Insulating roofs of a house extension

Roofs should be insulated to the Best Practice standards.

There are three common methods of insulating the roofs of domestic extensions:

• insulating at ceiling level (with an unheated loft above);

• insulating within the pitch of the roof (between the rafters);

• insulating a flat roof.

Where there is to be an unheated loft beneath a pitched roof, flexible insulation quilt may be placed immediately above the ceiling, between and over the ceiling joists, as shown in Figure 9. The insulation is supported by the ceiling lining (usually plasterboard or thermal board). In order to meet the Best Practice standard you must ensure the following.

• The insulation quilt should be in two layers, one between the ceiling joists, and the other across them, to prevent thermal bridging.

• The ceiling lining should be thermal board instead of ordinary plasterboard.

• The insulation material should not be compressed when it is tucked into tight corners.

• The insulation layer should be 270mm (approximately 10 inches) in thickness.

It is important to ventilate the roofspace, above the insulation, in order to reduce the risk of condensation. If the roofspace is to be used for storage, bearer boards should be placed across the existing joists, to prevent the insulation from being compressed.

Where insulation is to be placed within the pitch of the roof (between the rafters) the building regulations specify that a 50mm wide ventilation gap must be maintained above the insulation (and beneath the roofing felt and tiles), in order to reduce the risk of interstitial condensation.

Consequently, the thickness of any insulation placed between the rafters cannot exceed 50mm less than the depth of the rafters. Also, eaves ventilators and ridge or abutment ventilators must be installed, in order to admit ventilation air at eaves level and permit it to escape at the top of the roof.

The rafters are unlikely to be deep enough to contain sufficient insulation to meet the Best Practice standard (plus the 50mm ventilation gap).

Additional insulation can be provided in two ways:

• supplement the insulation between the rafters by using a thermal board, instead of ordinary plasterboard, for the internal ceiling lining, as shown in Figure 10; or

• adopt a form of construction called a ‘vapour balanced’ or ‘breathing’ roof.

Vapour balanced ‘breathing’ construction A vapour balanced roof construction is one through which moisture is allowed to permeate, removing the need for ventilation of the roof construction. The impervious roofing felt is replaced by ‘breather felt’, and the 50mm ventilation gap, the soffit and ridge ventilators and the polythene vapour barrier are all omitted. This simplifies the construction and leaves more space for the insulation.

Vapour balanced ‘breathing’ construction A vapour balanced roof construction is one through which moisture is allowed to permeate, removing the need for ventilation of the roof construction. The impervious roofing felt is replaced by ‘breather felt’, and the 50mm ventilation gap, the soffit and ridge ventilators and the polythene vapour barrier are all omitted. This simplifies the construction and leaves more space for the insulation.

If the roof is to be flat, a ‘warm roof’ construction should be used. This is the most common type of insulated flat roof construction, in which the insulation is placed above the timber structure and deck, with the waterproof external finish layer bonded directly to it, as shown in Figure

In order to meet the Best Practice standard it will be necessary to include a substantial thickness of rigid, high performance insulation.