Cavity wall insulation, which type do you need?

Uninsulated walls can account for over 35% of the heat lost from the average home, making it the biggest loser in retaining heat. The three main options to prevent this heat loss are:

The one that’s most suitable for you will depend on the structure of the walls.

How do I find out the structure of my walls?

Thanks to updated building regulations, standard houses built after 1990 will have wall insulation installed already. It might be possible to add more insulation, but it’s usually more economical to address the heat loss in other parts of the house.

Houses built between 1920 and 1990 tend to have an empty void or ‘cavity’ between the outer and inner layer of walls. Cavity wall insulation comprises filling this space between the walls with loose insulation. You should be able to tell that your house has a cavity if the external walls only show stretchers (the long side of the brick), or the wall is thicker than 260mm.

Houses built prior to 1920 have a solid wall structure, as there were no obligations to insulate. Walls less than 260mm thick are usually solid, and you’ll see brick stretchers and headers (the small end of the brick) in any exposed brickwork. Houses made of stone also have solid walls, along with other non-standard types of buildings (steel or timber-framed). The options for solid walls are external and/or internal wall insulation.


Cavity Wall Insulation

£5 – £6 per m2

The beauty of cavity wall insulation is it’s a quick, non-intrusive means of insulation. The operation will need to be carried out by a professional, but it’s clean and takes just a couple of hours to install for a mid-sized house. Holes are drilled between the bricks from the outside of your house, and loose insulation (typically mineral fibre or polystyrene balls) is injected into the cavity.

  • Cheapest method of wall insulation
  • Up to £225 per year in savings (EST)
  • Reduce carbon dioxide emissions by up to 1,120kg (EST)
  • 5-year payback period
  • No hassle to your household
  • Quick and clean to install
  • Requires access to all parts of external walls
  • Cavity must be greater than 50mm wide
  • Unsuitable for homes regularly experiencing driving rain

Internal Wall Insulation

£55 – £140 per m² (installer)

Internal wall insulation is essentially creating an inner lining for your house’s external walls. This is done by constructing a ‘stud’ wall (a few lengths of timber with semi-rigid insulation slotted in between) or fixing insulation boards to the wall known as dry lining. A vapour barrier and plasterboard/render are then used to cover the insulation.

Depending on the thickness of insulation you choose, you may lose up to 40mm of space from inside your house, so thinner insulation boards with a higher thermal resistance help to conserve space in the room. It is possible to insulate the walls in this manner yourself but it can be quite a laborious task, and contracting an installer will ensure that thermal bridging is prevented.

  • Can be DIY
  • Up to £425 per year in savings (EST)
  • Reduce carbon dioxide emissions by up to 1,870kg (EST)
  • Reduces floor space in rooms with external walls
  • Disruptive process, but can be done room by room
  • May not support heavy objects hung on the wall


External Wall Insulation

£55 – £180 per m2

You’ll need planning permission for external wall insulation, but it’s considered the most effective at preventing heat loss through the walls. It’s also the most drastic, as it involves completely changing the appearance of your house. Board insulation is fixed around the outside walls before it’s covered in a breathable render. This type of insulation also acts as a protective coating for the brickwork, which could extend the building’s lifespan.

If you’re concerned about the change in aesthetics to the front of the building, you can opt for brick slips (that replicate the appearance of conventional brick walls) or a hybrid of internal and external wall insulation – internal at the front, external at the back.

  • Up to £425 per year in savings (EST)
  • Reduce carbon dioxide emissions by up to 1,870kg (EST)
  • Little disruption to your household
  • Reduces draughts
  • Improves waterproofing
  • Prevents build-up of moisture from condensation
  • Can help to maintain external walls
  • Most expensive option (try to couple with other home improvements)
  • Requires planning permission


Problems with condensation

Condensation will cause complications if left untreated, leading to problems like wet walls, mould and even breathing difficulties. Since insulation is only effective when it’s dry, make sure there are no damp patches on the inside walls, or the external walls aren’t damaged (in which case, you may have to contact a professional to resolve this issue beforehand).

You also need to consider the effect adding insulation to your home will have on ventilation and the dew point – the point where hot and cold air meet. The solution to this may be as simple as opting for vapour-permeable materials (on breathable solid walls) or adding a vapour barrier, but you can discuss this in greater detail with your installer.


Requirements for the Renewable Heat Incentive

If you’re planning on receiving an income from the Renewable Heat Incentive (RHI) after installing a heat pump or biomass boiler, you’ll need to check that your last Energy Performance Certificate (EPC) recommended installing loft and/or cavity wall insulation. If it did, you’ll have to get these installed and have another EPC survey before claiming.


Savings sourced from the Energy Saving Trust (EST)
Indicative costs per m² sourced from the Department for Business, Energy and Industrial Strategy (BEIS), April 2017

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