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Solid wall insulation

Insulating your solid walls could cut your heating costs considerably, because solid walls let through twice as much heat as cavity walls do. The good news is they can be insulated – from the inside or the outside.

If your home was built before 1920, its external walls are probably solid rather than cavity walls. Cavity walls are made of two layers with a small gap or ‘cavity’ between them. Solid walls have no such gap, so they let more heat through. Solid walls can be insulated – either from the inside or the outside. This will cost more than insulating a standard cavity wall, but the savings on your heating bills will be bigger too.


How much could you save?

Type of solid wall insulation Saving per year Total cost including installation  Carbon dioxide saved per year
Internal Around £460 £5,500 to £8,500 1.8 tonnes
External Around £490 £9,400 to £13,000 1.9 tonnes

Estimates based on insulating a gas-heated, semi-detached home with three bedrooms.

You might be able to reduce these costs by carrying out the work at the same time as other home improvements. And you could spread the cost by not tackling all the house at once – see below for how to keep the cost down.


Internal or external insulation?

Internal wall insulation is done by fitting rigid insulation boards to the wall, or by building a stud wall filled in with mineral wool fibre. External wall insulation involves fixing a layer of insulation material to the wall, then covering it with a special type of render (plasterwork) or cladding. The finish can be smooth, textured, painted, tiled, panelled, pebble-dashed, or finished with brick slips.

There are advantages and disadvantages to both.

Internal wall insulation:

  • is generally cheaper to install than external wall insulation
  • will slightly reduce the floor area of any rooms in which it is applied (the thickness of the insulation is around 100mm)
  • is disruptive, but can be done room by room
  • requires skirting boards, door frames and external fittings to be removed and reattached
  • can make it hard to fix heavy items to inside walls – although special fixings are available
  • needs any problems with penetrating or rising damp to be fixed first.


Keeping the costs down

The costs we quote for installing solid wall insulation are for paying a company to come in, insulate your whole house in one go, fully redecorate and replace everything just as it was. Some people want exactly this, but it does cost a lot of money. If you're looking for a cheaper option, the best thing to do is to insulate a wall whenever you are doing something else to it anyway. Fitting the insulation work in with your other home improvements not only saves money on the job, it also spreads the cost of the insulation as you work your way round the house.

If you're planning a new kitchen or bathroom, this is an obvious time to fit internal insulation. But for all the other rooms, why not factor it in when you're next redecorating? You'll be clearing the room and making a mess anyway, so why not take the opportunity to improve your insulation while you're in there?

External insulation will also cost less if you do it when you're having other work done to the outside. If you're having a new roof, or painting the windows, or even having solar PV panels fitted, then you will probably have scaffolding up already, which can save a bit on the costs. And if you have rendered walls with damaged render, or brick walls that need re-pointing, external insulation may not cost you much more than you would need to pay for the repairs.

Lots of older houses have an attractive frontage which wouldn't be suitable for external insulation, but a much less impressive rear where external insulation could be just the thing. The front wall can then be insulated internally, one room at a time.

External wall insulation:

  • can be applied without disruption to the household 
  • does not reduce the floor area of your home
  • renews the appearance of outer walls
  • improves weatherproofing and sound resistance
  • fills cracks and gaps in the brickwork, which will reduce draughts
  • increases the life of your walls by protecting the brickwork
  • reduces condensation on internal walls and can help prevent damp (but will not solve rising or penetration damp)
  • is best installed at the same time as external refurbishment work to reduce the cost
  • may need planning permission - check with your local council
  • requires good access to the outer walls 
  • is not recommended if the outer walls are structurally unsound and cannot be repaired.



If you insulate a solid wall, you have to make sure it complies with the current Building Regulations. The main condition to meet is the thermal performance of the insulated wall - if you live in England or Wales then it must have a U-value of no more than 0.30 W/m2K. The U-value is a measure of how quickly heat will pass through the wall - as a rough guide you will need around 60mm to 120mm of insulation to achieve this, depending on what insulation material you use.

Normally your installer will ensure that the insulation is up to standard and will arrange approval from the local Building Control Office for you. If they are not going to do this, you should contact Bulding Control at an early stage to make sure you comply.

If you are planning to remove and replace more than half of the internal plaster or external render of a wall, or if you are dry lining a wall, then you have to insulate to this standard whether you were planning to insulate or not.

If you live in Northern Ireland or Scotland you should contact your local building control office for information on what you need to do to comply with the regulations.