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Internal Wall Insulation

The Importance of Wall Insulation in Houses

Buildings account for 40% of our energy consumption and in houses the majority of this energy is lost through the walls. 

Therefore insulation is one of the most cost effective improvements that can be made and there are various insulation options depending on how the home is constructed. In general, the walls in the home can be categorised into two distinct types. Modern style homes, usually built post-war (1945), are constructed with cavity walls. Older style homes are constructed with solid brick walls.

In a home with cavity walls, around a third of the heat which is lost escapes through the walls. Occupants can make savings around £135 per year* and see a return on investment within a couple of years by insulating their cavities.

However, solid walls can lose even more heat than cavity walls. Typically the total heat loss from an un-insulated house with solid external walls account for 45%. Insulating solid walls can be more expensive than cavity wall insulation. However, higher savings can be achieved through internal or external insulation for solid walls saving the occupant up to £445 a year* and internal wall insulation offers a quicker payback than external wall insulation.



Why Insulate a Solid Wall Property

Over a quarter of the UK’s CO2 emissions are generated by our homes. Of the estimated 25 million homes in the UK approximately 36% have been labelled as “hard to treat”, the majority of which are solid walled properties. Insulating all the solid wall properties in the UK would provide significant reductions in the amount of CO2 we emit and would also help to reduce an occupant’s fuel bills.

Save money on energy bills

Homes losing heat, lose money. A poorly insulated home will be costing the occupant significantly more in heating bills than a fully insulated property. Insulation slows the transfer of heat, reducing the amount of energy consumption in the home - keeping it warm in winter.

Eradicate fuel poverty

50 per cent of solid wall properties are occupied by people living in fuel poverty. Fuel poverty is linked to multiple deprivation and unaffordable fuel prices characterised by inadequate insulation and inefficient heating systems. Fuel poverty can be seriously damaging to people’s quality of life and can be particularly uncomfortable for the older generation, children and the disabled. Britain is said to have the highest number of avoidable deaths due to winter cold in Western Europe.

Help reduce CO2 emissions

By insulating the solid walls in these properties the environmental impact of providing space heating is significantly reduced. It is estimated that a properly insulated solid wall home could save approximatly 2 tonnes of CO2 per year when compared to a poorly insulated one.

Increase a home’s value

Insulating a solid wall home properly can add to a home’s market value by significantly improving its Energy Efficiency Rating. In the case of external wall insulation it can improve the overall aesthetics and internal wall insulation can improve the interior décor and appearance of a property.

Solid Wall Insulation Options

External wall insulation or internal wall insulation?

Solid walls can be insulated externally or internally. Each solution has its merits.

External Wall Insulation (EWI)

External wall insulation systems generally comprise an insulation layer mechanically or adhesively fixed (or both) to the existing wall and covered with a render coat. Timber boarding, concrete and clay tiles or metal cladding can also be applied.

An external insulation system can radically improve the appearance of a property and planning permission as well as Building Regulations compliance may well be required prior to installation commencing. Particular attention will need to be paid, for instance, to window sills, rainwater downpipes and gutters, and eaves.

Relocation or changes to the roofline may be required to accommodate the thickness of the system. External wall
insulation systems (such as the SWIP EWI System) are installed by specialist installers.

Internal Wall Insulation (IWI)

Internal wall insulation solutions usually involve the installation of metal or timber studs with insulation installed between the studs and then overlaid with a vapour control layer and plasterboard. Alternatively, a thermal laminate board or rigid insulation board plus plasterboard can be mechanically fixed to the walls. These systems should not be used to isolate or hide moisture penetration or damp problems in the existing structure.

In accordance with Building Regulations, solid walls should prevent moisture ingress arising from exposure to rain and snow without moisture penetrating to the inside and damaging the building. Insulating internally improves the thermal performance of the wall without affecting the external appearance of the building. However, there will be a small reduction in the internal floor area, typically only 1-2%.

The SWIP IWI System can generally be installed with minimal disruption to occupants. In a large number of cases, internal wall insulation could be the preferred option because it costs less to install and maintain than external
insulation systems, does not require scaffolding during the installation process, the existing appearance of the building is maintained and it provides flexibility during the refurbishment program.

