If you are planning to make use of the roof space in your home to create additional living space, you must make sure it is insulated to the standard required by the latest edition of the statutory Building Regulations. This is the case whether you are converting an existing roof, or creating habitable attic rooms in a new house.
It is less expensive to build a conventional cold roof, with the required insulation usually mineral wool placed in the ceiling joists above the top floor. However, a cold roof insulated using wool rolls is useful only for storage. To undertake a full conversion and create space, the insulation should be placed at rafter level to create a warm roof.
Insulation running in line with the rafters can be placed above, in between or below the rafters. Sometimes a combination of these is necessary to achieve the desired level of insulation. If insulation is placed exclusively between the rafters, they will have to be very deep, as 125mm of even the very best insulation, phenolic foam, will have to be used. There is also a danger of discolouration of the internal decoration caused by minor condensation forming at the position of the rafters, as they form a cold bridge.
Positioning the insulation over the rafters in a continuous layer solves the issue of cold bridging, but is only an option on new builds or renovations where the roof tiles are being stripped and relaid. Even then, the depth of insulation now required to meet the Regulations means that very large fascia boards and wide soffits are required.
Placing all of the insulation beneath the rafters solves the issue of cold bridging, but can present a problem in terms of restricted headroom. This problem can usually be designed out in a new build, but may be an issue when an existing roof is to be converted.
The most practical solution is to place most of the insulation between the rafters, with a smaller additional layer fixed to the underside of the rafters. Insulated plasterboard is useful for this purpose. Plasterboard is available with a wide variety of rigid insulation products bonded to it.
The most practical solution for loft conversions where the roof is not to be completely stripped is between- and under-rafter insulation with a 50mm ventilated airspace between the upper surface of the between-rafter insulation and the traditional sarking felt.
Step-by-step guide: Between Rafter insulation
Partially filled rafters:
- To maintain the required void above the insulation and to ensure the boards are flush with the bottom of the rafters, sidenail battens to the rafters in the appropriate position to provide a stop.
- The void must be 50mm deep and ventilated for constructions not using a breathable sarking membrane. Ventilation should be provided in accordance with Approved Document F, F2 (Condensation in Roofs) of the Building Regulations or Technical Standard K (Ventilation of Buildings, Regulation 23) of the Building Standards (Scotland).
1. Measure the space between the rafters before cutting the boards, as spacings will vary slightly.
2. Cut the insulation boards to size using a sharp knife or fin-toothed saw. We used Kingspan Thermapitch TP10 zero ODP which is a 100mm-thick rigid board.
3. Install the insulation flush with the bottom of the rafters, making sure that it does not fill the entire rafter depth. Ensure that the insulation boards fit tightly between the rafters.
4. Fill any gaps with expanding urethane sealant.
Fully filled rafters:
- Measure the space between the rafters before cutting the boards, as spacings vary.
- Cut the insulation boards to size using a sharp knife or fine-toothed saw.
- Install the insulation so that it is flush with the bottom of the rafters but does not fill the rafter depth.
- In all cases, ensure that insulation boards between rafters are fitted tightly.
- Fill any gaps with expanding urethane sealant.
- Measure up before cutting the boards.
1. Cut the insulation boards to size using a sharp knife or fine-toothed saw.We used Kingspan Thermawall TW56 zero ODP.
2. Where board dimensions allow, fix the boards at right angles to the underside of the rafters.
3. Boards should be fixed with galvanised clout nails or drywall timber screws, long enough to allow 25mm penetration of the timber. These should be placed at 150mm centres and not less than 10mm from the edges of the board along all supporting edges.
Things to remember…
- Ensure accurate trimming to achieve close butting joints and continuity of insulation.
- Ensure the continuity of the insulation at the ridge.
- To prevent a cold bridge, tightly pack flexible insulation material between the rafters and the cavity closer.
Types of Insulation
Polyurethanes: There are a number of plastic foam, polyurethane- type materials which are gas based the best known are Celotex and Kingspan and these are considerably better than airbased insulants such as mineral wool.Whilst they tend to be expensive, they are increasingly popular where space is at a premium.
Extruded polystyrene: This is denser than the expanded version and is widely specified under floors and where there is contact with the ground. Suppliers include Dow Styrofoam and Knauf Polyfoam.
Boards, rolls or slabs? The wool-type insulations are cheapest sold in rolls. The polyurethanes and polystyrenes are usually sold as boards: these can be used in any application, but are particularly good under floors. Boards are often fitted in sloping roofs but this requires a lot of cutting.Walls can be insulated with boards or slabs, a semi-rigid format which stiffens wool-based insulation.
Blown-in: Insulation doesn’t have to be dry fixed. Wool, fibres and plastics can all be blown into position via a nozzle. This is a widely used technique under roofs.Warmcel is a popular form of insulation, used broadly in timber frame houses and lofts. Its made of cellulose fibre, made from recycled newspaper.
Multifoils: Multifoils are a form of insulation using thin rolls of aluminium foil alternated with layers of foam padding. They promise good insulation levels, way in excess of their 25mm width, and are a popular way of insulating sloping roofs.
Natural materials: The use of more natural insulation materials is becoming popular. Sheeps wool is a wonderful material to use in place of the synthetic wools, made of glass fibre and mineral wool which, whilst being very cheap, are not pleasant to work with. Look out for Thermafleece.We are also beginning to see wood fibres and hemp being used in both roll and slab format, useful for insulating timber walls and roof spaces. Look for products such as Homatherm and Thermo-Hemp.