Modern houses are built with loft insulation, but older properties may be losing a quarter of their heat through the roof. Installing or topping-up insulation in a standard loft is an easy and cost-effective way of making a home more energy-efficient.
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For an accessible loft, the most straightforward method is to lay loft rolls of mineral, glass or sheep’s wool between and over the ceiling joists to a depth of 270mm. A layer is placed between the joists then another is laid at right angles to cover them. For topping up, the appropriate depth of loft roll is simply laid on top of the existing material. The loft door can be insulated, but it is better to replace it with an insulated trap door.
The second method, blown installation of fire-retardant mineral wool or cellulose, is for roof spaces with difficult access. The process usually takes no more than a couple of hours.
Finally, for flat roofs, expanded polystyrene insulation boards can be attached on or beneath the roof. These can be a bit more expensive and complicated to install, and can lead to condensation problems, so installation is best left to a professional.
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If the loft is used for storage and there is insufficient joist depth to accommodate the insulation, there are two solutions. The first is to infill between the joists with loft roll and then lay insulation boards and chipboard loft boards across the top. Alternatively, the level of the loft floor can be raised to accommodate two layers of loft roll, by fitting battens across the joists and nailing chipboard loft boards on top.
For either method, a gap should be left between the loft insulation and the boards to prevent condensation and compression of the loft roll, which will reduce its efficiency.
The floor of an existing loft conversion can be insulated to keep the rest of the home warm, with loft roll added between pitched roof rafters and insulation boards to false ceilings, walls and dormers to keep the loft room itself cosy. A 50mm gap is needed between insulation boards and roofing felt for ventilation.
Other things to remember
For DIY, choose products with an ‘Energy Savings Trust Recommended’ logo, insulate pipes and tanks and do not install loft roll beneath the cold water tank. Remember to wear protective clothing and a mask.
If the space is used to store perishables like photographs or clothes, the roof should also be insulated to keep more heat in the loft.
An insulated loft needs to be adequately ventilated around the eaves to prevent damp. If there is an existing damp problem it is best to get professional advice.
Costs and savings
270mm of new loft insulation could yield annual savings of £225 for a detached house or £135 for a three bedroomed semi-detached. The cost would be £395 or £300 respectively. It may be possible to get grants through the Energy Company Obligation (ECO) towards the costs. Check with your energy supplier.