Low Energy House - Rafter Insulation - House Insulation

To create a heated room inside the pitched roof of a house, the roof must be insulated at the house rafter level

Heat Loss through the Rafters of a House Roof

Headroom in a roof space is often limited so it is usual to fit insulation between the rafters. Because heat can be lost by conduction through the roof timbers themselves, additional rafter insulation is required above or below the rafter line. The relative thicknesses of the two insulating layers will depend on the thermal value of the selected rafter insulating material.

A Typical Solution for House Rafter Insulation

A typical method of insulating the rafter line is to fit rigid insulation board between the rafters.  The depth of the rafters will determine how much can be fitted between them.  If it is necessary to fit thicker insulation than the rafters will accommodate then battens can be fixed to the underside of the rafters to increase the effective depth.

Building Regulations - Target Emission Rates

Current Building Regulations require new dwellings to be compliant with an overall energy and carbon performance with the Target Emissions Rate (TER) based on the thermal performance of the whole building envelope.  Thermal U-values for individual elements are not, therefore, set for new houses.

In order to achieve the overall Target Emission Rate, most houses will require the pitched roof line U - value to be 0.20 W/m²K. The standard for replacement thermal elements in an existing dwelling is also a U-value of 0.20 W/m²K.

Avoiding Condensation in House Rafter Insulation

Converting an unheated roof space to a heated living spaces increases internal moisture levels and increases the risk of condensation occurring within the newly insulated roof. How this condensation is dealt with will depend on the construction of the roof.

Existing Ventilated Pitched Roof

Ventilated Construction

Where a traditional bitumen based sarking felt or other impermeable underlay is used under the tile/slate finish, it is necessary to provide cross ventilation to enable the water vapour to dissipate to the atmosphere and prevent condensation forming within the roof construction. In order to achieve this, a 50mm deep ventilated airspace must be created between the sarking felt and the cold side (in winter) of the insulation with air vents provided in the roof finish.

Vapour Control Layer

On completion of the installation a vapour control layer should be installed on the warm side (in winter) of the insulation in order to prevent water vapour from the dwelling entering the insulated construction.

Unventilated Pitched Roof

Unventilated roofs create a warm pitched roof space which does not require cross ventilation. This method of roof design is more efficient as no ventilation is introduced and consequently no cold air to infiltrate the roof structure.

Vapour Permeable Breather Membrane

Where vapour permeable breather membranes are used as an underlay to the tile/slate finish, any water vapour that does pass through the rafter insulation layer can disperse through the breather membrane underlay to the outside air.

Ventilation Path

Most types of breathable membranes allow the cold side of the insulation to be fitted in direct contact with the cold side (in winter) side of the insulation (it is advisable to check with membrane supplier prior to specifying). However, the outer side of the membrane must be adjacent to an air space to allow moisture vapour to escape to the outside of the building. The air space is usually achieved by the introduction of 50mm thick counterbattens under the tile/slate battens.

Insulation Materials that are Commonly Used for Rafter Insulation

Extruded Polystyrene Rafter Insulation
Extruded polystyrene rigid board insulation between rafters and over or under the rafter line

Rigid Urethane Rafter Insulation
Rigid urethane insulation board, with aluminium foil facing both sides, fitted between rafters and over or under the rafter line

Rigid Phenolic Rafter Insulation
Rigid phenolic insulation, with aluminium foil facing both sides, fitted between rafters and over or under the rafter line

Glass Wool Rafter Insulation
Glass wool quilt fitted between the rafters

Rockwool Rafter Insulation
Semi rigid slab of rock wool fitted between rafters
Rock wool quilt fitted between the rafters

Sheeps Wool Rafter Insulation
Sheep's Wool fitted between rafters

Insulated Plasterboard Rafter Insulation
PUR insulated plasterboard sheet under rafter line
PIR insulated plasterboard sheet under rafter line
Extruded polystyrene insulated plasterboard sheet under rafter line

All insulating materials must be fitted in accordance with the manufacturer’s instructions

House Rafter Insulation - Best Practice

The thickness of the rafter insulation may be increased to improve the thermal comfort of the dwelling and to save more energy. In the context of an insulation project, additional thickness of insulation can be installed at relatively little extra cost.

When an appropriate system of insulation has been chosen, it is important that the edges of the rafter insulation are lapped with the wall insulation to ensure continuity of the insulated building envelope