Internal and external solid wall insulation
Solid walls can be insulated either internally or externally. Internal wall insulation usually involves fitting insulation boards or building a stud wall filled with insulation. External insulation is added by fitting insulation to the outside of the wall and covering it with render (shown below) or brick slips which replicate the appearance of traditional bricks.
The method and suitability of solid wall insulation is dependent on a number of factors including: character of local area, age and construction.
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Age and construction
Solid wall houses are generally either traditional buildings constructed of permeable material or non-traditional buildings which have impermeable layers and frequently include a damp-proof course. Traditional buildings are constructed of porous materials which allow the transfer of water vapour through the walls letting the building ‘breathe’. Internal and external insulation can both affect the permeability of the walls and the point at which condensation will form. For this reason, the materials and structure of the building must be fully understood before insulation is fitted. Insulation systems can be permeable or contain a vapour control barrier to control the passage of moisture through the walls.
Character of the local area
If the house is semi-detached or within a terrace, the visual impact of external solid wall insulation will be obvious. It is therefore beneficial for neighbours to coordinate and install insulation on adjacent houses at the same time. This has the added benefit of reducing cost.
Solid wall insulation in listed buildings will rarely, if ever, be appropriate. Similarly, changes to the character and appearance from external wall insulation are unlikely to be acceptable if the house is in a Conservation Area, especially if the changes would be easily visible (ie:. on a front or side elevation). This issue should be checked with the local planning authority.
Benefits of solid wall insulation
There are many benefits to solid wall insulation which vary depending on the characteristics of the house. The main benefits are:
- Reduced heating bills. The Energy Saving Trust estimates on bills of £460 for a detached house, £270 for a semi and £180 for a mid-terrace.
- Houses will retain more heat and be more comfortable to live in.
- The insulation acts as a noise barrier.
- The improvements can increase the value of the home, particularly where the insulation improves the external appearance of the property.
Risks with solid wall insulation
The main risks of solid wall insulation occur when insulation is either not fitted correctly or has been poorly designed for the house. The insulation material used must be appropriate for the house type. For example, solid wall houses with permeable walls must either be insulated using porous materials to allow the walls to breathe or have a vapour barrier which is sufficient to prevent damp occurring within the walls causing decay of building fabric. Care must be taken during installation and after to ensure that the vapour barrier cannot be breached in any way.
When installing internal solid wall insulation, there is a risk of the formation of thermal bridges with cold spots. Thermal bridges occur at junctions where the insulation is not continuous. These will occur where the insulation layer is interrupted by another material or where the insulation is thinner at a certain points, for example:
- Between floors.
- At a wall-floor junction.
- At the junction of an internal wall with an external one.
This can cause damp to form in these cold areas. One way to avoid this from happening is to continue the insulation onto the room on internal walls. This is often referred to as ‘a return’.
The formation of thermal bridges is more likely to occur with internal solid wall insulation than external solid wall insulation as external wall insulation is more continuous around the exterior of the house than it can be inside.
Solid wall insulation will change the appearance of a property: the windows will appear deeper and the roof overhang will be reduced. In addition to this, external details such as downpipes, window sills and boiler flues will need to be extended or replaced. Decorative details on the wall will be covered over and, although it may be possible to replace some details, unique architectural features will be difficult and expensive to replicate. For these types of property, internal wall insulation is likely to be a better option.
Existing damp problems such as rising or penetrating damp must be sorted out before installing solid wall insulation.
Other factors with solid wall insulation
|Considerations||External wall insulation||Internal wall insulation|
|External appearance||Can improve the rain screen protection and appearance of a property and improve its value.||
No change in external appearance, this might be a benefit if the owner wishes to retain attractive external features. Potential to improve property value.
|Greater thermal comfort||
Effect of building as a thermal store is improved by reducing swings in temperature, for instance from solar gain. Warmer in winter, cooler in summer.
Increases the responsiveness of the heating system. Warmer in winter, cooler in summer.
Time taken to install
Four to five weeks. Dry weather is necessary for installation.
Four to five working days, depending on size of property and number of rooms being insulated.
|Disruption to household||Scaffolding will be required to access upper storeys, but otherwise minimal disruption to occupants.||
Would need to move furniture and fittings etc and might need to cut off services temporarily to some locations. Easiest if the dwelling is empty at the time of works. Redecoration required.
|Space||No loss of internal space but a reduction in the width of narrow external passageways could be an issue.||Loss of internal space – between 37mm and 110mm on external walls to reach a required u-value of 0.30.|
|Costs||Approximately £6,500 - £13,000.||Approximately £5,500 - £8,500.|
|Continuity of insulation||
Generally good continuity across plain wall elements although there might be some disruptions from adjoining garden walls, garages, lean-to, etc.
Will inevitably be breaks in the insulation continuity at internal partition walls, intermediate floors, etc.
|Planning||Planning consent is unlikely to be required unless the property is in a Conservation Area or there will be a change in the external appearance of the building e.g. from brick to render.||Planning consent is only likely to be required for listed buildings.|
|Building control||Requires consent under Part L of the Building Regulations.||Requires consent under Part L of the Building Regulations.|
Further information on the requirements for Planning and Building Regulation Consent can be found in our Frequently Asked Questions.
Further information on insulating your home.
Energy Saving Trust