Investigation of air flow in a twin roofs of residential houses

Some kinds of flat roofs with low height of ventilated air gap have certain problems with thermal and moisture regime. The lower surface temperatures of internal constructions in the air gap of roof and presence of hot and moist air in the air gap from indoor environment of building causes condensation of water vapour at the cold surfaces of roof internal constructions. This kind of flat roofs was used in our country at the end of the 20th century. The used construction of flat roof is split in two parts. Between them is ventilated air gap. The thickness of the air gap is 150 mm. Multilayer construction over the air gap protects the building against rain and snow. Multilayer construction under the air gap protects the building against the thermal losses. Many problems were at that time with the quality of work, with realization of roof penetrations by building equipments such as ventilation duct or canalization. The central ductwork for ventilation of flats (kitchen, toilet, and bathroom) enters to the roof chamber but the ductwork is not directly connected with the central roof air fan. In the roof construction are many leakages. These places represent the path of natural motion of warm and humid air from heated space into the air gap of roof. The aim of this article is to gain the view on the air velocity, internal surface temperatures in the air gap of existing flat roof and potential places of air condensation by using CFD analysis. We used commonly used CFD tool ANSYS CFX.