Thermal effects of air flows in building structures

Thermal effects of air flows in building structures have been analysed by computer simulation. Some laboratory experiments have also been carried out concerning natural convection in closed and semi-open cavities filled with fibrous insulating material. Three different flow systems in building structures have been studied: natural/forced convection in open/semi-open (permeable cold surface) 2-dimensional insulation structures, heating of infiltrating air in cracks and diffusive infiltration. The thermal effects of air flows are described by the Nusselt number defined as the ratio of the average heat fluxes with and without air flow (internal/penetrating convection or infiltration). According to our computer simulations extra heat transfer due to natural convection is insignificant in closed normal size wall structures, but it may rise up as high as to ten percent, if the cold surface of structure is well-permeable. Laboratory experiments show greater convection heat transfer than calculations, obviously owing to leaky joints and air gaps between the insulation boards and covering surfaces. The heating of infiltration air in cracks may rise up to 80 percent of the outside/inside temperature difference corresponding to Nusselt numbers 0,9 - 0,7. This means that the heat load of infiltration and transmission is overestimated by 10 - 20 percent. Dynamic insulation with the exploitation of solar energy seems to be, according to computer simulation, a potential and promising way to get a better thermal performance into building envelope.