Roof and rafter insulation for lofts and rooms in the roof

Ceiling level or rafter level?

There are two main places to insulate a pitched roof:

• Ceiling level (a cold loft). Insulation is laid between and over the ceiling joists. The rooms below stay warm, and the loft above is left cold and ventilated. This is the most common approach and works well if you do not use the loft much.
• Rafter level (a warmer roof space). Insulation follows the slope of the roof, fixed between and under the rafters. The loft itself sits inside the insulated part of the house, which makes it more useful for storage or as a room.

Rafter insulation usually means a larger area to cover and more detail to get right, so it is not automatically better. It is a different solution for a different use of the space.

Warm roofs, cold roofs and moisture

You may hear the terms warm roof and cold roof. Broadly, a cold roof has its insulation at ceiling level with a ventilated space above. A warm roof has its insulation at or above the rafters so the structure stays on the warm side. Fitting insulation from inside, under and between the rafters, sits somewhere in between and needs careful thought about moisture.

Warm, moist air from the house carries water vapour. If that vapour reaches a cold surface, it can condense. To limit that risk, rafter insulation usually needs a vapour control layer on the warm side, sealed joints, and the right ventilation or air path on the cold side, depending on the roof. Getting this wrong can lead to condensation and damp timbers, so we design the build-up before we start.

Why the roof underlay matters

Under the tiles or slates most roofs have an underlay. In older homes this is often bitumen felt, which does not let water vapour through, so the space beneath it normally needs to be ventilated. Many newer roofs use a breathable membrane, which allows some vapour to pass and can change the ventilation requirements. Some older roofs have no underlay at all.

We check which you have, because it affects how much air space we leave, whether ventilation needs to be kept or added, and which products are suitable. We follow the manufacturer's guidance for each material rather than assuming one method suits every roof.

Products and our process

Depending on the roof and the depth of the rafters, we may use rigid PIR boards, mineral wool, SuperQuilt multifoil with battens creating air spaces, or a combination. YBS states that SuperQuilt is designed to be used under rafters alongside other insulation such as PIR or mineral wool, with air gaps either side.

1. We survey the roof from inside, checking the underlay, rafter depth, ventilation and any signs of damp or damage.
2. We explain the build-up we suggest and give you a written quote.
3. We fit insulation, battens and vapour control layers neatly, sealing joints and keeping any ventilation paths open.
4. We tidy up and explain what has been done, including any areas we have left for a specialist.

Rafter insulation pairs well with boarding for storage and a good loft ladder if you plan to use the space regularly.

When a roofer or specialist is needed

We work from inside the roof space. If the survey shows slipped or missing tiles, a failing underlay, daylight where there should not be any, water staining, soft or damaged timbers, or signs of persistent damp, a qualified roofer or the right specialist should assess the roof first. We do not diagnose roof rot or structural problems, and insulating over them can make things worse.

Converting a loft into a habitable room is a bigger project that can involve Building Regulations, structural work, fire safety and planning considerations. If that is what you have in mind, you will need the right professionals involved, and we can talk about where insulation fits in.

Talk to us about your roof

If you are weighing up ceiling-level against rafter-level insulation, we can look at your roof and explain the options without any pressure. Call 07835 254 241 or email info@energyloftinsulation.co.uk to arrange a free survey.