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Question DetailsAsked on 9/16/2013

Roof ventilation: what problems might arise if net free area ratio is lopsided toward intake?

We have a small ranch with hip roof that needs new roof and new soffit venting. Our research tells us we should be aiming for something like a ratio of 60/40 intake/exhaust. My concern is, if we use perf vinyl soffits, will that be too much intake for ridge vent? Is overkill on intake ever ok? We've read that hip vents are a bad idea. Yes? No? Should we be looking at different soffit vent material, something with less net free area?
The outside walls of the house measure 24ft x 38ft, the ridge measures 13ft, and the soffits are 2ft wide all around the house.

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2 Answers

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The bad thing to have, as you seem to have discovered, is less intake area than outlet area, because that means the exhaust system will be intake-limited - you will not get as much air flow through the ridge vents as you could, and it will also put a slight vacuum on the attic when heated air is moving, resulting in it drawing air from the house below through vapor barrier penetrations.

The 60/40 ratio rule of thumb first appears in the literature, as far as I have seen, in a 1950's study in the Northeast that found, WITH normal "open" ridge vents (as opposed to mech type) and eave baffles installed in the eaves to prevent blocking of the eaves with insulation, that you actually needed a 60/40 ratio to overcome the eave/baffle zone friction and have a "balanced" system where intake friction equals outlet friction.

There is nothing wrong with MORE eave opening area as long as it is screened against insects and protected from rain entry. However, you can eventually reach the point, in theory, where the airflow would be so slow that you would not have ventilation occurring near the wall, where there is commonly a fair amount of the escapee air from the house, through attic penertration of wiring and leakage up through the wall envelope itself. You also want a moderate flow at the wall to prevent hot air from the outside wall surface, from sun exposure, from sitting in the eave area without moving, or damp air on rainy days condensing in the insulation and baffles, but that would probably take a ratio of about 80:20 or higher to happen. Therefore, in practical terms, leaving eaves wide open except for screening, as is common in many houses (mine included), causes no problems, and no mater how much open area a soffit cover has, the eave opening into the attic is usually going to be the restricting point for the airflow anyway.

Hip vents combined with ridge vents are generally a bad idea IF they can intercept the airflow to the ridge vents, because they can act as a short circuit, much like a gable vent can. If the hip area is isolated or in gable houses where a large portion of the attic has no ridge vent, however, then venting with hip vents IS called for. Commonly, only the top third or half or so is slotted through the sheathing to provide a high-point vent and the rest of the gable vent is just dummy vent, running the rest of the way down the hip for appearance purposes but not providing any ventilation, though I have seen them commonly cut of at the end of the active ventilation zone to prevent backup of water from deep snow accumulation or wind-blown water that could get in there and cause a permanently damp condition.

In your case, with the ridge being only about 13 feet long or 1/3 the total house length, I personally WOULD install hip venting for the top half or so of the hips, to ventilate the end thirds of the attic.

Answered 6 years ago by LCD

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Thank you for taking time to answer. Much appreciated!

Answered 6 years ago by Guest_9915459




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