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AKH 07-12-2011 11:51 PM

Attic ventilation project
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I live in one of those houses where the second story is totally contained within the roof. Attached is a rough cross-section - attic space in grey, proposed rafter baffles (to keep top attic insulation from blocking the diagonal passageway connecting upper and lower attics) in red. The upper attic is 30x12 feet, the two lower attics 30x3. Attic height is about 5 feet.

My main question: for the purpose of determining the amount of needed ventilation, how do I measure the attic? Do I treat is as 30x12, 30x18, or somewhere in between?

My plan is to install static roof vents. Don't want ventilation with moving parts, can't afford the labor cost for installing ridge vents. 60 NFA slant roof vents aren't hard to find. The attic is short enough that I can cut the vent holes and apply the roofing cement from inside the attic, but I'll need a ladder to trim the shingles and nail in the vent.

josall 07-13-2011 09:38 AM

I would use 18 x30, I would put in 6 roof vents and 8 8x16 soffit vents. It's best if you have equal amounts of NFA at the intake and exhaust.

AKH 07-15-2011 04:57 AM

Some sources say that it's okay if the soffit vents have more NFA than the exhaust vents - what do you think?

Installing two 144NFA vents is sounding preferable to installing six smaller vents.

josall 07-15-2011 07:13 AM

It's my opinion that more NFA at the exhhaust creates more draw at the intake. The hot air has to leave the attic in order to be replaced by cooler air. For sure a balanced system is good and you can't put to much ventilation in.

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