- Author
- Burch, D. M. | Seem, J. E. | Walton, G. N. | Licitra, B. A.
- Title
- Dynamic Evaluation of Thermal Bridges in a Typical Office Building.
- Coporate
- National Institute of Standards and Technology, Gaithersburg, MD Johnson Controls Inc., Milwaukee, WI
- Journal
- ASHRAE Transactions, Vol. 98, No. Part 1, 291-304, 1992
- Report
- Paper 3573,
- Keywords
- office buildings | abridges (structures) | steady-state | finite difference theory | heat transfer | computer programs | fasteners
- Abstract
- A finite-difference model is used to predict the steady-state dynamic thermal performance of thermal bridges in a typical office building. The thermal bridges evaluated include a built-up roof system with ceiling fasteners, a roof/wall interface, an insulated masonry cavity wall with metal studs, a floor slab that penetrates wall insulation, and a window frame/wall interface. The steady-state analysis reveals that these typical thermal bridges increased the overall envelope heat transfer coefficient for the office building by 33%. A thermal bridge is found to have a large effect when it has a large cross-sectional area that short-circuits the thermal insulation of the building envelope. In the dynamic analysis, a finite-difference model is used to numerically determine a complete set of conduction transfer function (CFT) coefficients for each of the thermal bridges. The mathematical procedure is to predict the heat-transfer response of a thermal bridge when it is excited by a ramp excitation function. The heat transfer response for a triangular pulse is subsequently obtained by superimposing the responses for three ramp excitation functions to form a triangular pulse. A recursive relation, employing a past-history CTD, is applied to the traingular-pulse response to determine first-order CTF coefficients. The validity of the CFT coefficients is demonstrated by accurately predicting the heat-transfer response of each of the thermal bridges to a diurnal indoor and outdoor temperature cycle. Mathematical procedures are described to remove the air film resistances from the numerical CFT coefficients, thereby permitting them to be incorporated into computer programs that predict space heating and cooling loads for buildings.