A simple model was implemented to simulate ceiling bypass heat transfer, the heat that is transported from the conditioned zone to the attic via miscellaneous inter-wall cavities in the conditioned zone that may be partially open to the attic, as for example around a fireplace unit. Natural convection in the cavity when the conditioned zone is hotter than the attic is assumed to be the main mechanism for the bypass heat transfer. The conductance, when the conditioned zone air temperature Tairc > Tairu, the attic air temperature:
where, the conductance follows a simple power law dependence on the temperature difference:
is
a coefficient depending on the cavity geometry. Although an exponent of
nbp on the order of 1/4 can be assumed for laminar convection, there is
no current empirical basis for determining the exponent. If the ACM rule of U
= 0.02Aceil were implemented, then nbp would be chosen as
zero.