Loft insulation

25% of heat loss is through the roof, however, before starting to lay insulation, make sure your slates, roof tiles, flashings, and chimneyheads aren’t leaking before you start as wet insulation is ineffective.

When installing loft insulation you need to get a balance between an adequate depth of insulation and good ventilation to help prevent damp. Check out your loft space before ordering materials.

Slate roof construction


Your loft needs to have the right amount of ventilation to allow the timbers to stay dry. Too little ventilation allows warm moist air to condense on the underside of the sarking which leads to mould growth. If you find mould growth on the underside of the sarking then you should check where you can add more ventilation, including introducing additional vents into the roof space. Too much ventilation, however, can allow more cold moist air into the loft, also increasing the likelihood of mould growth.

Air flow needs to get into the loft either through the slate or tiles or through a gap under the eaves. To check if your roof is ventilated through the slates or tiles, lift up the skylight window or roof hatch and check what material has been placed between the wood sarking and the slates or tiles.

A dark grey material like roofing felt is Type 1F bitumen impregnated felt. It does not allow ventilation through the roof covering (slate vents should be installed). If your roof has this type of material under the slates/tiles, then you should leave a ventilation space at the eaves and keep any insulation from blocking the flow of air into the loft.

Vapour open membrane (vapour permeable felt) is more flexible and usually white or green. These membranes act in the same way as goretex fabric – they keep out moisture whilst allowing air and vapour to pass through. Ventilation will naturally occur through the membrane and sarking. This should allow you to take the insulation tight into the eaves, right up to the wood sarking.

If you are having your roof re-slated or retiled, ask for a vapour open membrane (rather than Type 1F bitumen impregnated felt). Slate vents can also be installed.

Depth of insulation

The first few millimetres of insulation provide the most insulation and effectiveness tapers off as depth increases. The current advice is to lay 270mm depth of insulation but you may need to taper this down towards the eaves or, counter intuitively, reduce that depth to allow adequate ventilation.

Installing insulation

When installing insulation key points to note are:

  • avoid gaps in insulation as this leads to thermal bridging – colder areas where condensation will occur
  • depending on the depth of your ceiling joists, the first roll of insulation can be laid between the joists and the next roll, at right angles and across the joists
  • insulation should go below electrical wiring to prevent overheating and above pipes, so that they are kept warm and don’t freeze
  • if you use the loft for storage and have boards over the rafters, you can raise the rafter height with timber fillets
  • line the loft hatch to avoid thermal bridging
Types of insulation material

Glass wool or mineral wool insulation is good for loft spaces as it is economical and quite easy to lay. There are other types of quilted materials which are more eco-friendly, such as wool, hemp, and recycled plastic, which will do the same thing but may not be as cheap.

Blown cellulose can also be used. This is made from recycled newspaper, fire-proofed with boric acid. The advantage of using blown cellulose is that it fills all the voids between the joists. The slight disadvantage is that it can be blown around if there is air movement in the loft.

Insulating rooms in the roof

Insulating attic rooms in the roof with combed or sloping ceilings is a little trickier than insulating a loft, mainly as access holes need to be cut into the internal plaster work or existing ceiling plasterwork/plasterboard has to be removed first before the insulation can be installed.

No loft access?

If you can’t access the loft, it is possible to add insulation to the underside of the ceiling. This is hard to achieve successfully, so we don’t recommend this method and suggest you take professional advice before considering this.

Common or individual responsibility?

Adding insulation is treated as maintenance and is therefore generally subject to a majority decision of owners. If you are the top flat owner and you cannot get a majority decision then you may need to go it alone. It is unlikely that other owners will refuse consent for you to do this.

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