How to Sustainably Renovate Existing Homes for Lower Energy Use Part 2: Insulating External Walls

External walls typically account for up to a third of all heat losses from older uninsulated homes yet they are often overlooked in upgrading works due to the complexities involved in adding insulation to the entire external wall.   If you are looking to improve the thermal performance of your home, there are some easy first items to do such as draught-proofing windows and doors and adding additional insulation in the roof or loft-space.  Replacing windows and doors can be expensive and requires some care in choosing the right products and installer, but should be a relatively straightforward thing to have done.  Beyond these items heat loss though the external walls may be a major issue and a big contributor to high heating bills but upgrading this element requires consideration of the possible alternatives and will probably also involve significantly more disruption to your home.

Insulating External Walls

Before deciding how to insulate the external walls it is important to have a good inspection of the condition and construction of the existing walls first.  

  • Any damp problems will need to be rectified first as these could be increased when insulated.
  •  The particular arrangement of your walls and their junctions with roofs, the ground and other building elements will make some methods of insulation more effective and cost efficient and could rule out others.
  • It is important to have your walls inspected by a professional to identify the existing construction and assess the possible solutions.  Contractors often only specialise in one or two systems, so although they may be experts in their own products, they may not have much knowledge of other alternatives.  It may be necessary for you to talk to a number of different contractors or employ an independent consultant to help you.
  • It is important to consider wall insulation alongside other energy efficiency measures, as part of the ‘Whole Dwelling Assessment’ to work out what combination of improvements will work well together.  This improvement plan should also cover any further improvements that may need to be done in order to avoid unnecessary future complications-refer to Part 1 of these blogposts for more on this.
Different elements of a traditional house fabric that can be upgraded as part of a Low Energy Retrofit

One of the most important aspects of insulating is to achieve as much continuity of the insulation as possible.  A ‘cold bridge’ is any area in a building where a gap occurs in the insulation, for example where the roof insulation doesn’t quite meet the top of the wall insulation or where wall insulation is discontinuous because of a wall running into it.  As these areas will be colder than the main areas there is a greater risk of condensation forming.  With condensation comes the added problem of mould.

Methods of insulation

The vast majority of houses in the UK are brick, stone or concrete masonry construction so this article will concentrate on these but if your house construction is different, it will require a more specific solution.  There are three main ways of insulating existing external masonry walls:

  • Cavity Wall Insulation (CWI) inserted into the internal cavity of the wall (if there is one)
  • External Wall insulation (EWI) applied to the outside face of the building
  • Internal Wall insulation (IWI) applied to the inside faces of the external walls within rooms

These are discussed in more detail below.

How much wall insulation do I need?

Insulation is usually expressed in terms of a ‘U-value’, a measurement of the amount of heat that flows through the particular build-up expressed in W/m2, where the lower the value for a construction the more insulated it is. 

  • A typical uninsulated nine-inch-thick solid brick wall would have a U-value of over 2.0 W/m2 while a typical uninsulated eleven-inch cavity wall would have a slightly better U-value of around 1.6 W/m2.
  • A new house wall built to the current Building regulations must have a U-value of better than 0.3 or 0.28 W/m2, while a wall for a Passivhaus needs to be better than 0.15W/m2, and often these values will need to be substantially bettered.
  • To fully comply with the Building Regulations, cavity wall insulation must achieve a U-value of better than 0.55W/m2, and internal or external wall insulation should achieve 0.3W/m2, although lesser values may be acceptable where that figure is not achievable for technical, functional or economic reasons (such as if you can show that the insulation will not achieve a simple payback over 15 years or if it will take up more than 5%of the floor area of a room).

Cavity Wall Insulation (CWI)

If your external walls have a cavity and it hasn’t yet been filled with insulation, it’s often the least intrusive and usually the simplest method of insulating a wall.  It can give a significant improvement to the thermal performance of the wall without affecting the external appearance of the building or requiring internal changes.  it must be carefully considered however and the correct solution chosen for your particular conditions. 

  • There are many systems available including expanding foams and filling with blown particles such as polystyrene beads or cellulose but
  • it is important to get a solution suited to your particular building and installed by an approved and skilled contractor, trained in that particular system. 
  • They are usually installed by drilling a series of holes from the outside through the mortar joints in the outer leaf of brick or stone, then blowing the insulant material into the cavity. 
  • Some systems are better than others at filling around window openings and into complicated geometries so in order to get the insulation into all the nooks and crannies. 
  • Many systems should not be installed in walls that are exposed to driving rain as filling the cavity with insulation could cause routes for wind driven rain to get to the interior and cause damp.
  • Most uninsulated cavities will only be around 50mm, so the insulation thickness will be limited to that. 
  • Different insulation types vary considerably in their thermal efficiency and their suitability for different wall constructions and degrees of exposure
  • Insulating in the cavities can give continuity at areas such as across junctions with internal walls (which can be a major problem with internal insulation solutions) but many older cavity walls will have solid masonry or lintels spanning the cavity at some points such as around openings to doors and windows.  These will leave cold areas on the internal wall surface even if the cavity is completely filled.

