External walls: internally applied insulation

Description
Insulation layer between light-gauge steel channels or timber studs on the inside of walls, most commonly covered by foil-backed plasterboard (or plasterboard with separate vapour control layer) fixed to the channels or studs. Alternatively, composite boards of insulation and plasterboard are adhered and/or mechanically fixed to substrate.

Unit cost of installation
£2200 (to U value 0.35); Cost will depend on type of system used. Composite boards vary in price - for a given U-value, polystyrene-backed boards are cheapest and thickest while phenolic foam-backed boards are the most expensive and thinnest.

Bulk cost of installation
Data not available.

Indicative cost and carbon saving

Maintenance
None normally required.

Lifetime
Lifetime of the building

Technical suitability
Can be used in almost any situation and does not require specialist or registered installers. A continuous perimeter seal should be provided behind plasterboard dry linings to external walls. Systems are available that have been certified by BBA and BRE Certification.

Because insulation is applied internally some of the thermal mass of the structure is isolated from the internal atmosphere. This will tend to increase the responsiveness of the heating system, but also increase fluctuations in indoor temperatures over a complete daily heating cycle.

Detailing around external door and window frames can be difficult and if not dealt with effectively can lead to a lowering of surface temperature adjacent to the window or door frame and a consequent increased risk of condensation in those areas (although generally internal insulation greatly reduces the risk of surface condensation). A similar situation can occur where internal separating walls and floors meet external walls. To avoid the problem the insulation should be returned for a short distance along the separating wall, and on the underside of solid intermediate floors. The upper faces of solid floors are more difficult to treat, and may benefit from the addition of composites to provide a floating floor (see floor insulation). Suspended floors can be insulated near the wall with fibrous insulation, but that would require floor boards to be lifted and it would be difficult to incorporate a vapour barrier in this situation.

As with external insulation care needs to be exercised to ensure that air bricks are not blocked, and internal fixtures and fittings (radiators, electrical sockets, skirting boards etc.) should be removed prior to insulating the wall and subsequently re-fixed. Insulation should extend behind the skirting boards to within 5mm of floor to avoid cold bridges behind the skirting. Any penetrating or rising dampness should be dealt with before insulating the walls.

One advantage over externally applied insulation is that it is easy to treat individual rooms.

The thickness of internal insulation can be a problem in smaller rooms. In these situations flexible insulated linings could be considered.

Non-technical constraints
Installation can be disruptive to occupants. Rooms slightly reduced in size - can be critical in small rooms. Work should be carried out before wet central heating systems are installed.

Ancillary benefits
Can increase responsiveness of central heating systems by effectively isolating thermal mass. Leaves external appearance of dwelling unchanged.

Trigger points for work
Major internal refurbishment e.g. treatment for rising damp or replacement of internal plaster. (N.B. Building Regulations Approval is required in England & Wales whenever any thermal element is renovated, and a U-value of 0.35W/m²K would normally be required. However a lesser standard may be acceptable if it is not technically or functionally possible to achieve this U-value).

Performance Limitations

Technical Guidance Available
» CE17/GPG 138 'Internal wall insulation in existing housing - a guide for specifiers and contractors'
» BR262 'Thermal insulation: avoiding risks' BRE, 2002
» CE184 - Practical refurbishment of solid-walled houses
» BBA and BRE Certification certificates
» National Insulation Association (NIA) 01428 654011 - http://www.nationalinsulationassociation.org.uk/