Pitched roofs can be insulated between, below or above the rafters. It can be advantageous to combine more than one of these approaches – for example, by insulating between and below rafters. Permeable insulation often works well with old buildings, though non-breathable forms should not be discounted where adequate ventilation and detailing can be achieved. The use of sprayed foam or loose-fill insulation is inadvisable. Non-combustible insulation will be required for any fire separation.

Avoid insulating above recessed light fittings (unless they have a heat shield) and route any electrical cables outside the insulation to reduce the risk of overheating and fire

Positioning insulation between rafters where access is available may avoid having to increase the height of a roof by insulating above or, alternatively, reducing headroom or disturbing a historic ceiling internally by thermally upgrading below. The disadvantage, however, is that insulation can not always be inserted to the required thickness because of insufficient rafter depth, though this can sometimes be overcome by screwing timber battens below. There may also be a risk of ‘thermal bridging’ (cold spots) along the uninsulated rafters themselves but other approaches are sometimes limited. Soft, pliable insulation, including hemp, can be appropriate

Minimise possible condensation problems by:

• Providing sufficient ventilation above or below a breather or impervious underlay respectively. Special attention is needed where the roofline is interrupted by rooflights etc.
• Ensuring that insulation is fitted tightly.
• Lagging pipes and ducts where condensation could cause damage.

Bear in mind that underlay is not necessarily needed. Additionally, the batten cavity may not require a ventilation path when using permeable, air-open roof coverings (such as clay tiles or rough natural slates) so long as there is no ‘torching’ (mortar to the underside) or a likelihood of airflow through joints becoming restricted by moss or lichens.