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Here's one way to estimate the R-value of a radiant barrier based on
the air
gap and the emissivities and surface temps and the direction of
heatflow from
http://www.reflectixinc.com/pdf/RIMA_Handbook.pdf

10 SCREEN 9:KEY OFFIM HC(18,6)
20 DATA 0.359,0.184,0.126,0.097,0.080,0.068
30 DATA 0.361,0.187,0.129,0.100,0.082,0.072
40 DATA 0.363,0.189,0.131,0.101,0.085,0.075
50 DATA 0.364,0.190,0.132,0.103,0.087,0.078
60 DATA 0.365,0.191,0.133,0.105,0.090,0.081
70 DATA 0.366,0.192,0.134,0.106,0.092,0.082
80 DATA 0.360,0.204,0.169,0.179,0.185,0.189
90 DATA 0.366,0.267,0.223,0.233,0.238,0.241
100 DATA 0.373,0.247,0.261,0.271,0.275,0.276
110 DATA 0.380,0.270,0.292,0.301,0.303,0.303
120 DATA 0.387,0.296,0.317,0.325,0.327,0.326
130 DATA 0.394,0.319,0.339,0.347,0.347,0.345
140 DATA 0.381,0.312,0.295,0.284,0.275,0.268
150 DATA 0.429,0.381,0.360,0.346,0.336,0.328
160 DATA 0.472,0.428,0.405,0.389,0.377,0.368
170 DATA 0.511,0.465,0.440,0.423,0.410,0.400
180 DATA 0.545,0.496,0.469,0.451,0.437,0.426
190 DATA 0.574,0.523,0.494,0.475,0.460,0.449
200 FOR I=1 TO 18'read data table
210 FOR J=1 TO 6
220 READ HC(I,J)
230 NEXT:NEXT
240 T1=105'temperature of surface 1 (F)
250 E1=.03'emissivity of surface 1
260 T2=75'temperature of surface 2 (F)
270 E2=.8'emissivity of surface 2
280 L=2'air gap (valid range: 0.5-3")
290 LI=INT(2*L+.5)'length table index
300 HF=0'heatflow 0-down,1-sideways,2-up
310 E=1/(1/E1+1/E2-1)'effective emittance
320 TM=(T1+T2)/2'mean temp (F)
330 DT=ABS(T1-T2)'temp diff (valid range: 5-30 F)
340 DTI=INT(DT/5+.5+6*HF)'temp diff table index
350 HR=.00686*((TM+459.7)/100)^3'radiant conductance
360 R=1/(E*HR+HC(DTI,LI))'US R-value (ft^2-F-h/Btu)
370 PRINT T1,E1,T2,E2
380 PRINT L,HF,R

T1 (F) E1 T2 (F) E2

105 .03 75 .8

gap heatflow US R-value

2" 0 (down) 7.146456

With more than one space in series (eg double-foil foamboard spaced away
from a basement wall), we can't just add R-values. We only know the
overall
temp diff, so we have to iterate to find a solution. It's no surprise
that
the FTC prohibits makers from advertising R-values for radiant barriers
to avoid confusing the public.

Nick


Rockwool has a set value and can be compounded. Makers claim all sorts
of wild claim for radiant barriers. To bottom line, what is it the
equivalent to in rockwool in thickness?

I think that is exactly what Nick has done. But remember fiberglass
blankets are temperature independant (mostly). Radiant barriers are
dependant on the temperature (emisivity is T^3) and the air space can be
treated as a more conventional "R" value. Note that Nick has commented
the code.

Run that for a higher delta temp and you will get a higher R, just a
lower delta temp gives a lower.

Jeff