Cavity Wall Insulation

Approximately a third of all the heat lost in an un-insulated home is lost through the walls. Cavity wall insulation insulates the space between your home’s inner and outer wall, and can save you up to £135 a year on your heating bills.

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Do I have Cavity Walls?

Cavity wall insulation is intended for use with external walls with a gap, or cavity, between them. But how do you know if you have cavity walls?

  1. If your home was built in around 1932 onwards, it is highly likely that the external walls have cavities, although new homes (built in the last 10 years, from 1995) will already have the cavity insulated
  2. The pattern of the brick will give you a strong clue; if there is a cavity the bricks tend to be all laid lengthways, whereas solid walls have bricks alternately laid length ways then end-on (across the wall rather than along it so that one end is on the internal side of the wall and the other on the external side)
  3. Check an external doorway to see how thick the wall is – if it’s more than 30cm (11.5 inches) thick then you probably have a cavity wall.
  4. If you are in any doubt, your installer will be able to confirm exactly what type of wall you have and recommend the most suitable method of insulation.

If you don’t think you have cavity walls, remember that solid walls can be insulated too. Find out more details about Solid Wall Insulation here.

Do I already have cavity wall insulation?

You can check to see if your cavity wall has already been insulated by looking under your external window frames – if there are holes in the wall, you already have cavity wall insulation installed.

Will it work in my home?

Generally, the answer is yes, but check the following:

  1. Your external walls must have unfilled cavities (houses built in the last 10 years typically will have their cavities filled already)
  2. Timber framed walls with a studwork cavity, metal framed walls, concrete walls, and stone walls with uneven cavities are unsuitable for filling
  3. The house shouldn’t be more than 4 storeys tall
  4. The cavity should be at least 50 mm wide – your installer will be able to confirm this
  5. The brickwork or masonry should be in good condition
  6. If your house was built before 1940, it is likely that it doesn’t have a VDPC around the windows (Vertical Damp Proof Course), meaning that standard cavity insulation will cause condensation problems. If this is the case, you should install a VDPC or alternatively get solid wall insulation fitted.
  7. Dirt inside the wall will also cause condensation to occur, so you should check that the wall is clean before going ahead.
  8. If your wall in is a position where it is exposed to driving rain it might not be suitable as the rain may cause condensation inside.

If one or more of your walls has these issues, your house is not suitable for cavity wall insulation. Any gaps in cavity wall insulation lose heat more quickly than the same area of unfilled cavity wall, making the insulation a lot less effective. If your cavity wall is unsuitable for treatment, you can insulate it instead using solid wall insulation, which should be eligible for either partial or full funding under the ECO scheme, the grant scheme running alongside the Green Deal. Details of how to apply to this scheme have not yet been announced.


Installation is quick and simple for a professional installer and should not take more than a couple of hours, leaving no mess behind. Bigger homes might take a little longer.

As for choice of insulation material, mineral wool is the most common material used but is known to shift if not installed well, creating ‘cold spots’. Cellulose beads are the best material for staying put and reducing ‘cold spots’, with cheaper polystyrene beads and injected foam a close second.

If your cavity wall is a stone wall, you need a breathable (natural) insulation material such as sheep’s wool, hemp or paper (cellulose), which needs to be inserted slightly off from the wall (25mm) to prevent condensation issues.

Ensure that your installer is a member of a recognised professional body. We will only recommend certified installers. Installations by certified installers will be guaranteed for 25 years by the CIGA (Cavity Insulation Guarantee Agency)

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Costs and Maintenance

The Energy Saving Trust estimates that the cost of installation is around £100-350 for a 3 bed semi-detached house, though you may well be able to get your cavity wall insulation FREE through the HHCRO (Home Heating Cost Reduction Obligation) scheme, part of the ECO grant scheme. To find out if you qualify and sign up visit our HHCRO Free Insulation page.

There is no maintenance over and above what you would have to do to the exterior of your house anyway.


The annual saving for a 3 bed semi would be in the region of £110, and some estimates are more, which means that your costs will be paid back within 1-4 years, or immediately in the case of free insulation.

How much can you save?


What guarantee of workmanship should I look out for in a cavity wall insulation installer?
Cavity wall insulation installers can sign up to a professional code of practice, such as those provided by the Cavity Insulation Guarantee Agency, which guarantees the work for 25 years. The installer should carry out post-installation checks on both the inside and the outside of the property, and you will be asked to sign a form to show you are happy with the work.
I want to use my loft for storage, how can I do that if it is insulated to the depth recommended?
You or your installer can instead lay down innovative loft boards called Space Board. These are insulation boards made from Polyfoam extruded polystyrene designed to be placed on top of the ceiling joists instead of the 170mm mineral wool insulation. You should still aim to lay down 100mm of standard insulation between the joists.
Why wasn't my house built with cavity wall insulation and the right amount of loft insulation?
Houses comply with the building regulations that apply at the time the property is built. Since the oil crisis in the early 1970s, the insulation standards in the Building Regulations have been progressively upgraded.
I already have some loft insulation - do I need to install more?
If your loft insulation is 100mm or less you would certainly benefit by having it topped up with more insulation to make it up to a 270mm thickness. The Energy Saving Trust estimates that top-up loft insulation can save the average householder as much as £60 per year at current energy prices.
What is loft insulation made of?
Typically, loft insulation is a mineral wool made out of glass or rock fibers. You can also get natural loft insulation made of sheep's wool, hemp or cellulose which will remove the need for protective equipment,
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