Everything you need to know about loft insulation

Loft insulation is one of the most effective ways of achieving a warmer home, which is cheaper to run and more energy efficient.

The recommended depth of mineral wool fibre is 270mm or 12”. The insulation is brought into the loft in rolls and first laid between the joists and then across the joists to achieve the required depth, leaving access to services (water tanks, etc.).

The Energy Saving Trust says that insulating the loft space to a depth of 270mm will save an average terraced household with no existing insulation in place £145 per year and an average detached house £315 per year on energy bills.

Typical costs for getting loft insulation installed in a three bedroom semi-detached house by a professional contractor are approximately £400. Mineral wool insulation can also be bought from DIY stores or builders merchants and installed by a householder with reasonable DIY skills. Note that this form of insulation isn’t suitable for lofts with small loft hatches or limited space, where insulation blown into the space using special equipment by a professional installer would be more appropriate.

Loft insulation shouldn’t be compressed as this reduces its ability to insulate. If you need to create a boarded area for storage, you can either raise the height of the joists to fully accommodate the insulation, or use thinner insulating material.

All lofts that are insulated at the loft floor level require adequate ventilation to prevent condensation forming, especially in the winter months, as this can saturate the insulation and potentially cause damage to roof timbers and ceilings. This is achieved by vents installed either at the soffits (the horizontal boarding along the edge of the roof, behind the guttering), ridge tile vents, vent tiles and gable end wall vents. Sometimes a combination of different types of vent is needed.

It is also essential that all pipework and water storage tanks are insulated, as the loft area will be much colder once the recommended levels of insulation are installed on the loft floor, as heat will be prevented from rising into the loft area.

An alternative approach is to spray insulating foam directly onto the underside of the roof and between the rafters. This isn’t recommended because the roof isn’t a heated space, so this method offers little insulation benefit and can trap moisture in the rafters which can cause them to rot. The presence of spray foam insulation can affect the ability to get a mortgage on a property and once it’s applied it can be a very difficult and costly job to remove.

Loft floor insulation should not be laid in lofts that are boarded over or floored. If there are any doubts regarding the strength of the ceilings or joists directly below the loft areas, or if any possibility of hazardous substances could be present, seek advice from a qualified installer.