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7 Posts
Go with the ridge vent

A ridge vent (properly installed) is the best option for almost any gabled roof. It's passive, has no moving parts, needs no electricity, works day and night.

My research suggests that Shingle Vent II is currently the best, and that's what I'm getting for my 2300 square foot "loft bungalow".

A ridge vent is perfectly situated to evacuate the most heat from your attic, because:

a) it's at the highest point and,

b) it spans the whole top of your house (attic)

All it needs is the same thing that any other type of roof vent needs, to work effectively:

- enough INTAKE ventilation from the bottom edges of the roof and,

- an unobstructed path for air movement from eaves to ridge, between each and every pair of trusses or rafters.

Think about it.

Hot air rises. Its natural tendency is to exit through a hole at the very top of the enclosed space (in this case, a long narrow slot), as long as compensating volume of air is allowed to enter from somewhere else in that space to keep the flow going. That flow will happen. It's basic physics. Problems and sub-par functioning occur when:

a) Your inlets are higher than the area of the eaves - hot air in the attic still rises to exit the ridge vent, but it's mixed with outside air that came in partway up the contained volume; basically, you are short-circuiting part of the flow that you need. The air that enters a high inlet vent does not pass through the full volume of the attic and so does not pick up heat from that full volume. It competes to some extent with the large volume of overheated air that it bypasses.

b) The venting is unbalanced - a two-inch wide slot along the length of your roof needs the equivalent of two one-inch wide slots, one at each eave, the full length of the roof. Any less than that much input surface area results in less-than-optimum exhausting. You can have _more_ input venting than output, but you should never have _less_ input vent area.

c) The lower venting is intermittent. The best flow is from a full-width slot at the bottom feeding up toward a full-width slot across the top. If you skip the lower-level venting between some pairs of trusses or rafters, then you allow that entire area of roof deck to remain much hotter than the areas where air is moving bottom to top and drawing heat with it.

Mushroom and tower vents (like Venmars) are limited by their locations, in two ways:

a) They are not quite at the top of the roof, so there's always a dead (hot) space above them, unlike a ridge vent.

b) They leave large swaths of roof unventilated, therefore large areas inside the attic are stagnant or have less-than-ideal movement of hot air. Usually, there's a six-to-twelve-foot strip up each side of a roof that gets poorer venting because the tower vents (or worse, the poor flat little mushrooms) are grouped more toward the middle... guess where the shingles start to fail...

Power vents have their own problems. Lower-level input venting is just as critical for them as for any other style of vent. They make up, somewhat, for not being the full width of the roof, but they have moving parts and they need power.

If you don't have your intake vents low enough (many people install a power vent on the roof, but leave their gable vents uncovered), you have an exaggerated version of the short-circuiting air-flow problem. Air comes in high, flows directly to the power vent, and gets blasted out. You get a small hurricane between the gable vent and the power vent, leaving the rest of the attic space stagnant. And hot. Rain and dirt are sucked in along with the unintended high-entry-point input air from the gable vent. The same can happen if you leave old mushroom vents in place. They need to be removed (and the holes sealed, of course) for the same reasons.

People with poor or NO lower-edge input vents make the mistake of installing a power vent, thinking (mistakenly) that if it's got enough power, it'll be able to overcome the inconvenience and just suck any hot air out of the attic... the brute-force method. Doesn't work. Invariably the houses with no proper lower-edge inlet vents are old and leaky. So the power vent in the attic sucks mightily and the input air has to come from somewhere, so it gets sucked out of your house. The house becomes a low-pressure zone. This whole arrangement is bad for three reasons:

1) Outside air (usually hot air) rushes in through every crack and crevice, heating your indoors when you least need extra heat in there.

2) Household air being sucked up into the attic carries moisture from cooking and bathing. That moisture gets into the insulation and/or it condenses onto the underside of your roof deck when nighttime cools the outside. Mould and rot happen.

3) A power vent that isn't matched with sufficient, proper input venting is under stress, and its motor burns out.

Go with a ridge vent and find some way to get the same amount of unobstructed input venting to match. Proper attic venting must be balanced. That product you mentioned sounds like a good one to try.

For the ridge, I like the Shingle Vent II because it's got good baffling (testing showed no entry of 110 mph horizontally blown rain), because it works as well as the others in still-air (windless) conditions, and it has that extra deflector to provide additional suck (Bernoulli effect) whenever a bit of wind blows across it.

Good luck. Don't take my word for anything. It's not gospel. But if it makes sense and meshes with what you hear and read elsewhere... take the hint.... :wink:

- kevin

7 Posts

Lots of triangle and geometry sites on the web.
For example:

For that one, you'd specify angle "A" as 22 degrees, and side "b" as the ceiling of your kitchen. The calculator fills in the length of "c", the roof. But of course, add some extra length to "b" to account for overhang/eaves.

If you meant the height of the rafters, rather than their length, then that's a different story. Two-by-twelves on sixteen-inch centers oughta be plenty tough enough. Or engineered joists/rafters. Whether you add additional depth would depend on how much insulation you want to apply. But leave air space between insulation and roof deck, bottom-to-top - from the soffit to where your new sloped roof will join the wall of the house. Then provide exit ventilation. Second best would be several flat mushroom vents just below where the new roof meets the house. Best would be a "vertical wall flashing vent" located AT the join line - similar to a ridge vent, but adapted to installation where a roof butts against a vertical wall. This will keep your kitchen from turning into an oven in summer, and will keep your shingles from cooking and curling.

Sounds like a project.
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