Numerical analysis of additional heat loss induced by air cavities between insulation boards due to non-ideality

Abstract

This study uses the numerical steady-state three-dimensional CFD calculation method to assess parametrically the thermal transmittance of a building envelope with partially and fully penetrating air cavities within 20 cm thick layer of insulation. Heat flux measurements from climate chamber experiment were used to validate numerical model and estimated heat flux at different height of penetrating air cavity. Focus is on the effect of different cavity geometry and cavity location within a building envelope, including the effect of temperature difference across the building envelope. Fully penetrating horizontal air cavities in the wall construction and vertical air cavities inside the floor construction were found to have an insignificant effect on thermal transmittance. The vertical air cavities in the wall and the roof construction significantly increase the convective heat transfer through the insulation layer, well over the normative correction levels defined by EN ISO 6946. The effect is strongly dependent on the temperature difference and cavity thickness. At 40 K temperature difference, the penetrating air cavities with 5, 10 and 20 mm thickness will increase the thermal transmittance up to 10, 33 and 59% respectively; the simplified procedure in EN ISO 6946 was unable to describe this effect adequately. For an upward heat flow in the roof construction, the effect was even more adverse. Using dual layer insulation with rebate, tongue and groove edge or expanding montage foam to tighten the external side of the air cavity, additional heat loss is reduced roughly by half and the dependency of additional heat loss on the temperature difference between internal and external environments is decreased.