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Re: [Phys-l] Insulation R-value



Correction: Units of R are F-deg (of temp difference) per (BTU/hour/sq-ft)

So R-19 means that a difference of 19 Fahrenheit degrees across the surfaces
of this material will produce a transfer rate of 1 BTU/hour per square foot of surface area.
R-30 is better because it requires a 30 degree difference to produce the same energy transfer rate.

Note that this may not reflect the over-all (internal air to external environment) transfer, due to air boundary layers, convection , radiation, etc.

Al Bachman

From: "ALVIN BACHMAN" <bachman_28@msn.com>
Reply-To: PHYS-L Maillist <phys-l@carnot.physics.buffalo.edu>
To: phys-l@carnot.physics.buffalo.edu
Subject: Re: [Phys-l] Insulation R-value
Date: Fri, 17 Feb 2006 16:57:57 +0000

R is related to the reciprocal of k.

dQ/dt= k A (T1-T0) / L = A (T1-T0) / R

For a set of slabs (same R) , of conductivity Ki and thickness Xi, R = SUM(
1/KiXi )

Units of k are W/(mK) and common units of R are BTU per sq-ft per hour per
F-deg,
which is why everyone drops the units and says R-19 or R-30, etc. )

Al Bachman