Is External Wall Insulation A Good Idea?

Is External Wall Insulation A Good Idea?

As a homeowner, you may be wondering whether installing external wall insulation is a good idea, and here we examine the pros and cons.

External wall insulation isn’t cheap, but it will reduce heat loss and your home’s energy bills.

However, there are some disadvantages of external wall insulation that we will look at later.

The main issue is that if your property was built before the 1920s, then it’s likely that you don’t have cavity walls and will have solid walls instead.

  • A cavity wall has two layers that have a small gap, or a cavity, between them;
  • A solid wall will have no such gap so you cannot use cavity wall insulation.

Despite this, a solid wall can be insulated either from the outside or the inside.

It’s a more expensive process than having to insulate a cavity wall, but there are potentially big savings to be enjoyed on your heating bill.

So, the first step is to determine which wall type you have.

What is external wall insulation?

For a home with solid walls, external wall insulation will make your home more thermally efficient.

Usually, external wall insulation consists of an insulation layer that will be fixed to the existing wall.

It’s then finished with either cladding or render to help protect the wall from the weather.

For a home with solid walls that don’t have the option of installing cavity wall insulation, this is the method you will need for insulating the wall.

You need to consider carefully before installing it because while it may appear to be a good idea, there are some issues with external wall insulation, and these are mainly around how your property will deal with moisture.

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The advantages of installing external wall insulation

The most obvious advantage of installing external wall insulation is to help reduce the heat loss from your property and consequently reduce your energy bills. You will also:

  • Reduce draughts;
  • The insulation will not reduce the internal floor space;
  • The insulation will contribute to the home’s thermal mass;
  • Insulation will boost sound resistance;
  • Improve weatherproofing;
  • Increase the wall’s life;
  • Reduce internal wall condensation.

The installation of external wall insulation can also be carried out without disrupting your home and once complete, the improved warmth will deliver comfort.

How much will external wall insulation cost?

External wall insulation is not a cheap process so you need to fully assess if the savings in the heating bill and the gain in comfort are worth the money.

This is one of the big issues, and you need to consider how much external wall insulation will cost before undertaking it.

For example, if you want to insulate a three-bedroom semi-detached home, then it could cost up to £10,000.

In comparison, it will cost around £8,000 to fit internal wall insulation.

The costs involved will depend on the materials used and whether the installation firm is using a proprietary system.

Whichever route is suitable, you will be needing to put up scaffolding and then remove any exterior cables and pipes, and then replace them afterward.

Building experts will highlight that one way of reducing the cost of installing external wall insulation is to include it as part of a larger property revamping project.

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How much could I save fitting external wall insulation?

Along with making your property warmer, there are some big energy bill savings to be enjoyed when fitting external wall insulation. For example:

  • A detached home could save £425 per year;
  • A semi-detached home could save £255 per year;
  • A bungalow could save £170 per year;
  • A mid-terraced property could save £160 per year;
  • A mid-floor flat could save £125 per year.

These cost savings provided by the Energy Trust are based on insulating a home with gas heating.

Does external wall insulation need planning permission?

It’s important that you check with your local planning authority because external wall insulation will change your home’s external appearance.

For those properties, this means having planning permission and you should begin this process before approaching insulation firms.

You also need to understand that if your home is a listed building or in a conservation area, then it’s unlikely that you will get planning consent.

However, in most cases, the installation of external wall insulation for a domestic property will be classed as a permitted development so you should have no problems.

Will external wall insulation cause damp problems?

As we will see later in this article, there is a potential issue with external wall insulation causing damp problems in your home.

Moisture may enter either from the external wall when it rains, for example, or from the activities that are undertaken within a property on the internal wall, for example, washing, cooking, and drying clothes.

Since most homes have cavity walls and are likely to have cavity wall insulation, you need to appreciate that before this housebuilding development, builders had to choose whether to build either:

  • A wall that would prevent moisture on both skins (i.e. impermeable);
  • A breathing wall so moisture will penetrate – and then be evaporated away.

It’s important to understand that builders installing a solid brick wall will usually tend towards building an impermeable wall, while a stone wall usually tends to be a breathing wall.

In comparison, for homes with a cavity wall, this will be impermeable since the cavity creates a barrier that will prevent any moisture from penetrating the inner wall and the home since any rainwater that enters will be evaporated by the cavity’s air movement.

Also, the insulation materials used today will be clad or rendered to your home to help prevent rainwater from penetrating.

While these materials work to prevent water ingress, they don’t stop any moisture from reaching the wall’s internal surface so you may need to consider improving your property’s ventilation to move any moist air to prevent damp problems.

I have solid stone walls; can I use external wall insulation?

Even if you have solid stone walls you can install external wall insulation.

For anybody with a property that has solid stone walls and wants to know whether you can install external wall insulation, then the answer is yes.

Also, some homes with a solid stone wall may have a small cavity between the interior and exterior wall that is filled with rubble.

So, any moisture that forms within the cavity will either evaporate away because of the air movement.

It is also worth considering that a stone wall will usually be a ‘breathing wall’ and you need to maintain the breathability that will ensure the wall operates as it is designed to do.

Essentially, you’ll need to use natural insulation such as cork or wood fibre and use lime as the external render.

For a solid brick wall, which type of external wall insulation is best?

Need to know which type of external wall insulation is best? Then check your bricks.

That’s because not all bricks are the same since the permeability and the quality of them will vary.

For example, it is fairly common to see bricks that are ‘spalling’, that is a brick with its surface flaking off.

This tends to happen because, in the freeze-thaw process, the moisture that penetrates a brick will freeze, and when it thaws, this will lead to flaking.

As such, external wall insulation will probably not have much impact on the wall’s performance – except to improve the thermal performance.

I have cavity walls; can I use external wall insulation?

We have already seen that a cavity wall will usually be ventilated which is why builders create the cavity in the first place.

So, any warmth from the property that penetrates the internal wall to enter the cavity will then be exhausted by the cavity’s ventilation.

This then means that installing external wall insulation will not have much of an effect since most of the heat lost will disappear in the cavity before reaching the external skin.

Perhaps one issue to consider is that if the external skin enables rainwater to penetrate into the cavity, you should consider external insulation and use a render that is weatherproof to help stop the rainwater from entering and boost the wall’s thermal barrier.

How long will external wall insulation last?

Depending on the installer, most manufacturers offer a warranty of 25 years, but the insulation has to be installed correctly.

This is a minimum term, so you should expect your insulation to last longer than that.

There shouldn’t be any problems with external wall insulation if it has been protected from the weather so a waterproof final render coating will be crucial.

If you need to contact installers of external wall insulation, the National Insulation Association has a list.

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What are the disadvantages of external wall insulation?

There are some issues to consider for the disadvantages of external wall insulation and the main one will be its cost.

You need to consider carefully whether the investment in this type of insulation will deliver a boost to your home’s value and a reduction in energy bills. The disadvantages also include:

  • If the insulation will change your home’s appearance, you may need planning permission;
  • External wall insulation may not be suitable for a home in a conservation area, or for a listed building;
  • The insulation is thick, and this may lead to issues around sills, eaves and windows.

The other major consideration is that the insulation may lead to problems with condensation within your home which, in turn, could result in damp problems.

Is external wall insulation a good idea?

Essentially, there are some issues to consider before you think about investing in external wall insulation because the financial benefits could be impressive.

This type of insulation can be expensive and will boost energy efficiency and lower your property’s heating bills.

There are also some attractive ways to improve the look of your home and when compared with internal insulation, the benefits of having external wall insulation outweigh the cost and inconvenience of having it fitted.

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