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Q: Thermal Resistance of a building material ( Answered 5 out of 5 stars,   0 Comments )
Question  
Subject: Thermal Resistance of a building material
Category: Science > Physics
Asked by: markthearch-ga
List Price: $20.00
Posted: 08 Aug 2004 04:40 PDT
Expires: 07 Sep 2004 04:40 PDT
Question ID: 384966
I need to convert the following physical properties information for an
insulation material in the US, to an Australian 'R' value equivalent.
density is 7.37 kg/cubic metre, thermal resistance (90 days at 23 deg
celcius) is 0.671m2. deg celcius/W.  The assumed thickness of the
material is 90mm.  Please note that to convert USA 'R' values to
Australian 'R' values, you must multiply by 0.176.  Thanks

Request for Question Clarification by smudgy-ga on 08 Aug 2004 06:26 PDT
Hi markthearch,

For what thickness of material is the given thermal resistance? Is
this where we are to assume 90mm thickness?

Thanks,
smudgy.

Clarification of Question by markthearch-ga on 08 Aug 2004 15:38 PDT
Hi Smudgy,

Below is information from the manufacturer....does this tell you....is
it the BTU.in?  Therefore suggesting inch?

ASTM D1622 - Density (core)			7.37 Kg/m3
						(0.46 lb/ft3)
ASTM C518 - Thermal Resistance 90 days @ 230C	0.671m2.0C/W
						(3.8 ft2.h.0F/BTU.in)

Cheers,

markthearch
Answer  
Subject: Re: Thermal Resistance of a building material
Answered By: smudgy-ga on 09 Aug 2004 12:19 PDT
Rated:5 out of 5 stars
 
Hi markthearch,

I hope you find the following answer satisfactory; if you have any
questions, please request a clarification before rating and I will do
my best to resolve any outstanding issues.

I looked up some information on ASTM C518, which is a thermal
resistance testing standard, and it seems that the numbers you
provided are probably indicating the thermal resistance of an inch
thickness of material. This is supported by the Imperial units given
for thermal resistance, which seem to be a "per-inch" unit as you
indicated. Also see note at the end of this answer.

It turns out that the numbers you have given for thermal resitance are
exactly the R-values of the material in both Imperial and SI units.
According to several web sources I have found, R-values in metric are
given in units of m^2*C/W, and in English units of (ft^2*h*F)/(BTU).
So that means that the English/US R-value is about 3.8 per inch of
material, and the metric R-value is about .671 per inch of material.
This also coincides with the known conversion factor between US and
metric R-values: when we convert .671 metric R to Imperial R, we get
3.8125.

Now an inch is approximately 25 mm; if we have 90mm of insulation we
have about 3.6 inches, so the R-value of a 90mm thickness is 3.6 times
the R of a single inch. So a 90mm thickness of the material has a
metric R of approximately 2.416.

I hope you find this answer satisfactory; if not, please feel free to
request a clarification, and I will do my best to answer any questions
you have.

Good luck,
smudgy.

Here is some information on the units of R-value in both English and metric units:
Metric and Imperial R-value:
http://www.efunda.com/units/convert_units.cfm?From=903
http://www.efunda.com/units/convert_units.cfm?From=464

General information on Imperial R-value and calculating total R-value
http://www.roofhelp.com/Rvalue.htm

This website gives the units of R-value as (F?hr?ft2)/Btu:
http://www.insulation.org/metalbuilding/pages/resources/resources.html#9
Having inches in the denominator of the listed thermal resistance on
your product seems to indicate that each inch of the insulation
provides an (Imperial) R-value of 3.8.

Google searches performed:
<ASTM C518>
<"r value" insulation equation>
markthearch-ga rated this answer:5 out of 5 stars
Very prompt response, great to also show methodology in determining
answer.  Thank you!

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