Certain modeling techniques and compliance assumptions applied to the proposed design are fixed or restricted. That is, there is little or no freedom to choose input values for energy compliance modeling purposes. However, there are other aspects of energy modeling where some professional judgment may be necessary. In those instances, the compliance software user must decide whether a given input is appropriate and will be matched by the actual installation.
Enforcement agencies have discretion to question a particular input if the permit applicant cannot substantiate the value with supporting documentation, cannot demonstrate that appropriate judgment has been applied or cannot demonstrate that the actual installation matches the input.
Simplified modeling approaches can be used if the predicted energy use of the proposed building is not affected or if the proposed energy use increases, reducing the compliance margin when compared to a more explicit and detailed modeling assumption. That is, simplification must reflect the same or higher energy use than a more detailed model and reflect the same or lower compliance margin when comparing the standard and proposed TDV energy and HSE.
Any unusual modeling approach, assumption, or input value should be documented with published data and, when applicable, should conform to standard engineering practice.
Example 12--1
Question
Three different sized windows in the same wall of a new one-story office building are designed without exterior shading, and they have the exact same NFRC-rated U-factors and SHGC values. Is it acceptable professional judgment to simplify the computer model by adding the areas of the three windows together and inputting them as a single fenestration area?
Answer
Yes. For a simplified, two-dimensional, geometry model, the compliance software will produce approximately the same energy results whether the windows are modeled individually or together as one area because the orientation, fenestration U-factors and SHGC values of the windows are identical.
However, if overhangs and side-fins are modeled, the correct geometry of fixed shades must be modeled for each window.
For detailed, three-dimensional, geometry models, the location of windows on walls affects the daylighting energy calculation and this affect must be considered before making the simplification in the example.
For reference, to help determine if you’re using a detailed or simplified approach, a wall in a simplified model will be entered as an area, orientation (i.e., North, South, East, West), height, and width. In a detailed model a wall is entered as a series of points (i.e., x, y, z coordinates) to place the wall in three-dimensional space relative to other surfaces in the building.