Adapted from "Prediction of the thermal performance of single and multi-airspace reflective insulation materials" by Andre O. Desjarlais and David W. Yarbrough, from ASTM STP 1116--Insulation Materials, Testing and Applications, Ronald S. Graves and Donald C. Wysocki, editors, 2nd volume, October 1991, pp 24-43:
20 DATA 2.5,5.09,7.5,1.4959,-0.608,6.359e-2,6.5572e-3,2.7129e-4 30 DATA 3.5,4.68,7.82,-1.3771,.2989,5.93e-3,1.2676e-3,8.7056e-2 40 DATA 2.5,4.35,7.2,-2.2234,0.6784,-3.028e-2,1.6481e-3,2.3528e-2 50 DATA 0,0,0,0,0,0,0,0 60 DATA 2.5,3.49,6.8,-.862,.2912,-3.863e-4,.16645,-1.397e-2 70 DATA 2.5,4.14,6.9,-.3,.0381,2.409e-2,1.5679e-2,-3.4369e-2 80 FOR I=0 TO 5 90 READ GR0(I),GR1(I),GR2(I),A(I),B(I),C(I),A2(I),A3(I) 100 NEXT I 110 DIR=0'heatflow direction 120 IF DIR=-1 THEN DIR$="down" 130 IF DIR=0 THEN DIR$="horiz" 140 IF DIR=1 THEN DIR$="up" 150 DIAG=0'0 for up or down, 1 for diagonal heatflow 160 IF DIAG=0 THEN DIAG$="no" ELSE DIAG$="yes" 170 I=2*DIR+DIAG+2'index (0-5, skipping 3) 180 T1=30'surface 1 temp (F) 190 T2=70'surface 2 temp (F) 200 DT=ABS(T2-T1)'temp diff 210 TBAR=(T1+T2)/2'mean temp 220 HR=.00686*((TBAR+460)/100)^3'radiation conductance (Btu/h-F-ft^2) 230 E1=.05'surface 1 emittance 240 E2=.05'surface 2 emittance 250 E=1/(1/E1+1/E2-1)'effective emittance 260 L=3.5'airspace width (inches) 270 LGR=LOG(DT*L^3)/LOG(10)+3.4146-.004359*TBAR+3.6441E-06*TBAR^2'log10 Grashof 280 IF LGR=LOG(GR1(I))/LOG(10) THEN LNU=A(I)+B(I)*LGR+C(I)*LGR^2:GOTO 320'eq 5 300 LNU=A2(I)*(LGR-LOG(GR0(I))/LOG(10))^2 310 LNU=LNU+A3(I)*(LGR-LOG(GR0(I))/LOG(10))^3'equation 6 320 NU=10^LNU'Nusselt number 330 K=.0003053*TBAR+.1575'air conductivity (Btu-in/F-ft^2-F) 340 HC=K*NU/L'convection conductance 350 R=1/(E*HR+HC)'US R-value (ft^2-F-h/Btu) 360 PRINT "gap (in):",L 370 PRINT "dir:",DIR$,"diag:",DIAG$ 380 PRINT "T1 (F):",T1,"T2 (F):",T2 390 PRINT "Tmean (F):",TBAR,"dT (F):",DT 400 PRINT "E1:",E1,"E2:",E2 410 PRINT "Eeff:",E 420 PRINT "R-value:",Rgap (in): 3.5 dir: horiz diag: no T1 (F): 30 T2 (F): 70 Tmean (F): 50 dT (F): 40 E1: .05 E2: .05 Eeff: 2.564103E-02 R-value: 2.461726
Nick