Cavity Walls Insulation

This page answers questions about cavity wall insulation. For other types of insulation, go to the Insulation FAQ.

How much will I save from cavity wall insulation?

According to analysis of actual total gas bills before and after installation (National Energy Efficiency Data), savings are typically about 10%. However you could save more or less depending on the shape of your house and how you set your thermostat - you may find you are much warmer than before.

Can I get a grant for my cavity wall insulation?

See our Finance page for information on grants for energy efficiency such as cavity wall insulation.

How can I tell if my cavity walls have been filled?

If your home was built in 2000 or later you should have had cavity wall insulation installed from the start, as required by building regulations.

If it was built before then it may have been filled on construction or later. If it was filled later you should be able to see the holes drilled (and now filled in) to allow this – about 1m apart. If you are unsure you may be able to investigate by looking down into the cavity from up in the loft space or looking behind the electricity box, electric switches where you may be able to see the insulation within the cavity.

You can ask a registered installer to survey your home. They will drill a small hole in your external wall to see if your walls are hollow or filled. They can assess this using a boroscope.

Is cavity wall insulation suitable for all houses?

Insulation must be carried out by a registered installer who will carry out a survey of your property and advise you. Your home is likely to be suitable for cavity wall insulation if:

  • its external walls are unfilled cavity walls
  • your cavity is at least 50mm wide
  • the masonry or brickwork of your property is in good condition

Cavity wall insulation cannot be installed if:

  • There are any signs of damp. Cavity wall insulation can help to prevent condensation but all other damp problems must be cured before it can be installed.
  • The cavity width is less than 50mm or the property is timber framed.
  • The cavity has any rubble inside or the damp course has failed.
  • Ventilation is inadequate and a vent cannot be fitted.
  • Your home is in an area prone to wind driven rain and is not sheltered. This is not a problem in Cambridge. There is a map here ( showing which regions are vulnerable.

If you cannot fill your cavity walls for any reason you can treat them as solid walls instead and insulate them on the inside or the outside.

I live in a timber framed house, can I have Cavity Wall Insulation?

Unfortunately no, the timber frame requires ventilation around it or it may rot. Installing cavity wall insulation may prevent the ventilation. Other types of insulation are possible for timber framed houses but may prove expensive.

How is cavity wall insulation installed?

Cavity wall insulation is blown into the cavity from the outside of a house. Every part of the wall must be filled with insulation, so it's important that the installer can reach all your external walls.

If your home's external walls are joined to another house, the installer will need to insert a cavity barrier to contain the insulation, so your neighbours aren't affected.

To insulate your cavity walls, the installer drills small holes around 22mm in size at intervals of around 1m in the outside wall of your home. With specially designed equipment, they then blow insulation into the cavity. Once all the insulation is in, the installer fills the holes in the brickwork so you'll barely notice them.

What are my options for filling cavity walls? How do I decide which to use?

There are a number of different products, or systems, each of which must pass stringent technical requirements laid down by the British Board of Agrément or alternative independent approvals body. All systems are "fit and forget" and require no servicing, maintenance or adjustment. They should be guaranteed by the Cavity Insulation Guarantee Agency. You can also check for a recommendation by the Energy Saving Trust – their recommended insulation products will be guaranteed for at least 25 years.

There are three main systems in common use, and all (except for UF Foam) can be used in all areas of the UK. CIGA has registered installers for all three types:

  • Mineral Wool insulation is a cotton wool like material produced from either rock wool or glass wool, and is used in the majority of installations.
  • Polystyrene Bead is made up of small expanded polystyrene beads normally injected with a binder.
  • UF Foam cavity wall insulation was introduced into the UK in the late 1960's and involves the injection of a water based chemicals system that produces an insulating foam in the cavity.

All three of these materials offer similar savings but each has different qualities - a registered installer can advise on the most appropriate material for your home.

My cavity walls were filled 10 years ago – are they OK?

Cavity wall insulation is guaranteed for 25 years and the British Board of Agrément say that insulation should last the life of the building.

The easiest way to check cavity walls for problems is to use a thermal imaging camera. Here in Cambridge, CCF has thermal cameras for you to borrow: you need to attend an (online) training session first so you know how to to use them. See the CCF web page for info.

My cavity walls were filled some years ago and the filling has sunk – can I top it up?

The easy answer to this is ‘it depends!’ It is important that you find out exactly what type of cavity wall insulation you have installed.

If you have a newer house it is likely to have been installed into the cavity as blocks of insulation. There are cases where this has not been installed to the recommended density, but unfortunately in these cases it is unlikely you can top up.

If the house is older and cavity wall insulation has been installed it may be possible to top up. The key thing is to top up with the same material. By mixing insulation materials you run the risk of causing damp. It is also very important to ensure that the cavity is in good condition. There are a lot of cases in older houses when building standards were not as high where the cavity has been bridged, for example from building debris thrown down the cavity at the time of construction; or alternatively ensuring that the cavity ties have not rusted. By topping up insulation in these cases you may create damp.

If an installer tells you it is not possible to top up it is worth confirming why this is the case. Sometimes it is not possible because they do not provide a compatible system but another installer may be able to do it. However in some cases it may not be possible because of the type of insulation that has been installed.

I have heard that filling cavity walls can cause damp. Is this true?

Cavity wall insulation should never be installed if there is already any damp in your walls. Damp issues must be treated by a professional who specialises in damp prevention before cavity wall insulation is installed.

However assuming that your property has no damp issues, it is sufficiently sheltered from wind driven rain and the cavity is in good condition (which will be assessed when your property is survey by a registered installer) the cavity wall insulation will not cause damp. It contains a water repellent and so will not allow migration of damp across a cavity. All types of cavity wall insulation have been tested, assessed and approved by the British Board of Agrément or the British Standards Institution.

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