A Study of the Roof Environment in Four Domestic Buildings
Author(s): Brian Ridout, Iain McCaig, Soki Rhee-Duverne
This report presents the findings of a project carried out to investigate the behaviour of heat, moisture and air in domestic pitched ‘cold roofs’ under the influence of internal and external environmental loads. The aim of the project was to better understand the factors affecting the risk of condensation and moisture accumulation, the influence of roofing underlays and air and vapour control layers (AVCLs) on roof environments, and the role of ventilation in managing the risk of condensation. The four buildings selected for this study represent a range of typical domestic roofs. Two had undergone recent energy saving renovations, including the introduction of AVCLs at ceiling level, cellulose insulation (hygroscopic) and vapour-permeable roofing underlays. A third roof had no AVCL at ceiling level, glass fibre insulation (non-hygroscopic) – which was increased in thickness during the monitoring period – and no roofing underlay. The fourth roof had no AVCL at ceiling level, glass fibre insulation and impermeable BS 747 Type 1F roofing underlay. The four roofs were monitored continuously over four years. Sensors were installed to provide an upward profile of air temperature and relative humidity from the room below the roof space to the roof covering. This included sensors below and above the insulation in the roof space, at high level and between the roof covering and the roofing underlay (where one existed). Surface temperatures on the underside of roof coverings were also recorded, as were air velocities at low and high levels within the roof space. In one roof, the moisture content of timber was recorded, too. Weather stations were used to monitor exterior conditions. Due to the large amount of data obtained, the analysis presented in this report is limited to comparing the roof environments during winter (January/February) and summer (July/August). The data demonstrated a seasonal consistency in the environments in each of the roofs over the monitoring period. In winter, the relative humidity in the roofs was high and all the roofs were liable to moisture absorption in porous materials and condensation on impermeable surfaces to varying degrees. Condensation was generally a transitory event, with moisture evaporating and rejoining gases in the air as the roof environment warmed as the day progressed. Daily evaporation of absorbed moisture and surface condensation requires a decreasing vapour pressure gradient away from the surface. If the relative humidity within the roof space remains high, there is little, if any, gradient and the rate of evaporation declines. During prolonged periods of very cold weather, the moisture content of porous materials increases and condensate on impermeable surfaces accumulates until it forms droplets. In some buildings, where moist air from the building enters the attic and/or there is restricted air exchange between the attic and the exterior, this phenomenon might be exacerbated by adding insulation. This appeared to be the case in Building 4 ‘Long Compton’.
- Report Number:
- 6/2022
- Series:
- Research Report
- Pages:
- 130
- Keywords:
- Environmental Monitoring Traditional Construction Condensation Ventilation Insulation Roof Environment Cold Roof Moisture Risk Moisture Accumulation Climate Data Gathering Measurement Risk Assessment
