Thermal resistance of ventilated air-spaces behind external claddings; definitions and challenges (ASHRAE 1759-RP)

Abstract

The presence of an airspace within a building envelope is known to have a varying contribution to the overall thermal performance of the wall assembly due to the combined effect of convection and radiation in the air cavity. In particular, the thermal resistance of a ventilated airspace can vary significantly depending on multiple environmental and thermo-physical parameters. Although the thermal resistance of enclosed air-spaces in the building structures has been thoroughly investigated in the literature, it has not been defined for a ventilated cavity. This paper aims to introduce the plausible definitions of the thermal resistance of a vertical ventilated airspace behind external cladding systems. Both theoretical and applied formulations are provided and compared. The energy balance method is used to model the steady-state heat transfer through two types of traditional external wall systems (i.e., brick and vinyl siding) in summer and winter conditions. A range of air exchange rates in the cavity is examined, and the effect of the presence of reflective insulation in the airspace on the thermal resistance of the air gap is analyzed. The results show that the presence of a ventilated cavity in the wall assembly can influence the thermal performance of the building envelope. In particular, the effective thermal resistance of a ventilated airspace behind a brick cladding wall could be between 0.17 and 1.85 times the thermal resistance of the cladding in the range of air change rate in the cavity from 0 to 100 1/h. The effective thermal resistance of the ventilated air gap behind vinyl siding could reach up to 9 times the thermal resistance of the cladding.

CFD Analysis of Different Ventilation Strategies for a Room with a Heated Wall

“…Air cavities behind the external claddings function as solar chimneys and increase the thermal resistance of the wall. Rahiminejad and Khovalyg [26] reported that the total effective resistance of a cladding and a cavity was up to nine times the thermal resistance of the cladding alone. Particularly, the total resistance increased with the induced flow rate in the cavity.…”