Internal Roof Insulation with Pavatex Wood Fibre

Internally insulating a roof with wood fibre insulation is carried out in retrofit projects when the roof covering does not need to be removed. If the roof tiles or slates are being removed during the renovation, we strongly recommend that you insulate above the rafters – see External Roof Insulation. Roofs are insulated at rafter level when the attic area is being used as a living space or where the property has vaulted ceilings. It is more sensible to insulate the loft floor if the loft space is not being used.

It is critical to keep a 50mm well-ventilated space above the insulation to allow all the water vapour to evaporate away safely. Pavatex wood fibre insulation is breathable, hygroscopic and capillary-active so water vapour can pass through the insulation from areas of high humidity to lower humidity – generally from inside to outside. This contributes greatly to keeping the roof timbers and building fabric dry. Wood fibre buffers moisture so keeps buildings at an optimum humidity level which results in a healthy indoor climate. This greatly reduces the chances of mould, wet rot or dry rot occurring.

If there is adequate room height, we recommend that Isolair Multi or Pavatherm boards are installed below all the rafters because this will reduce cold bridges through the rafters. This heat retention is very considerable because the rafters typically take up 9 to 15% of the entire roof surface and the heat will escape through these timber elements if not insulated.

Because Isolair Multi boards are water-resistant, we recommend placing a thin layer of this between rafters that are at least 100mm deep so that any condensation from below the roof covering will run down the surface of the Isolair Multi boards and evaporate away. An alternative is to drape the Pavatex ADB Breather Membrane under each rafter and up along the rafter sides, across the cavity and back down the side of the next rafter. It is critical to keep a 50mm ventilated cavity above the membrane. Then these ‘pockets’ are fully insulated from below with Pavaflex or Pavatextil P flexible insulation.

Converted attic areas of buildings tend to overheat during the summer because there is virtually no thermal mass in a lightweight roof covering. On warm sunny days, there will be considerable solar gain through the windows leading to high temperatures in attic spaces. Most insulation materials have low thermal mass so they can’t store this excess heat effectively. However, wood fibre has a high density and a high Specific Heat Capacity, so these attributes contribute to high thermal mass. This excess heat from the sun will be stored for between 10 and 12 hours when it will then be released as temperatures drop, whereas other insulation materials can only provide 5 to 8 hours lag time. This keeps buildings a few degrees cooler during the warmer summer months. The reverse happens in winter when the heat from the central heating system will be stored in the wood fibre layer and released at night-time as temperatures drop. This is particularly crucial when the loft space is used as living accommodation.

Acoustic insulation through the roof will be greatly improved with the addition of dense wood fibre sarking boards so will reduce external noise from rain, traffic or airplanes.