CROSS Safety Report
Rotting of rafters likely due to spray foam insulation
This report is over 2 years old
Rafters had rotted in areas to 40-50% of their original depth due to spray foam insulation having been installed some years previously, says a reporter.
Key Learning Outcomes
For construction professionals:
Be aware of the risks of moisture and condensation. Whenever insulation is added to a tiled rafter roof, the ventilation requirements must be considered
Consider carrying out a condensation risk analysis to ensure the correct form of construction / insulation is detailed to prevent condensation occurring
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The rafters shown in Figure 1 had rotted in areas to 40-50% of their original depth due, says a reporter, to spray foam insulation having been installed some years previously. The spray foam insulation may have been applied due to water ingress from the tiles, lack of ventilation, or both. What was supposed to be retiling of a roof, turned into replacement of all the rafters.
Expert Panel Comments
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The public are constantly being pressed to add more insulation in lofts but if this is done without adequate ventilation, the risks of condensation rise hugely. Occupiers sometimes complain of leaking roofs when actually it is condensation in the loft space exacerbated as windows have become more draught proof. The safety risk is that the rot is in structural members that may never be accessed for inspection.
Whenever insulation is added to a tiled rafter roof, the ventilation requirements must be considered. In a traditional construction, ventilation is provided to the roof space to prevent condensation occurring and it may be that the sprayed insulation blocked this. Even in a ‘warm roof’ construction, ventilation is normally needed above the vapour barrier.
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