Question-and-Answer Resource for the Building Energy Modeling Community
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Particularly for existing buildings there needs to be a qualitative examination. You'll be able to gauge the envelope's construction type -- is it made of materials that would tend to yield higher or lower infiltration rates? Are there many penetrations and joints? Are thermal images of the windows available when the HVAC system is on and when it is off?
Then also assess the construction or maintenance quality which may be a factor that you can observe...was the envelope well-sealed originally, and has it been maintained?
In some cases you may be able to acquire trend data or deploy data loggers to show how temperature changes when the HVAC system is off. In past experience this will mainly be useful as backup for your assumptions in cases where the infiltration rate is very high...in those instances the client probably has some idea that the building is leaky.
For most cases it will be difficult to confirm or deny the constants in the more detailed formulas, but by comparing the utility bills and maybe the space temperature trends you can partially calibrate.
This works in conjunction with knowing the other energy using systems in the building -- the more of those that are well-known, the less energy use that is eventually left over to vary as infiltration or process loads.
Lastly, run a sensitivity analysis to determine how important infiltration is to the results. (And your other major unknowns that were assumed to have a certain value.)
