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attic ventination test
The average daily max temp is 92.7 F in July in Houston. On a still sunny day,
a square foot of light-colored south shingles might absorb 0.2x250 = 50 Btu/h and lose heat to outdoor air with a 2 Btu/h-F conductance, which makes the equivalent shingle temp 92.7+50/2 = 118 F, so we might have something like this, with attic air temp T (F), viewed in a fixed font: T 0.5 | R1 | ----- -----www-------www------| -- |---| | | RATT ----- | 118 | I --- | - outside|inside | | - If 1250(118-T)/1.5 Btu/h that flows into the attic equals I that flows out and I = 16.6Asqrt(H)(T-93)^1.5, with lesser vent area A ft^2 and height diff H in feet, A = 190/144 = 1.3 ft^2 and H = 8' make T = 118-(T-93)^(2/3)/13.5 = 117.4 F, after 2 iterations. With A = 8 ft^2 (eg 2 2'x2' gable vents), T = 114.5. Not a big difference, esp since the larger soffit area will add about 40% to the flow in the first case, which isn't counted above. A radiant barrier can help by increasing the R1 above. Not much, altho larger vents help. A thermal chimney would also help (how much?), by increasing H. It might have a transparent south side or roof to collect sun and raise the "chimney" air temp and bouyancy and flow in summertime and collect wintertime heat. 20 ASR=1250'south roof area (ft^2) 30 RSH=.2'shingle reflectivity 40 TA=92.7'outdoor temp (F) 50 GSA=2'shingle-outdoor conductance (Btu/h-F-ft^2) 60 TTS=TA+250*RSH/GSA'Thevenin equiv shingle temp (F) 70 FOR RATT=1 TO 4 STEP 3'R-value of roof (1-4 for radiant barrier) 80 KROOF=ASR/(1/GSA+RATT)'roof coefficient 90 H=8'vent height difference (feet) 100 FOR A=8 TO 32 STEP 8'vent area (ft^2) 110 C=16.6*A*SQR(H)/KROOF 120 T=TTS'initialize attic air temp (F) 130 TL=T'save previous value 140 IF TTA THEN T=2*TA-T'reflect up 150 T=TTS-C*(T-TA)^.666'find new value 160 IF ABS(T-TL).1 GOTO 130'iterate 170 PRINT RATT,A,T 180 NEXT:NEXT RATT A (ft^2) Tattic (F) 1 8 114.2156 1 16 111.3843 1 24 109.0334 1 32 107.0474 4 8 109.0334 4 16 104.0215 4 24 101.0059 4 32 99.13989 Nick |
#2
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attic ventination test
The average max temp is 92.7 F in July in Houston. On a still sunny day,
a square foot of light-colored south shingles might absorb 0.2x250 = 50 Btu/h and lose heat to outdoor air with a 2 Btu/h-F conductance, which makes the equivalent shingle temp 92.7+50/2 = 118 F, so we might have something like this, with attic air temp T (F), viewed in a fixed font: T 0.5 | R1 | ----- -----www-------www------| -- |---| | | RATT ----- | 118 | I --- | - outside|inside | | - If 1250(118-T)/1.5 Btu/h that flows into the attic equals I that flows out and I = 16.6Asqrt(H)(T-93)^1.5, with lesser vent area A ft^2 and height diff H in feet, A = 190/144 = 1.3 ft^2 and H = 8' make T = 118-(T-93)^(2/3)/13.5 = 117.4 F, after 2 iterations. With A = 8 ft^2 (eg 2 2'x2' gable vents), T = 114.5.... A radiant barrier can help by increasing the R1 above. ... A thermal chimney would also help (how much?), by increasing H. It might have a transparent south side or roof to collect sun and raise the chimney air temp and bouyancy and flow in summertime... .... and make the average attic temp only 2 F above the outdoor temp. 20 ASR=1250'south roof area (ft^2) 30 RSH=.2'shingle absorptivity 40 TA=92.7'outdoor temp (F) 50 GSA=2'shingle-outdoor conductance (Btu/h-F-ft^2) 60 TTS=TA+250*RSH/GSA'Thevenin equiv shingle temp (F) 70 RATT=4'R-value of roof (1-4 for radiant barrier) 80 KROOF=ASR/(1/GSA+RATT)'roof coefficient 90 H=8'vent height difference (feet) 100 A=8'vent area (ft^2) 110 C=16.6*A*SQR(H)/KROOF 120 T=TTS'initialize attic outlet temp (F) 130 TL=T'save previous temp 140 IF TTA THEN T=2*TA-T'reflect 150 T=TTS-C*(T-TA)^.666'new attic temp (F) 160 IF ABS(T-TL).1 GOTO 130'iterate 170 C=16.6*A*SQR(H*(T-TA))'airflow (cfm) 180 PRINT C,(TA+T)/2'airflow and average attic temp (F) 190 C=1000'airflow estimate (cfm) 200 T=TA'average attic temp estimate (F) 210 TC=TA'average chimney temp estimate (F) 220 DT=KROOF*(TTS-T)/C'attic air temp gain (F) 230 T=TA+DT/2'average attic temp (F) 240 MA=H*.075*530/(460+T)'attic air mass (psf) 250 TAT=TA+DT'temp at top of attic (F) 260 CSUN=.9*250*32'sun into chimney (Btu/h) 270 CGAIN=CSUN-(TC-TA)*32'net chimney gain (Btu/h) 280 DTC=CGAIN/C'chimney air temp gain (F) 290 TC=TAT+DTC/2'average chimney temp (F) 300 MC=H*.075*530/(460+TC)'chimney air mass (psf) 310 MB=MA+MC'total bouyant air mass (psf) 320 MO=2*H*.075*530/(460+TA)'outdoor air mass (psf) 330 DP=MO-MB'bouyancy force (psf) 340 C=1975*A*SQR(DP)'airflow (cfm) 350 IF ABS(C-CL).1 THEN CL=C:GOTO 220'iterate 360 PRINT C,T,TC airflow attic temp chimney temp (cfm) (F) (F) 1518.035 100.8667 H = 8' vent height difference 1496.56 94.82298 99.28098 H = 8' + 2'x4'x8' solar chimney Nick |
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