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Old 08-04-2008, 05:02 PM   #1
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hip roof ventilation


Need advice on PAV's for a very shallow hip roof. (average height probably about 2 1/2 feet) My house is about 2000 sq feet. A long skinny 1950's ranch with about a 3 ft overhang with soffit vents. I had two older "lifetime warranty" attic ventilator fans installed by the previous owner (I've been here 13 years so they were old) One caught fire and did some damage. After finally settling the insurance, I'm about to get the roof fire damage repaired (and more unfortunately since the roof was old too)and am not sure what fans to get. (My roof has very little slope and very little ridge length partly due to "big hips" and partly due to a large rectangular three flue chimney--two fireplace, one furnace-- stuck in the middle of the house interrupting the ridge so a ridge vent doesn't seem the best to most people I talk to.) I was going to get two new fans each rated for about 1150 sq feet. The old fans were close to the ridge and probably about 25 feet apart. One was about two feet away from the chimney --slightly below the ridge--and the other is in line with it horizontally at the other end of the roof ridge near the beginning on the "hip". I don't know the old fan ratings and can't find out as they were old and are not made anymore. I have talked to everyone in town and no one knows what I should do. Finally someone suggested I call Air Vent tech support which I did. Twice. Both times I was told that my attic (which is probably slightly bigger than 2000 sq feet because of the overhang) is not large enough to support two of even their smallest (1150) fans. That the fans will work against each other and will rattle alot before they burn out. I don't want another fire obviously as that was very scary as smoke detectors don't go off when the roof is on fire. (I also have a Federal Pacific breaker box that I will upgrade but can't now--that may have contributed to the fire as the breaker may not have thrown--I am having the breaker replaced now) I also don't know my "net free area". I have 16 soffit vents that are 6 x 12 (general venting area, not the size of the frame) with screening behind them. That works out to 8 square ft but it is not net free space since there is insect netting and also vent "bars". Air vent said use one big (2100 sq ft) fan and install a continuous soffit vent that is 9" wide. That would be a big hassle and by my calculation that only provides at the very very most 10 sq feet of intake --a gain of 2 feet over what I have--but that's not "net free" either. (My house is about 60 x 32 but I have an attached long screen porch where there is no soffit so that's how I came up with 10 feet) My contractor doesn't know what to do but he says he thinks the siting for one fan will be a problem. The chimney is close to the middle of the house's length so the fan couldn't be very close to the middle of the attic without being awfully close to the chimney which seems not good for air flow. Someone else said "if it ain't broke.." in other words, if you had two that worked for years, put two back. But I also had a fire! And the fans I had may have been less powerful. The company that makes the Air vent fans says don't put two even if you had two so.... Someone at the hardware store said to put in a bunch of "punch in" round soffit vents in addition to my rectangular vents. I suppose I could but that still doesn't solve the problem of where to put the fan/s/

I saw the CFM/300 calculation for venting on another post but that yields net free space needed. Also, it doesn't address the two fan situation. I know I don't want air just being exchanged between the two fans. I also know I don't want conditioned air pulled into the attic. I know the square feet x .7 to calculate CFM but am not sure what to do with that re: two fans or one fan.

I live in the south and it is hot as blazes in the attic without ventilation. I can feel the heat up when I stand up and I never could when I had working fans. I have plenty of insulation (holding heat now!) but not blocking the vents.
Thanks for any advice.

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Old 08-08-2008, 03:21 PM   #2
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hip roof ventilation


god do i hate hipped roofs.

having genuflected some, if I were you, I'd go with 2 fans. The odds of a fire again are super low anyway!

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