Combination Installations

In many instances (especially mid terraced houses), the optimum solution may be to install a combination of both
external and internal wall insulation. An EWI system may suit the back of a house where appearance is less important or already compromised by single storey extensions, outriggers, soil stacks, rainwater downpipes and boiler flues
etc. The front of the house can be insulated with the SWIP IWI System, which maintains aesthetic quality of the
existing facade, while also delivering high levels of thermal performance and a flexible installation solution.


Advantages of internal over external wall insulation

Solid external masonry walls can be upgraded in two ways, either internally or externally. There are several reasons why internal wall insulation may preferred to external wall insulation for upgrading solid masonry walls:

  • It costs less to install than external insulation
  • It is easier to maintain than external insulation
  • No scaffolding is required
  • The external appearance of the building is maintained so it can be installed in conservation areas
  • Materials are readily available
  • It can be installed on a room-by-room, single façade or whole house basis, as part of a full refurbishment plan
  • Installation is not delayed by bad weather

Thermal Comfort

An internal wall insulation system enables comfortable room temperatures to be achieved more quickly than with an external wall insulation system. Heating time periods can be reduced, which, in turn reduces heating costs particularly in intermittently heated buildings such as dwellings.

Taking the Opportunity

An ideal opportunity to install internal insulation on a solid wall is when other work is already required, e.g. when existing plaster is crumbling and needs replacement, when the decorative finish is being removed, or when rewiring or installing central heating, or during total refurbishment. The existing wall should be examined and any remedial work, e.g. the insertion of a damp proof course or the repair of overflowing guttering, carried out before the insulation system
is installed. It is important that the internal insulation system is not used to hide or isolate damp or wet walls.

Taking advantage of any opportunity to improve the energy efficiency of a dwelling and upgrading un-insulated external walls provides a number of benefits including:

  • Reducing CO2 emissions
  • Reduced fuel bills
  • Increased thermal comfort for the occupiers
  • Reduced risk of condensation and mould growth
  • Reduction of fabric damage and maintenance costs.


SWIP Internal Wall Insulation System

SWIP IWI system components SWIP Internal Wall Insulation provides a system based approach to internal wall insulation. The system has been tested and certified to demonstrate that all components work together as designed to deliver the required performance. The SWIP IWI System is designed for upgrading existing solid (or cavity) external walls. It consists of thermally engineered composite studs and insulation slabs, which can be combined to provide greater thicknesses of high performance thermal insulation than achievable by using a single thickness.

SWIP Studs are a composite of high performance extruded polystyrene insulation and Oriented Strand Board (OSB). Traditional internal wall insulation which uses traditional timber or metal studs leads to thermal bridging through the studs. This needs to be compensated for by increasing the thickness of the overall system. The innovative use of the thermally insulated SWIP Studs within the SWIP system prevents this and results in an overall thinner system. The SWIP IWI System has been designed to simplify the process of upgrading existing solid (and cavity) masonry walls whilst delivering high levels of thermal performance. Typical U-values.

U-values (W/m2K)

SWIP Stud thickness (mm) SWIP Stud with SWIP Batt SWIP Stud with SWIP Batt & 35mm SWIP PIR Laminate SWIP Stud with SWIP Batt & 50mm SWIP PIR Laminate
65 0.43 0.29 0.25
95 0.30 0.22 0.19
2 x 65 0.24 0.19 0.17


System Components

  • SWIP Stud – Extruded polystyrene bonded to Oriented Strand Board (OSB)
  • SWIP Batt - Water repellent glass mineral wool slab
  • Plasterboard fixing – Drywall screws
  • Sealant – SWIP multi purpose sealant
  • Plasterboard
  • Fixing to masonry - SWIP IWI fixings manufactured in accordance with BS1210
  • Wall plugs – SWIP IWI wall plugs
  • SWIP Vapour Control Layer – vapour resistance of 260MN.s/g
  • SWIP Reveal Boards – Extruded polystyrene bonded to plasterboard. Extruded polystyrene with a cement screen either side



Thickness (mm) Width (mm) Length (mm) Thermal conductivity (W/mK) Thermal resistance
65 50 2400 n/a 1.63
95 50 2400 n/a 2.54


Earthwood SWIP Batt

Thickness (mm) Width (mm) Length (mm) Thermal conductivity (W/mK) Thermal resistance
65 555 1200 0.035 1.85
95 555 1200 0.032 2.95