attic ventination test

How do I test for adequate attic ventilation?
I was looking at this sight and got curious about my attic ventilation. http://www.airvent.com/homeowner/products/staticRoof-specs.shtml
I was looking at the roof vents and there are only three for a 2500 sqft single story house. The vents that I have looks like these
http://www.airvent.com/homeowner/products/productImages/staticVents-sqMetal.jpg
I estimate the free area at 190 sqin free area total for the three. There are no ridgevents or gable vents to help out. There is a continuous soffit vent around the eave of the house and I calculate the area as 2000 sqin total. More than enough
I suspect the there is inadequate ventilation on the roof vent side and I'd like to test for this.
Is temperature the big deternining factor.
This is in Houston, TX where HEAT is a big problem.
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mook Johnson wrote:

Hi, Stick your head in there and see how hot/cool in there.
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OK its HOT. How hot should it be if the outside temp is 90 degreesF?
The house is not shaded.
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mook Johnson wrote:

Ask to see the "federal guidelines" they rely on.

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mook Johnson wrote:

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------| --> |---| | | ----- | 118 inside I --- - | -
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.
Nick
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With inadequate venting your beams and deck would mold and rot.
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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 ASR50'south roof area (ft^2) 30 RSH=.2'shingle reflectivity 40 TA.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.6*A*SQR(H)/KROOF 120 T=TTS'initialize attic air temp (F) 130 TL=T'save previous value 140 IF T<TA 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
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... and make the average attic temp only 2 F above the outdoor temp.
20 ASR50'south roof area (ft^2) 30 RSH=.2'shingle absorptivity 40 TA.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.6*A*SQR(H)/KROOF 120 T=TTS'initialize attic outlet temp (F) 130 TL=T'save previous temp 140 IF T<TA 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.6*A*SQR(H*(T-TA))'airflow (cfm) 180 PRINT C,(TA+T)/2'airflow and average attic temp (F) 190 C00'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 C75*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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