I am curious about the zone map developed by LBL that is used for calculating Building Tightness Limits (BTL)  The map is ubiquitous, but pretty crude.  It certainly doesn't correspond to avg wind speeds where I live.  Are there any other factors incorporated in that map? 

Using the wrong zone can result in a 15-20% error in the BTL

For reference, I'm attaching some maps from Montana

Thanks, Jim

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I'm chuckling as I remember reading an article years ago about the development of the LBL number. I have looked since and not been able to find that article so I cannot reference it. If anyone knows its whereabouts I would appreciate a link. Anyway, I'm a numbers person, so when the developer stated his accuracy estimates of -50% and +100% I quickly added this to my list of guesstimates that we base all of our very accurate results upon. After question the accuracy of almost every step we take, I finally decided that accuracy is not their target. It is consistency. I was looking at a rater certification at the time and what they wanted was for all auditors who might rate the same home, they should all get the same results. Thus, it mattered less that the number was wrong, and more that everyone arrived at the same answer.

If you want a correct number, use YOUR best guess. If you want to get an answer that others would also get, stick to where the pencil falls and use the LBL number you find. If it falls on a line, state which one you used and why.


I've forwarded to Max Sherman and Iain Walker.  They may get back to us.

 To the technical question asked, the map does include wind speed, but stack effect--i.e. indoor-outdoor temperature differences--is the bigger driver.  The zones follow a combinations of wind and temperature.

I am the father (or at least grandfather) of all of this.  I wrote a paper to show how the old rule-of-thumb (divide by 20) did not work and even if you did it right there would be big errors and generalizations.This was obviously the wrong thing to do, because those numbers got turned into the BTL.

Finally, finally, the BTL is dead.  ASHRAE, RESNET and BPI have moved on to do things in a much better way.  No one should use BTL anymore as these approaches are superior.

We are, however, coming out with a new (color!) map. It does not have BTL and it does not have zones, but it does have a single value for over 1000 weather stations in the country.  This paper has been accepted, by ASHRAE's HVAC&R Research Journal, but I don't know when it will come out.  It is possible to calcute a BTL from these numbers, but I am not going to say how. :)



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