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We need to put a hole from our attic to the outside, one way or the other. Before making any holes, I hope to hash out the details. Here's the goals:

  1. Make sure attic is properly vented

  2. Direct bathroom vent outside (it currently terminates into open air of the attic)

First objective:

I know we have a ridge vent, as I've seen the light and clearly heard the outdoors through it while working in the attic. The underside of the roofing deck is exposed, and while the roof is expected to need full replacement in 5-10yrs (there are a few layers of shingles as I understand it, so we'll need to strip them all and lay new roof), the roofing deck seems very old but in very good shape: solid wood boards tongue & grooved.

I recall being told we have sofit vents from an inspector or friend, but I need to double check. They'd be obviously visible where the roof overhang meets the exterior wall if we have them, right? Based on something on the internet, I have the idea that if I have sofit vents along the length of the attic sides and have a ridge vent, then attic ventilation is good to go and a gable might only make things worse. So, do I need to install a gable vent?

We don't have any gable vent yet. Only one gable is exposed to outdoors anyway, and on the other end the attic terminates at an interior wall of a living room that was redone to have a vaulted ceiling with exposed rafters.

Last detail is, how's the attic temperature? Well, it's hot in the summer but it's not too crazy, so there must be some ventilation. It's cool in autumn/spring and bitter cold in the winter, but warmer than outside. In the one winter we've had this house, we get some ice dams in parts of the roof, building up on the long roof overhangs, but nothing too bad yet. Maybe I should take measurements of attic temperatures before making a decision about this?


Second objective:

This is pretty straight forward: bathroom vent needs to vent outdoors. What's less certain is if that can be through ducts to a hole drilled in the gable with a louver vent covering, or if it needs to be up through the roof. I understand this depends on distance and whether we have a gable vent or not, which ties into objective 1. As for distance, the roof is within 5' of the bathroom vent's attic-side, whereas the gable is more like 15'-20' of duct work to get to (gotta go up and measure that more precisely). I suspect venting it out the gable is easier, but given it's the only outdoor-exposed gable and it's a bit far, maybe it's not the right way to do it, especially if we need a gable vent for attic ventilation on the same gable?

So: Do we need a gable vent, and if it's uncertain, how can be check to be sure? Depending on if we need a gable vent or not, how should the bathroom vent be routed outdoors?

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Gable venting is not a good idea for your attic. Gable vents work well when there are adequately sized vents on opposite sides of the attic, so air can flow across the whole interior space. This design is impossible in your attic because you only have a gable opening at one end.

Ridge/soffit venting is a good solution for your attic (and more popular than gable venting in new construction for good reasons). The ridge vent allows warm air to rise out of the attic, and the soffit vents supply replacement air from the outdoors. These vents are likely visible from outdoors, but they can be subtle; often there's simply a perforated panel installed in the soffit rather than distinct vents.

Adding a gable vent to your system would make it worse. Replacement air could come in through the gable vent instead of the soffits, and airflow would suffer in the parts of your attic further away from the gable.

From your description of the roof decking, you probably don't have a severe ventilation problem. Signs of that would include mold formation and rot in the roof decking, but your roof decking sounds fine. If you do need to improve ventilation, check your soffit venting. Are there vents serving each rafter bay? Is the path from vent to attic unobstructed? Sometimes vents are blocked by over-eager insulation installers; depending on the situation, you might be able to address the issue by installing plastic/foam baffles designed to ensure a path for air, or simply by cutting away or moving insulation. Improvements to ventilation can definitely help with ice dams, but it's hard to suggest particular improvements for your situation without locations & pictures. In particular, as you mention having a vaulted ceiling, I'd suspect your ice dams have more to do with that roof and its connection to the vented roof. If your ice dam issues are sporadic, consider getting a roof rake and clearing snow buildup in the risk areas; and when you get your roof replaced ensure these areas are thoroughly covered with ice & water membrane under the shingles.

For the bathroom vent, either approach is fine as long as it's within the manufacturer's spec for what your fan can handle.

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