- posted on September 3, 2006, 7:50 pm

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.

- posted on September 3, 2006, 7:59 pm

Hi, Stick your head in there and see how hot/cool in there.

- posted on September 3, 2006, 8:21 pm

OK its HOT. How hot should it be if the outside temp is 90 degreesF?

The house is not shaded.

- posted on September 4, 2006, 2:23 am

mook Johnson wrote:

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

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

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- posted on September 4, 2006, 1:14 pm

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

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

- posted on September 4, 2006, 1:29 pm

With inadequate venting your beams and deck would mold and rot.

- posted on September 4, 2006, 7:04 pm

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

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

- posted on September 5, 2006, 1:39 pm

... 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

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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