Association and the Cavity Insulation Guarantee Agency (CIGA) both represent their members consisting of manufacturers and installers.  CIGA issues an independent 25year guarantee for cavity wall insulation installed by registered installers who are also members. 

Usually, cavity wall insulation lasts anywhere from 20 to 80 years. So it shouldn’t need replacing soon. In fact, most insulation needs replacing not because it has deteriorated but because the technology has improved so much since it was installed. 

Q. Will cavity wall insulation stop damp? 

Damp caused by cavity wall insulation is very rare, but it does happen.  Usually, it occurs if there’s a combination of the following factors. 

  • The weather in your area is such that the house is exposed to severe winddriven rain. Usually, this is in Northern Ireland, most of Wales, most of Scotland and the west of England. 
  • You haven’t got any trees or other buildings to shelter your building, and you live in an exposed location. 
  • Your external walls are in a poor condition, poorly built or sloppily maintained. Usually, this shows cracks in the brickwork or render. 

The Building Research Establishment publishes in its official guidance that there is a good chance of moisture bridging the cavity if your home meets the above conditions. If you choose the correct insulation then the air within the cavity can breathe and you’ll experience far less condensation and damp transmission. 

Q. How do you fix damp in cavity wall insulation? 

Houses with cavity wall insulation will usually become damp if the outside brick leaf isn’t weatherproof and the damp condenses onto the insulation within the cavity. There are two methods you can use to fix this problem.  

  • A resin-based textured wall coating for external walls. Firstly, the contractor does all visible repairs such as cracks, holes, loose rendering and repointing etc. Then, apply the exterior coating. Your walls will be weatherproof and won’t normally need painting for 20 to 25 years. Depending on the brand you choose you’ll also receive a guarantee for about 20 years. 
  • A damp proof wall coating. This application protects the outside wall from rain. And, as it’s colourless, it won’t change your home’s appearance. This is especially important if you live in a listed building. 

Q. What happens when cavity wall insulation gets wet? 

If you have poorly maintained walls or if the cavity wall insulation isn’t installed correctly, you can often experience condensation and damp penetration. The rain soaks into and through the outside layer of bricks and reaches the edge of the cavity. This then comes in contact with the insulation and can either transmit the moisture across the cavity or allows the moisture to condense onto the insulation granules. Next, the water drips down to damp proof course level. If there is a large amount of water which occurs with poorly maintained brickwork, the water can be absorbed by the insulation which then sags and slumps. Thus, losing its insulating effect. 

Q. Can I remove cavity wall insulation myself? 

Yes, although its much easier to use a specialist company because you need to know the procedures and have specialist equipment. Also, it’s difficult to find a waste site that will accept a large amount of insulation material. 

Bricks must be removed at intervals along the wall and the insulation removed using a large and powerful vacuum machine. If you have mineral or glass fibre insulation, the vacuum machine sucks the fibres out of the cavity. However, if you have polystyrene beads or urethane foam, you must first break up the solid mass using a specialist machine before it’s sucked out. 

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Q. What size is a cavity block? 

These are sometimes called ‘hollow blocks’. They used to be quite common when constructing houses. Usually, they are 18 inches long by 9 inches wide by 9 inches high. As you might expect, the blocks are hollow giving a ready-made cavity. However, this isn’t a true cavity because the gap is bridged by the block walls every 9 inches.  

Q. How do you insulate a cavity block wall? 

The best and most cost-effective way to insulate a cavity block wall is to inject the walls with foam insulation. Because of the hollow cavities within the blocks, there is a series of hollow cylinders from ground level to roof level. This is a simple job, usually taking less than one day and it definitely improves the thermal performance of your walls.  

Q. How much does cavity wall insulation removal cost? 

This cost will vary depending on the type of insulation you wish to remove. On average, however, the price is between £20 and £25 per m2. For a typical semi-detached house of about 80m2, the price will be about £1,600 to £2,000 plus VAT. Remember, you’ll have to pay a bit more if you live in London. 

Q. Does cavity wall insulation cause Mould?

Yes, it can. But, only if your wall is in poor condition. Let’s think about it. You’ve filled the wall cavity with a solid material. The same cavity that’s there to stop the movement of moisture from the outside to the inside. It seems obvious that the moisture will travel across the gap now its filled. 

The way to prevent this is to repair and fully maintain your outside brickwork and coat it in either a coloured resin based textured coating for exterior walls or a clear waterproof dampproof coating. 

Find a cavity wall insulation specialist near you

There are multiple benefits to this type of work, but it’s complex work that should only be carried out by a registered professional. To find out the average cost of cavity wall insulation in your area, simply fill out the form at the top of this page and we’ll find you 3-4 specialists. Comparing quotes only takes a minute and best of all, it’s totally free of charge.

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