Exterior wall insulation.
Is it Right for You?

Exterior wall insulation (also referred to as external insulation) is a way of insulating the main walls of your home from the outside and is commonly used in homes that have solid walls with no cavities.

In the UK the majority of homes built before 1920 - 1925 will likely have solid walls.

This obviously makes it more difficult to insulate the walls as there’s no cavity into which you could add blow in insulation.

However, while it does make things a little more difficult, it doesn’t mean it can’t be done. You still have two options when it comes to insulating your solid walls:

  1. Internal wall insulation - As the name suggests this form of insulation is applied to the inner surface of the main exterior walls of your home.

  2. Exterior wall insulation – Again fairly obvious, but this is where insulation is applied to the outer surface of the main exterior walls.

In this article I’m going to look at external wall insulation. Why you might choose it over internal insulation and look at the main advantages and disadvantages of choosing it.

How do you know if you have solid walls or cavity walls:

As mentioned above if your house is in the UK and was built before 1920-1925 then it probably has solid walls.

The easiest way to confirm this is visually, if you have un-rendered brick walls and you can see the ends as well as the sides of the bricks then you have a solid wall.

If your walls are rendered, then you’ll have to measure the thickness of the external wall. This is easiest done at a doorway or window opening. If it’s 230-250mm you probably have solid walls although some early cavity walls were 250-270mm (a modern cavity wall would be 300mm plus).

Also don’t assume that all your walls are the same, my house has a combination of 250mm solid brick walls along with 600m solid stone walls.

Why you might want exterior wall insulation?

The main reasons for going down the exterior wall insulation route are as follows:

  • The walls of your home don’t have a cavity to insulate – In older homes this is common and means if you want to insulate your walls you’re restricted to either insulating internally or externally.

  • You don’t want to lose floor space by insulating internally – If you were to insulate internally you’d have to apply the insulation to the inner face of the main external walls (typically by building a timber frame and installing rigid foam insulation followed by a vapour barrier and finishing it with plasterboard).

    This means you’d typically lose 100-150mm of floor space (depending on the type of insulation and the u-value you want to achieve). Then if the room has two or more external walls you’ll lose that space on two or more walls.

    This might not be a major concern if you have large rooms but if your house has quite small rooms it can make a big difference.

  • You want to minimise cold bridging - Cold bridging is where cold is transferred right through the solid walls of your home from the outside to the inside. Masonry is a good conductor of heat so the cold passes through the wall quickly.

    Cavities were introduced to minimise this as the cold struggles to get across the cavity as air and insulation are poorer conductors of heat than masonry.

    However even in cavity properties cold bridges can exist. For example at window heads and sills cold bridges can occur as these “bridge” the cavity allowing the cold to get across to the inner wall.

    External insulation minimises the occurrence of cold bridging as it wraps the house in one large sheet of insulation separating the cold air from the masonry walls and thus minimising any cold bridges (a bit like putting a coat on your house).

    Whereas internal insulation still allows some cold bridges to remain at wall junctions etc. as can be seen in the following diagrams.
internal insulation/cold bridgingInternal Insulation Allowing Cold Bridge
Esterior Wall Insulation/No Cold BridgingExternal Insulation Eliminating Cold Bridge
  • You want to use the thermal mass of the main walls to your advantage (almost like as a type of storage heater) – If you were to insulate your main walls internally you hold the heat inside the property. Meaning the masonry walls never really warm up.

    However if you insulate externally the large thermal mass of the masonry walls slowly heat up. Then after your heating turns off the walls slowly give out that stored heat and because there is insulation on the outside of the wall the majority of the heat is redirected back into the house. See diagram below.
External Insulation takes advantage of thermal massHeat Sync Diagram - External Insulation
Internal Insulation - No Thermal MassHeat Sync Diagram - Internal Insulation
  • You want to refresh/renew the exterior of your property and increase its thermal efficiency at the same time – Due to the method of installing exterior wall insulation it means that you'll end up with a fresh newly rendered or brick slip finish which will make your home look brand new again.

    When adding exterior wall insulation the insulation is chemically and mechanically bonded to the existing surface of your walls (assuming they are in good condition). Then they’re typically finished in an acrylic or rendered finish.

    These finishes can have colour pigments mixed in meaning you won’t need to paint the exterior. This alone can save you a lot of money in redecorating costs over future years.
External Insulation - Typical InstallationExterior Wall Insulation - Typical Application

Advantages of exterior wall insulation:

The advantages of exterior wall insulation are fairly well detailed above so I’ll just list them again here:

  1. Avoids cold bridging.
  2. Won’t cause you to lose valuable floor space internally.
  3. Let’s the thermal mass of the walls work as a heat sync.
  4. Means your home will look new again from the outside.
  5. Can reduce future external redecoration costs.
  6. Increases the thermal efficiency of your home dramatically.

Disadvantages of exterior wall insulation:

  1. External insulation is expensive (the last quote I saw was around £100/m²). This is typically around twice the cost of insulating internally Although internal insulation will involve extra costs such as relocating radiator pipes away from the walls, replacing flooring, relocating fireplaces out from walls and redecorating etc.

There are several things to consider when deciding if exterior wall insulation is right for you, namely.

  • Is maintaining internal floor area a priority?
  • Can you afford it?
  • Would it be beneficial to update the exterior of your home and reduce future redecoration costs? Or alternatively are you about to redecorate internally, in which case insulating internally might be right for you.

My opinion is that exterior wall insulation has more plus points over internal insulation with only one disadvantage (and granted it’s a biggie), cost.

But that said, if the interior of your home is good and you weren’t planning on redecorating the additional costs that need to be added to the cost of insulating internally could well push the cost of internal insulation close to the cost of external insulation.

If cost is the only thing stopping you, have a word with your installers and they should be able to tell you if there’s any grant assistance available in your area.

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