Questions on Roof vent for bathroom exhaust fan


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Old 05-02-17, 12:56 PM
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Questions on Roof vent for bathroom exhaust fan

I saw this on another forum and it got me thinking. That person didn't get answers, but they clearly didn't know to post here

I have a bathroom exhaust fan which is vented into the attic. (bad, I know) We just had the roof replaced. I was planning on doing a soffit/eve vent, but I've been reading left and right that it is a bad idea and to do a roof vent.

I saw a youtube video from ThisOldHouse, https://www.youtube.com/watch?v=PqrZWd_CQIE. They talk about using a 4 1/4 drill to cut into the roof. This roof kit - Broan Roof Vent Kit-RVK1A - The Home Depot, seems to be a 4 in vent. But the instructions indicate that I should cut a 5 in hole. http://www.homedepot.com/catalog/pdf...3689e340f0.pdf

So my questions -
1. Are the instructions wrong? I can't see why it would be a 5 in hole.

2. Should I buy insulated ducting?

3. Can this be installed without a second person? I can put a nail in the spot from the inside, cut from the outside, but is it a constant back and forth? There's a spot in the video where the guy has the other guy pass the ductwork up to nail it to the roof. Any tricks to do it solo?
 
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Old 05-02-17, 01:41 PM
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Peel back and set aside 3 rows of shingles from the top. Cut your hole from the outside, install the vent, reinstall your shingles and then return to the inside to attach your duct. Make sure you take accurate measurements so that you do not place your hole directly on a roof rafter. Peel back the tar paper and look for nails holding the sheathing in place and go for the center between nail lines.
 
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Old 05-02-17, 01:57 PM
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The way I do it is from the inside I try to figure out the best place to install the vent (the higher up the better) and use a drill bit not a nail. Once the drill bit goes through the roof I disconnect the bit and leave it sticking out of the roof.
Get up on the roof and find the drill bit and remove all the shingles in that area.
A properly install vent should look like this when done.
https://www.google.com/search?q=roof...gHzYEOmnrPRdM:
Notice the shingle detail.
 
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Old 05-02-17, 03:58 PM
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I cringe when people consider punching a hole in an otherwise non leaking roof. Do you have the option of exiting a gable end or soffit? Much better and controllable.

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Old 05-02-17, 05:35 PM
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I agree about punching holes in the roof. But everything I've read said that a soffit is a bad idea. (A soffit is the overhang of the roof, right?) so the exhaust hose would basically make a right angle for the air?)
 
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Old 05-02-17, 06:17 PM
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And, what problem does that present?
 
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Old 05-02-17, 06:28 PM
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Even with a backdraft guard, moisture will come back in
 
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Old 05-02-17, 06:37 PM
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You are supposed to let vent fans run for 20 minutes after you are done in the bathroom. That will remove any residual moisture. Once that air is outside it's dispersed, it won't be coming back in. I wouldn't cut a perfectly good roof either, if I could possibly avoid it.

Question...Why didn't you have the roofers put in a vent?
 
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Old 05-02-17, 07:17 PM
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There is an attic exhaust vent. This fan was installed, without proper venting since the roof was on tap to be replaced. It was missed. My fault really.
 
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Old 05-03-17, 05:48 AM
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I deal with attic ventilation as part of my energy consulting and I'm on the fence. A well ventilate attic that is air sealed between that space and the house below should have no difficulty handling the periodic small amount of moisture a soffit vent can deliver. I say small because once outside it is mixed with outside air which immediately lowers its moisture ratio. By the time it wanders back inside it should do no harm.

The key I think is the area above a soffit vent couldn't be more ventilated so any moisture going into the attic should keep right on going.

If you have the typical amount of recommended attic ventilation I would avoid the extra hole in the roof. Then if you are concerned just keep an eye on that area as you can always go back and drill that hole.

Just to add, I think the majority of disasters are with fans vented into the soffit area as opposed to below the soffit.

Bud
 
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Old 05-03-17, 07:04 AM
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sounds like soffit isn't so bad. Any suggestions to which soffit vent to go with?

Also - how exactly would the moisture impact things? if it goes into ducting, wouldn't the moisure just follow the ducting? or stay in it until it blows out again?
 
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Old 05-03-17, 07:27 AM
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If you are confident your attic has good venting, like soffit and ridge or gable vents then you should be fine. My only concern with the unit chandler posted is the deflection of air to each side, that is right where the vents will be. I did a quick search and saw the Lambro under eave vent at many sites. It doesn't look like a Cadillac but does direct the air away from the house.

Bud
Forgot to answer your moisture question. Keep the duct sloped to the outside and bury it in insulation, being careful to not block the vent space above it. Being covered in insulation and using one of the timers as Gunguy mentioned that keep the fan running for 10 or 20 minutes after it is turned off should eliminate any moisture inside the duct.
 
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Old 05-03-17, 10:09 AM
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I cant really gauge confidence in the roof venting. There is only the roof attic fan that I know of. It is connected to a humidistat and thermostat, so it should handle humidity.

I've not heard of these timers, but I'll will be interesting. I have a combo fan/light/speaker, so 20 minutes of "on time" may have some unintended results.

I figure that with insulated ductwork, downward angle should help. What would you say is the Cadillac of soffit exhaust fans?
 
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Old 05-03-17, 12:21 PM
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An attic with only an exhaust fan doesn't work very well so there should be other vents, either in the soffits, on the roof, or other. Using humidity as a control for a fan in the attic is questionable as well. Powered attic ventilators are generally an attempt to reduce the heat in an attic not specifically humidity. And in the winter neither heat nor humidity serve as a good control indicator.

You will need to investigate what you have for ventilation. If no soffit vents then there would be little concern about moisture coming back into the attic. You should resolve your ventilation plans before you install the bath vent.

Bud
 
 

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