One of wood fibre insulation's distinctive qualities is in its ability to achieve high rates of decrement. Addressing decrement delay, particularly in roofing, is to deal with the very common problem of the (usually) summer over-heating in roof spaces.
In more conventional roof construction types, heat is transferred rapidly from the absorbent roof finish through to the interior. By using wood fibre boards, the transfer of heat can be significantly delayed until the hours of darkness when overheating is less of a problem.
Perhaps the optimum way of utilising wood fibre insulation is shown in the example below. It shows wood fibre sarking as the windtight layer applied immediately over the tops of the rafters.
Allowing for a substantial depth of insulation over the tops of the rafters will effectively raise the temperature at that location above the dew point and so reduce the risk of condensation.
Flexible wood fibre bats are friction mounted as infill between the rafters to provide the bulk of the insulation.
Vapour control and airtightness is achieved through the use of OSB (with additional taping of joints) protected by a service zone behind the plasterboard finish.
Compared with petrochemical insulation, the sections can be quite substantial - particularly when looking at achieving low u-values. The effect is to raise the ridge height - which might be a point of concern in urban situations.
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