Issue 5 - Reducing heat loss through Roofs (Part 2)

Issue Date : 2nd June 2009

Welcome to the Newry Democrat’s renewables and energy saving section brought to you by Kellys Renewable Energy Solutions (www.kellys.uk.com).  This week we shall look at ways of reducing heat loss through your roof. Up to 25% of the total heat loss of a house is through the roof. This week we will focus on a pitched roof with a room in the attic. This type of roof is called a warm roof.

Pitched Roofs with a Room in the Attic

The insulation of houses with attic rooms is quite complex because there is limited space available above the living accommodation, so the insulation must follow the line of the rafters for at least part of their length. When the insulation is placed under the rafters, the headroom is reduced which may be an important consideration.
Another important consideration is when placing insulation between the rafters it is important to leave a gap above the insulation to preserve the cross-flow ventilation in the roof.

We shall look at a typical example of insulating a standard room in the attic with 100mm rafters. At Kellys we use 50mm Polyisocyanurate insulation (rigid foil back insulation) between the rafters while still maintaining the 50mm air gap for ventilation above the insulation. The ceiling could be underlined with another layer of insulation (the more the better) which eliminates the movement of heat through the rafters. An average thickness of internal insulation would be 50mm. Finally a vapour control layer and plasterboard are normally added. The roof should finish with a U-value of 0.18 (W/m²K). A quick note to remember is the lower the U-value the less heat loss through a specific area.

When insulating to the underside of the rafters, always follow the manufacturers’ instructions and technical notes to determine if your roof may be at risk from condensation.