Although filling the cavity can significantly improve the thermal insulation of the wall and therefore reduce heat loss, on its own it will not get near the levels of insulation required in new homes.  To increase the insulation of a wall further it may be used in addition to either internal or external wall insulation.

Most cavity insulation materials have the possibility to create problems if poorly installed.  Cavities were included in house walls to separate the wet outer leaf of masonry from the inner dry leaf, so if the cavity fill insulation creates any pathways between these leaves this can lead to damp penetrating to the interior.  These problems are usually down to installation issues including Incorrectly spaced installation holes, poor mixing of the insulation or poor continuity of installation:

  • These can create ‘shelves’ within the cavity or cracks in a foam that will allow water to track across the cavity to the inside of the wall where it can cause damp spots.
  • Where insulation hasn’t fully filled all the cavity leaving gaps cold spots can be left in the walls, often at corners or around windows or doors.  This can be checked by use of an infra-red camera and some contractors will offer this as a post installation inspection to show the insulation has been correctly installed.

External Wall insulation (EWI)

If you don’t mind changing the appearance of your home and if this is allowed by the Planning Authority, external wall insulation (EWI) would often be the preferred option for ease of installation, cost and for not reducing internal floor area in your rooms (as you would if insulation was added to the inside of your walls). 

Cross section of a traditional solid wall with External Wall Insulation

With EWI you are wrapping an ‘overcoat’ of insulation around your existing walls, which will have a weatherproof finish added to the outside.  This means that the look of the house will be substantially altered so the brick or stone will be completely covered by insulation then a new finish applied over this, typically a ‘render’ (external plaster).  Other finishes can be applied over the insulation such as timber boarding or other forms of metal or panel cladding to give a different look but the existing masonry surface will always be completely covered over.

It is easiest to achieve a good EWI result on a detached home where it can be entirely wrapped in the new insulation.  If your house is semi-detached or part of a terrace it will adjoin another property that is not being insulated at the same time, so adding external insulation to your house may leave an awkward junction where the new finish of your insulated wall stops at the property boundary line and meets the unimproved wall of the neighbour.  This may look odd but can also result in cold areas within your home due to your neighbour’s areas of cold wall just beyond your new insulation.

Plan of a typical attached house with External Wall Insulation

This insulation used is typically a high performance foam board such as graphite enhanced expanded polystyrene (EPS) which has a good cost to thickness ratio, is easy to install without gaps and provides a fairly solid base for a render finish.  There are a whole range of alternative foam boards some of which are thinner but more expensive.  Other insulation materials are available such as mineral wool or wood fibre board which can have the advantages of lower embodied carbon and more vapour permeability, but as these typically are less efficient insulators they need to be much thicker to achieve similar insulation values and may not be as robust if knocked.

Sand cement render finishes are cheap but are prone to cracking and failure and shouldn’t be used on insulation.  Usually, a thin coat acrylic render system is used as these can accept some movement without cracking, can be self-coloured so they don’t require painting and can even have a silicon finish that repels dirt.  There are also more traditional lime-based renders which can also accept some movement without cracking and are vapour permeable, so are suitable for use on vapour permeable insulation. The most important thing is not to randomly mix and match products but to use render and insulation as a complete system tested by a manufacturer to be used together.

The installation of EWI systems can be very straightforward and is done entirely from the outside.  The complete wall is covered in foam insulation boards attached to the existing walls with plastic pins, including thinner boards around the reveals of openings for windows and doors.  Most systems then require a layer or mesh sheet to achieve a good adhesion of the render coat (or coats) which are then sprayed on.  A through coloured finish coat will not require additional painting and reduces future maintenance.

A EWI installation will add least 100mm thickness onto the outside wall, so requires this amount of room at property boundaries and really needs the existing roofs to have at least this depth of overhang to give a water-resistant top detail to the wall insulation.  Anything fixed to the walls such as rainwater pipes will also need refixing spaced off the wall and most window cills will need to be replaced to suit the added insulation thickness. 

Internal Wall Insulation (IWI)

There are many situations where it is not desired to completely change the external appearance pf your home as dramatically as external wall insulation will.  Internal wall insulation can be fixed to all the internal faces of your external walls then finished with plaster or plasterboard.

