I am involved in a QC program that straddles two of the four zones BPI uses to identify LBL "N" factors. Has anyone ever seen a map with enough resolution that could help determine whether a project lands in one zone or another? My area of concern is the division between zones 3 and 4 in southern California.
Has anyone correlated the LBL N-factor zones with California Energy Commission climate zones?
With a requirement to meet ASHRAE standards, the 15% factor difference between zones could be significant.
If you are in doubt, just use the middle value common to both zone 3 and 4: 23.
The whole formula isn't all that precise anyways. The N factors for each zone are a range of four numbers - that should tell you something. Using the lower number will allow you to seal up a house tighter without having to do ASHRAE 1989. But to meet ASHARE 62.2 - 2010, you probably have to install some kind of ventilation anyways - the only question is what cfm rate to set the fan at.
If you want to be more precise, use ASTM 779 and ASHRAE Standards 119 and 136 to calculate your own N factor instead of using that burry map.