I was looking at this sight and got curious about my attic ventilation.
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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
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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.
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.
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 T.1 GOTO 130'iterate
170 PRINT RATT,A,T
180 NEXT:NEXT
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