• British Standard 5250 Code of practice for the Control of Condensation in Buildings provides the best advice on what should be done. If the insulation is placed between and above the horizontal ceiling joists, the thicker the insulation the longer the corrugated rafter trays that helps to maintain an air space between the insulation and the underside of the underlay. The air space allows ventilation air in and out at the eaves, to remove the cool air containing excess water vapour before it can condense on the cold hard surfaces in the roof, such as metal truss plates.

At the eaves where the roof insulation meets the outer wall, the space diminishes down to the thickness of the rafter, which in most cases will be less than 100mm. The depth of the rafter tray may further reduce the rafter depth, which can be between 25 and 50mm. This may be restricted still further by the wall plate being notched into the rafter. The resulting space left for insulation may be less than 50mm. The risk of condensation forming at this cold bridge area is high.

• If the insulation is positioned between the rafters, parallel with the roof tiles the quantity of insulation may be greater in thickness than the depth of the rafter. This may require an additional counter batten to be fixed to the top or bottom of the rafter to increase the insulation space. Alternatively a more expensive rigid insulation may be used that being a better insulator can be thinner to achieve the same thermal performance.

• What is more likely is rigid insulation board placed above the rafters and counter battens fixed through it into the rafters below. Because each timber rafter is below the insulation there are fewer cold bridges and joints, making a thinner board more efficient.

At the ridge the insulation board needs to be mitred and sealed to prevent a V shape cold bridge occurring. At the eaves the rigid insulation needs to maintain continuity with the wall and will need infill slabs to stand between the rafters to prevent a cold bridge at the wall plate. With this arrangement any services, such as soil vent pipes or roof windows that penetrate the rigid insulation will need to be sealed to prevent a cold bridge.

The use of vapour permeable underlay to improve the thermal performance of the roof will inevitably change the roof ventilation requirements. If you do not have roof space ventilation below the underlay then it will be needed above to ensure that there is a safe route for it to escape into the outside air. To meet the ever more stringent performance requirements, tiles are made to tighter tolerances (less gaps) to allow them to be laid at lower rafter pitches. Less gaps means the risk of water vapour condensing on the underside of the tiles and slates, or worse still the timber battens, the greater the need for batten space ventilation from eaves to ridge. Providing ventilation above the underlay is best done with a 25mm over fascia vent at low level and a continuous dry ventilated ridge system. If that is not possible, suitably spaced vent tiles with shortened or no underside pipes.

The use of vapour permeable underlays will allow the water vapour to pass into the batten cavity where it will condense on the underside of the roof tile/slates. Without ventilation at that point the battens and counter battens could be affected in the long term.

With the search for more efficient use of energy to heat buildings and hot water, we will see more buildings with Solar Water Heater or Photovoltaic panels that produce electricity, located on the upper parts of south facing roof slopes. This is likely to result in more systems being installed during construction or re roofing. At present the systems available are not liked by planners as they are not visually sympathetic to our traditional style of building, but some have been designed to be integrated into the roof hiding all the brackets and pipes. This trend combined with more compact housing units, may result in complete roof slopes being nothing other than Solar Water Heater or Photovoltaic panels.