Cross section of a traditional solid wall with Internal Wall Insulation
  • Careful bespoke design and installation is required to suit your particular house in order to avoid cold spots and to control water vapour movement within the wall to prevent damp and mould.
  • IWI is the most complicated method to achieve high levels of insulation due to the details that may be required to achieve continuity of insulation.  Unless carefully considered, cold areas can be left where there can be gaps in the insulation.  This can happen where internal solid walls (or floors) meet the external wall, if there are fireplaces in the external walls, and at windows the insulation needs to return into the opening to create a continuous layer of insulation up to the window frame.
  • Installing IWI is very intrusive as it will require removing and refixing anything fixed to inside face of your external walls including skirtings and other trims, electric sockets and wall lights, and if the room has internal features such as ceiling coving or cornices these will probably need to be removed. 
  • Adding the thickness of the wall insulation internally means that some area will be lost from your rooms, and this can be quite substantial when better U-values are required.
  • It is easier to install well in empty houses or as part of major retrofit as it may require removal of areas of ceiling and floor to achieve the critical continuity of installation.
  • It may be easier to achieve a good internal wall insulation (IWI) result on a home attached to another, as you can isolate your house from your uninsulated neighbour. 
Plan of a typical attached house with Internal Wall Insulation

There are two main types of IWI installation using either a vapour permeable or vapour sealed approach.  Each of these requires different materials and a different installation methodology.

Vapour Impermeable: In order to keep the internal insulation thickness to a minimum most insulated plasterboards use a foam insulation that is completely vapour impermeable, but if any water becomes trapped within the wall construction it can lead to damp and even mould growth. 

  • Using these impermeable insulants it is necessary to form an impermeable vapour control layer across the whole internal wall surfaces to prevent water vapour from the relatively humid interior getting through holes or cracks in the lining and ending up condensing within the construction. 
  • It can be difficult to achieve this continuous barrier in an existing building as where floors or internal walls meet the external wall surface that is being insulated.  A lot of wrapping and taping can be required to seal around floor joist ends etc.
How a traditional wall deals with moisture

Vapour permeable:  Some insulation such as woodfibre boards mineral wool or lime-calcium-silicate boards are vapour permeable which means that they have the capacity to diffuse any damp trapped within the construction into the inside of the building.  Current practice favours using vapour permeable construction in existing buildings where possible as it is working with the vapour permeable characteristics of the existing construction and while it still needs to be cut tightly around floor and ceiling joists it requires less complex sealing.  It does still have a number of issues that need to be considered:

  • Woodfibre is a much less efficient insulator than foam so you will either need increased thickness to achieve the same U-value or have to settle for less insulating. 
  • It can also be more difficult to achieve an adequately insulated detail to window opening reveals as it can be difficult to get enough thickness of woodfibre into the reveal without overlapping the entire window frame width. 
  • Woodfibre insulation is usually installed tight to the existing wall surface to avoid leaving unventilated voids so the wall surface may need re-finishing to achieve a flat and level background. 

A typical construction would have the insulation boards adhered to a plaster finish on the wall, then another layer of vapour permeable plaster on the boards as the internal finish. 

  • This sort of construction will also allow any damp in the external wall to come through it and so it relies on the inner face of the external wall to be completely dry, even if it is exposed to driving rain.  There are some vapour permeable treatments such as brick creams (not impermeable silicon waterproofing treatments!) that can be applied to the outside face of the wall to reduce the water in getting into the masonry. 
  • It also needs a well-ventilated internal environment to remove the moisture from inside the house so It’s good in a house with whole building mechanical ventilation. 

To achieve a Building Regulations compliant internal wall insulation lining achieving a U-value of 0.3 Wm2K, on a typical solid brick or stone wall it would require quite a substantial loss of internal floor area:

  • Impermeable construction: Adding a damp proof membrane to the inside face of the existing wall then 25mm timber battens and a foam backed plasterboard of 95mm totalling around 120mm overall thickness.
  • Permeable construction:  Using woodfibre boards instead of foam boards, it would need around 140-160mm total to achieve 0.3W/m2k,

Which wall insulation is right for me?

  • If you have a cavity and your walls aren’t too exposed cavity wall insulation should be the simplest and easiest to achieve a significant improvement, but won’t achieve very high levels of insulation unless it’s combined with additional IWI or EWI
  • If you don’t mind completely changing the look of your home (and if you are allowed to under Planning rules) EWI is often the most efficient, economic and less disruptive solution for achieving a highly insulated home.
  • Where EWI cannot be used IWI can achieve good results if you can accept the internal disruption. but requires careful design and installation.  A vapour permeable solution would be preferred if practical.

Sometimes neither option is entirely right and it’s necessary to use a combination of strategies.  If it is undesirable to change the appearance of the house internal IWI might be required for the front of the house but the simpler and more efficient EWI applied to the exterior of the walls around the back

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