[Greenbuilding] Precision for Air Permeance numbers (was Re:insulating on sheathed roof)

[Bob Korves]

----- Original Message ----- 
From: "Robert Tom" <ArchiLogic at yahoo.ca>
To: <greenbuilding at listserv.repp.org>
Sent: Thursday, January 03, 2008 4:48 PM
Subject: [Greenbuilding] Precision for Air Permeance numbers (was 
Re:insulating on sheathed roof)


> On Thu, 03 Jan 2008 17:29:04 -0500, Bob Korves <bkorves at winfirst.com>
> wrote:
>
>> I have a problem with is the fact that the Canadian government test
>> results are listed in six significant figures.  Anyone who has taken a
>> quantitative analysis class would know immediately that this sort of
>> testing is not reportable to six significant figures.
>
>
> I've never taken a "quantitive analysis" class but I don't have a problem
> with the 4 decimal place precision of the test data in the report.
>

Four decimal places but six significant figures.

http://en.wikipedia.org/wiki/Significant_figures

describes quickly about significant figures.

Basically, one should only use the number of digits that are known with some
certainty.  So, for example, when someone builds a test wall, puts it in a
chamber and tests the insulative qualities of it there is always uncertainty
as to exactly how much heat was applied to one side of the wall, how much of
the heat might have leaked out of the test system (entropy), the measuring
equipment is not 100% accurate, etc.  So we list the results of the
insulation test as R 3.2/inch, not
R 3.18765891/inch because we certainly do not know it with that precision
even if that is the final calculation of the math.  Actually, it is correct
to only use the number of significant digits that can be defended as
accurate in each step of the work.

We might list the 28 day compressive strength of a certain concrete as 3,000
psi, but not as 3,044 psi.  We don't know it that accurately, even if that
was the actual test result number.

Trivia and a bit off topic, yes, and sorry for wasting your time, but that
is how it is correctly done.
-Bob Korves