Low Energy House - Top-Up Loft Insulation - Loft Insulation

The performance of an insulating material relies on the amount of air trapped within it. If that air is kept dry and still then less heat can get into or past the material and the higher its insulating capacity will be

Existing Loft Insulation

If your loft space is already insulated there will probably be 50mm or 100mm thickness fitted between the joists. To comply with current standards more insulation will be required.

In normal circumstances, it is advisable to top-up with insulation of the same type as that already fitted. If you want to use two different types of insulation you should consult the manufacturers of both types of insulation, as condensation can sometimes occur at the junction between two materials.

Loft Insulation and the Cold Roof

Ventilated cold roofs usually have ventilators fitted in the eaves and in the rafter feet to allow the outside air to flow naturally across the loft space. The free passage of air helps remove condensation, prevents moisture entering the insulation material and stops mould growth and rot affecting the timbers.

Loft Insulation and Roof Ventilation

Where an existing cold pitched roof is already ventilated, care must be taken not to block the vents with the new or top-up insulation. Where an existing cold pitched roof is not ventilated, a system of eaves vents should be fitted to provide an air flow across the loft space.

Loft Insulation and a Sealed Roof - No Ventilation

If an existing cold pitched roof is being fitted with a new slate or tiled roof covering then it can be changed from a ventilated roof to a sealed roof by the inclusion of a sealed breather membrane at rafter level. In such cases, the loft floor can usually be insulated and no ventilation is required.

Top-Up Loft Insulation

If the existing insulation is in good condition it can be left in place and another layer added to it to bring the thickness up to the level of the top of the joists. A further layer of insulation should then be laid across the joists at right angles to reduce heat loss through the joists themselves. The top layer of insulation should have a minimum thickness of 200mm (8 inches).

Building Regulation Targets for Loft Insulation

Whether you are installing new loft insulation or topping-up old insulation you should aim to achieve a minimum U-value of 0.16 W/m²K. typical specification required to achieve this with all new insulation would be a total 250mm (10 inches) thickness of a mineral fibre. In the case of top-up it could be as much as 300mm (12 inches) total thickness.

Typical Loft Insulation

Mineral fibre blanket or loose-fill are the most commonly used insulation materials in loft floors. Glass wool and rock wool are both mineral fibres.

Blanket insulation is supplied in approximately 400mm and 600mm (16 inches and 24 inches) wide rolls and a variety of thicknesses. Loose fibre fill is supplied in bags for hand filling or can be blown into position, by a registered installer, using specialist equipment. (Guarantees for this work are available from the registered insulation installer). Where loose fibre is used, whether it is top-up or not, it will form one continuous layer of insulation.

When you have chosen an appropriate system it is important that the edges of the loft insulation are lapped with the wall insulation to ensure continuity of the insulated building envelope. Make sure the insulation does not block the air vents at the eaves