Condensation,heat transfer and ventilation processes in flat timber-frame cold roofs

Abstract

An experimental study of the heat and moisture transfer processes in an insulated flat timber cold roof, 4.4 m long x 2 m wide, has been carried out under controlled steady state winter conditions and wind speeds up to 5 m s^-1. The experiments were performed with intact, perforated and absent vapour barriers on the ceiling. The cavity between the insulation and plywood roof deck was ventilated via openings of various sizes at the ends of the roof. Cavity air speed, relative humidity and vapour pressure were measured, as well as moisture content in the timbers and the heat flow, temperatures and thermal properties of the roof. In accordance with current standards the roof was designed to have a 50 mm clear cavity, but in practice this was effectively 10 mm, thus restricting ventilation. The results show that vapour barriers and ventilation are of paramount importance if the airspace is to be prevented from reaching saturation conditions, resulting in dangerous levels of moisture in the timber deck and support elements. The minimum requirement for ventilation slot area was found to be an insufficient safeguard at low wind speeds, particularly in roofs without vapour barrier protection. At high wind speeds substantial heat losses from the cavity, accompanied by consequent increases in the thermal transmittance of the roof, were measured. Predictive models are given for the vapour pressure distribution and heat losses from the cavity.