adding a vent> what size? Flat roof

Old 05-03-10, 02:00 PM
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adding a vent> what size? Flat roof

Here's the info. I'm in Philly, in a row home, with a typical flat roof. Well, not truly flat, but you know. So I'm prepping the crawl space for blown in insulation, but before I blow in the insulation, i'd like to make sure I'm vented properly.
House is 15'x50' = 750 square feet.
The back of the house has two 4"x8" vents in the brickwork, but none in the roof. So I was thinking of putting a mushroom or a turbine vent in the front, which is also about 2 feet higher than the rear.
I'm leaning towards the mushroom, because I'm worried that a turbine will create too much negative pressure, and cause air to be sucked in from the living space. I'm figuring the mushroom would be limited by the vents in the wall.
Should I go this way, and if so, what size?
I've also heard that putting a mushroom at the low end of the roof, and a turbine at the high end is another option.
Old 05-06-10, 06:11 PM
Join Date: Jan 2008
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Has this roof gone without a vent for the last 100 years already with no damage? Just a thought.
If you insist on a vent then what ever you go with make sure the opening is at least 8" higher than the roof or it will leak.
That all said id go unpowered turbine, They have a nice high shaft the turbine sets on. And it wont suck all the air out of your home.

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