SR70 Performance of roofing systems with and without underlay (2007)

The performance of roofing systems with and without underlay has been investigated. Aspects of roof system performance investigated were:

• condensation under conditions of night-sky radiation
• solar driven moisture transfer
• the degradation of thermal insulation value due to wind penetration of insulation
• weather penetration.

The presence of building paper was found to increase the amount of condensation appearing on the cladding under conditions of night-sky radiation, contrary to conventional wisdom. However, the levels of condensation observed with underlay, although higher numerically, were not sufficient to cause dripping off the cladding and onto the underlay, so in practice these higher levels of condensation are probably not important. Solar-driven moisture transfer was found not to be a problem.

Literature searches and calculation suggested that loss in roof thermal insulation value due to wind penetration into roofing insulation, more likely if the roofing system has no underlay, was not highly significant. Calculations indicated that underlays improve the weather penetration performance of concrete tile roofs, improve a little the performance of metal tile roofs and do not improve the performance of long-run galvanised or precoated steel roofs.

Whether this improvement is significant, how much improvement occurs and for what roof geometries and weather conditions cannot be accurately determined by the approximate methods used.