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Solid and Cavity Wall Insulation

Did you know that about a third of all the heat lost in an uninsulated home goes out through the walls? That's not only lost heat, but lost money you could have used to save on your heating bill.

That's why insulating your walls could cut your heating costs considerably all-the-while making your home more comfortable.

Our guide will explain the difference between solid wall and cavity wall insulation, so that you can work out the type of insulation that suits you. By properly insulating your walls, you'll retain heat and cut down on the cost of your bills.

What type of walls have I got?

Before deciding on what kind of insulation to go for, you'll first need to work out what kind of walls you have:

Solid walls:- There are two types of solid wall insulation, interior solid wall insulation (inside wall insulation) and exterior solid wall insulation (outside wall insulation).

If your home was built before the 1930s, you likely have solid walls. But how else can you tell if you have solid walls? A quick way to know is to check the length of the bricks in the. wall. Solid walls usually have bricks of unequal lengths, while cavity walls have bricks that look more or less the same. Solid walls also tend to be less than 260mm in thickness.

Cavity walls:- Cavity walls consist of two walls with a gap - or cavity - in-between. The outermost wall is usually made of brick, while the innermost wall is constructed of brick or concrete block. the two walls are held together with metal wall ties.

If your house was build after the 1930s, then you probably have cavity walls. Nowadays, many new energy-efficient properties will have cavity walls that have been pre-filled with insulation material.

If you have solid walls:

If you've determined you have solid walls, below is a breakdown of costs and other things you can expect:-

Cost of solid wall insulation

According to the Energy Saving Trust, external wall insulation for a semi-detached house costs around £13,000. For an internal wall, the costs are a lot less, coming in at around£7,000.

Can I get free solid wall insulation?

Yes. If you receive certain benefits, you might be eligible for an insulation grant from the government to help with some or all of the costs in insulating your solid walls.

Differences between interior and exterior solid wall insulation

Cost may be one of the deciding factors in which type of insulation you go for, but there are other things to consider, too. The table below highlights the key differences between internal and external solid wall insulation:-

Type of solid wall insulationProfessional Required?Planning permission?Appearance of the houseDuration of work
InteriorNot AlwaysYes, for listed buildings.Won't change4-5 days
ExteriorYesNo, unless the building is in a Conservation AreaWill change4-5 weeks

What are the benefits of solid wall insulation?

Solid wall insulation could save you about £320 on your annual energy bills. Using the Energy Saving Trust's cost estimates, insulating an external wall would take roughly 40 years to pay for itself.

While this may feel like an age, there are some additional benefits to consider: You could quickly improve your home's Energy Performance Certificate rating, its resale value, plus have a warmer property each winter.

Solid wall insulation also provides a layer of protection against noise from rooms within the property, as well as outside.

How to insulate a solid interior wall

To insulate the internal solid walls of your home, your installer will:

  • Fit rigid insulation boards to the wall or add a stud wall and fill in the gaps with fibre insulation, natural or mineral wool, or your chosen insulation material.
  • If you're using rigid walls, they'll be fixed to the wall with plastic or adhesive. Then, the installer will add extra fillings to hold the boards in place and seal the joints between the boards.
  • For stud walls, you'll be given the choice of either metal or wooden frames, which will then be attached to the walls. The insulation material will then be inserted into the gap between the frame and the wall.

How to insulate a solid exterior wall

Although external wall insulation requires the skill of a professional, below is a breakdown of what's involved:-

  • Your wall will need to be checked for signs of damp. If any is found, the work will need to be delayed until the problem is fixed.
  • First, a later of insulating material will be attached to your wall. Then, the material will be covered - either with render (plasterwork) or cladding.
  • After, you'll need a finish applied to the walls to make them visually appealing. Depending on your taste, this could be stone, glass, tiles or wood finish. Aside from being visually appealing, the finish also adds an extra later for heat retention.

If you have cavity walls:

If you have cavity walls, below is a breakdown of costs and other things you can expect with installation:-

How cavity wall insulation works

A Cavity Insulation Guarantee Agency-qualified professional can insulate your cavity walls by drilling holes into the outer wall and filling the cavity (gap) with insulation materials. Once these have been filled, your walls will prevent heat transfer in your walls, helping keep the heat indoors.

What types of cavity wall insulation are there?

The most common cavity wall insulations in the UK use:-

  • Blown mineral fibre or wool:- your installer will do this by heating and spinning the insulating matter to create a fibrous material which is then blown into the cavity.
  • Polystyrene pellets (granules or beads):- the chosen pellets are pumped into the cavity along with an adhesive. Once inside, the two materials form a solid bond that prevents heat transfer.
  • Urea-formaldehyde foam:- after pumping the foam into the cavity, it expands and becomes firm within a couple of minutes. Your installer should return in a week to seal the walls after the foam has hardened.

Cavity wall insulation cost

Cavity wall insulation is one of the cheapest energy-saving measures you can install in your home. According to the Energy Saving Trust, the process costs between £300 and £750, depending on the size of your property. With an average saving of £245, the cost of the insulating your cavity walls should pay for itself in one to three years.

Can I get free cavity wall insulation?

Yes. If you receive certain benefits, you might be able to get a government grant for insulation to help with some, or all, of the cost of insulating your cavity walls. 

How long does cavity wall insulation take?

Cavity wall insulation usually takes between 6-12 hours to complete, and there should be no need to vacate the property while the work takes place.

Benefits of cavity wall insulation

As well as the previously mentioned benefits, other advantages of cavity wall insulation include:

  • Improving your home’s Energy Performance Certificate rating and resale value 
  • Reducing your carbon footprint 
  • Mould prevention. The insulation prevents moisture from moving through your walls

For more energy-saving tips, check out our guide on how to save energy at home.

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