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Old 07-27-2011, 04:26 PM   #1
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Attic ventilation


Hello and thank you for taking the time to answer my question. I am looking to install additional ventilation in my attic to decrease the temperature during the summer months and to remove stale air ( I dont think I have an issue with moisture)... The volume of the attic space is approximately 1750 cu ft (32ft L, 22ft W, 5ft H).. The volume is actually a bit less, since the roof is sloped on all four sides, not just two. There are soffits installed, but no soffit vents, and there is a gable vent w/ mesh screen at the front of the house. The gable vent is roughly 16in x 16in, which would give me about 1.8 sq ft of intake/exhaust area (minus a few points from the screen).. I know that I will have to install soffit vents if I install an attic fan, but what I would like to know is if I should install the soffit vents first and see if that alone would give me the desired airflow, before going ahead with attic fan installation.. I would also like to know if a 1000 cfm fan would be overkill for such a small attic space, especially since it is not airtight.. Going by the attic size calculations, to perform 10 air exchanges in an hour in a 1750 cu ft space (17500 cfh), I would need only a 300 cfm fan (17500/60).. I also have an old chimney which has some cracks in the bricks, so I do not want to start pulling air from unwanted areas... Also, ridge vents are not an option for me... If you need any more information, please let me know.. I can also provide pictures as well.. Thank you in advance..

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Old 07-27-2011, 04:58 PM   #2
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Attic ventilation


How much isolation and do you have any idea how hot is getting on these warmer days? I personally believe there is no concern with oversizing the fan unless the noise bothers you. The air seems to heat up faster than it will exchange. I don't have sofit vents but my exhaust fan is mounted just to side of the ridge and draws air in from the gable vents, obviously, soffit vents would be better. I would maximize your insulation first (R30-R60) depending on where you live.

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Old 07-27-2011, 07:33 PM   #3
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Attic ventilation


Might want to repost in the roofing section. Some of the guys on here might know the answer but in general HVAC crews don't deal with attic ventilation.
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Old 07-27-2011, 09:25 PM   #4
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Attic ventilation


FHA recommends a minimum of at least 1 square foot of attic ventilation (both intake and exhaust) for every 300 square feet of attic space, I tthink you wiill be better served witth something closer to 1 sq.ft. of vent for every 150 sq.ft.. of attiic space. For venting to work, cool air must come in the bottom, it heats up on its way up the roof and exists out the top. You can't suck air out the top if cooler air doesn't come in somewhere else. You have 704 sq.ft. of attic space which would require 4.69 sq.ft. vent. So you divide that in half, 50% goes to intake at the soffit level and 50% goes to exhaust at the highest point in your attic.

2.35 sq.ft. = 338.4 sq.in./ NFA Net Free Area
So if you buy 8x16 soffit vents with an NFA of 56 you will need 6 or 7.
If you buy Roof vents with an NFA of 60 you will need 6

If your gable vent is at the highest point of your roof you can use it in your exhaust numbers if it is somewhere between the highest and lowest point you would be better to close it off.. you don't want to short circut the air flow from low to high.
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