Well placed insulation reduces the risk of condensation. Poorly placed insulation increases the risk.
In general since condensation forms on cold surfaces, insulation by keeping the internal surfaces warm prevents condensation forming on them. However the layers of the construction on the 'cold' side of the insulation become that much colder.
Condensation risk is reduced provided that the combination of insulation material and vapour control layers does not allow significant quantities of moisture to arrive at the materials on the 'cold' side of the insulation. The rule of thumb is that the significant vapour control layer should be on the warm side of the insulation.
Should the significant vapour control layer be placed on the cold side of the insulation then condensation risk will increase dramatically. This has often be done inadvertently when a new ceiling is added under an existing roof. The dew-point is then 'dragged' below the previously successful, if energy inefficient, roof. This often results in sudden floods - particularly after a long cold spell when condensation collects as ice under the roof surface.
Keeping things simple is the best approach - but not one that is always possible. As soon as there are a number of materials which can be regarded as insulants (including timber), and a number of materials which prevent the flow of water vapour then calculations are necessary.
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