Where to cut roof vent for range hood

grandmumSeptember 5, 2013

Hooking up a range hood. I have a roofer coming to cut the hole in roof for the exhaust cap. The duct work my son is going to run, roofer will not do that portion.

What the best way to assure the hole is cut in the best place for my son to run the ductwork? Should he construct the duct work in advance or allow the roofer to determine the best place accoring to the roof and then run it?

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snoonyb

If you have penetrated the ceiling, cover the hole to prevent debris created by the roof being cut, from falling through.
Select a center point between two rafters, as close to vertical, above the ceiling penetration, and drive a 16p nail through the roof sheating and the roof material to mark the location.
Inform the roofer of the method.

    Bookmark   September 5, 2013 at 11:10AM
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rwiegand

Consider venting out a side gable wall rather than making a hole in the roof with all of the attendant problems of roof penetrations. If it needs to be on the roof consider what the vent looks like from the outside-- think about putting on the back side of the roof or a less seen side rather than front and center.

I think it's better to upsize the fan and duct diameter a bit to accommodate a longer run than install an eyesore in the middle of your roof. Straight up is most efficient, but may not be best.

    Bookmark   September 5, 2013 at 1:14PM
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grandmum

Thanks for the tips. It has to go thru roof, its in the back of the house and looks OK aestically.

There is a 3/14x10" piece of ductwork already in place for the old exhaust. We plan to add the adapter for a 6" round.
Im guessing to mark the hole in the center between 2 joists. The roofer will put cap in with tailpiece. We will only need about 32" of duct between roof and adapter if we go directly above.

Would flex aluminum suffice for such a small run (im guessing no).

also:

There is some insulation touching the 3 1/4"x10" ductwork as it enter the attic space thru the wall cavity.

Do any provisions need to be made for clearance from insulation when using standard single wall sheet metal? Again this is for basic range hood kitchen exhaust.

    Bookmark   September 5, 2013 at 2:53PM
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snoonyb

There is no need to separate the insulation from the ducting as it is not connected to a gas fired appliance.

Do not use flex-a-duct as grease will accumulate.

    Bookmark   September 5, 2013 at 8:33PM
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grandmum

We are having a hard time finding a quality vent cap for this.

Would it be best to use one that has a tailpiece that comes into the attic or will one that the 6" pipe itself goes thru the roof?

Can anyone suggest what we should be looking for? Not much to choose from at the home centers. Or just trust what the roofing contractor will have on hand?

    Bookmark   September 6, 2013 at 7:53AM
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snoonyb

Since aesthetics are not a concern, the standard "T" top which the vent pipe fits into, and is the least restrictive, will suffice and in all probability, what the roofer will supply.

    Bookmark   September 6, 2013 at 9:23AM
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grandmum

Thanks, I can picture a "T-Top" vent. However, do they have a damper or screen. We are in a cold climate and squirels and birds are a concern.

The manufacture for the range hood calls for their roof vent with damper and screen (Broan 634). I purchased one but may return it if there is a better option.

Take a look at the link and tell me if you think it will last and maybe suggest a way to secure the screen in there better (it just is held into place without screw or glue, squirel could push it thru maybe. )

Here is a link that might be useful: broan roof vent

    Bookmark   September 6, 2013 at 10:16AM
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snoonyb

After closer inspection you'll find that the screen has been spot welded.
Even on the street side it wouldn't appear offensive when painted to correspond with the roofing material.

In your case, where you may experience an occasion of blowing snow, the roof damper is a good idea. I'd keep it and have the roofer install it.

Just as a point of clarification, all of the venting appliances I install have the damper at the appliance and any bird, rodent, or leaf intrusion prevention are fabricated on site and included in the total price of the installation.

    Bookmark   September 6, 2013 at 1:48PM
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grandmum

Thanks again for your help, much appreciated.

    Bookmark   September 6, 2013 at 3:32PM
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grandmum

I had to remove some insulation to get at the 3 1/4"x10" duct that comes from my kitchen into the attic. There is a dead space in the chase in the wall where the duct enters the attic.

Do I just lay some insulation flush with the rest of the attic insulation on top and leave the dead space empty or do I have to stuff insulation into the chase?

    Bookmark   September 6, 2013 at 3:40PM
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snoonyb

Insulating the chase is not necessary, however, it can act as a vibration damper.

    Bookmark   September 6, 2013 at 4:10PM
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grandmum

So just lay it flat with the rest of the stuff paper side down. As mentioned before no need to worry about insulation touching ductwork.

You have been a great help my friend.

    Bookmark   September 6, 2013 at 6:35PM
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snoonyb

And the price was right.

    Bookmark   September 6, 2013 at 6:45PM
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grandmum

Hey thanks again, the job is complete. Roof vent looks nice and range hood duct is functional.

However, again about insulation.

I have unfaced R30 insulation within 6-8 inches of the duct coming out of the ceiling joists. Should I place the insulation tight up against the vent or keep the gap?

    Bookmark   September 22, 2013 at 11:20AM
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snoonyb

Right up against it is fine.You are not direct venting products of combustion so there is no danger of fire.

    Bookmark   September 22, 2013 at 11:54PM
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grandmum

Thanks again. I went back up today and put it in contact with the vent pipe.

Now the insulation covers the ceiling bypass that the vent comes into the attic from.

My best regards!

    Bookmark   September 26, 2013 at 12:51PM
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