internal insulation (commonly referred to as 'dry-lining walls') involves fixing insulation to the inner surfaces of your external walls and covering it with a vapour barrier layer and plasterboard or, alternatively, composite boards of plasterboard backed with insulation and incorporating a vapour barrier.

Types of insulation material used include expanded polystyrene, fibreglass and polyurethane boards. Polyurethane tends to be more expensive, but its thermal performance is approximately 50% better than polystyrene or fibreglass, so a 25 mm (1 inch) thickness of polyurethane will have roughly the same insulating performance as a 38 mm (1.5 inch) thickness of polystyrene.

The undesirable condensation that may occur on the walls is prevented by installing a vapour barrier (e.g. a polythene sheet) on the warm (room) side of the insulation. Composite boards for internal insulation generally incorporate a vapour barrier. It is important that the vapour barrier be well sealed at wall, floor, ceiling, door and window junctions, around light switches and at all other breaks in the insulation.

One of the main disadvantages of internal insulation is the loss of room space; this may be minimised by using high performance insulation products that are thinner. While this is often a more affordable option than installing external wall insulation, the loss of space and potential necessity to take out and re-fit fitted kitchens and appliances can result in people choosing the external insulation option.

When installing internal insulation, the junctions of internal walls and floors with external walls may ‘short-circuit’ the insulation, allowing heat to escape. The presence of such ‘thermal bridges’ will reduce the effectiveness of internal insulation which is not a case by external wall insulation systems.

Homeowners availing of any wall insulation grants under the Home Energy Savings scheme should aim to achieve a U-value of 0.27 W/m K or better (i.e. lower).

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