Installing bathroom vent to roof - duct run
kesler_f
June 4, 2013 in Design Dilemma
Hello everyone,
I wanted to ask the ductwork experts here for some help. I'm a homeower/DIYer here with a few questions. I'm having a roof cap installed on my roof for a new bathroom vent. I'm in the process of purchasing what i need. The bathroom sits under an attic crawl space and the duct will run through this space to the roof.

Questions
1. if the attic crawl space is already insulated, is their a need to insulate the duct? I'm using 4" metal round, no flex pipe.

2. the route for the duct will be going straight up, i read that 90 bends should be avoided when possible bc it limits flow. 45 bend are best....is this true?

3. since the duct needs to run up, should the duct run perfectly straight up or have it go up on a slope?

4. since i will be using 4" round metal duct, no flexing pipe........how does it get supported? and should it? I see pictures of flexing pipe just hanging from the roof opening to the vent unit with no support. I wonder if metal duct is installed in the same way or if it should have support?

Manu thanks in advance for those who can help.
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PRO
Harding Construction & Sustainable Solutions
Let me first address your question about insulation on the pipe. The reason exhaust pipes are insulated in unconditioned spaces, your attic, is because they carry warm and moist air from your home to outside. If this pipe is not insulated the heat carried in the air transferred to the pipe will cause condensation and potentially cause water damage at your ceiling or in your attic.

In regards to the duct, I would use two 45's to create a 90 and use rigid duct work. I would also immediately jump from a 4" to 5" or 6" duct to allow more volume of air travel. The rigid duct work, just like the 45s will decrease static pressure, the equivalent of high blood pressure, on the fan, the equivalent of the heart. If you do these things, the fan should run efficiently, use the least amount of power needed if variable speed, and run at a quieter volume.

If you haven't purchased the fan yet I would look into the Panasonic Whisper Series or other Energy Star rated fans.

If you have 8' ceilings the general rule of thumb is that you need to size the fan 1 CFM per square foot minimum for the size of the room.

I also install fans on humidity sensors and vacancy sensors instead of a switch so it takes user behavior out of the question and you don't have to worry about it running once you have left the room

Good luck!
June 4, 2013 at 8:39PM     
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PRO
Harding Construction & Sustainable Solutions
One more thing, the shorter and straighter the duct run the better
June 4, 2013 at 8:40PM     
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Harding Construction & Sustainable Solutions
Typically in steal duct work every 3 to 4' but there is a little more leeway with rigid duct work.
June 4, 2013 at 8:41PM   
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kesler_f
H.C.S.S.....thanks, this info is very helping considering i will be undertaking the duct connection/ buy materials myself to save money. But i'm having a roofer come by and cut the hole on my roof, install and flash the roof cap. I have a new vent already installed......just waiting for the duct work.

I would like to ask further advise since i'll be doing thisas a DIYer?? Do and Don't.

My main concern is proventing air leaks from bad connections, insulating and proper support since i plan on using rigid duct.

1.Regarding insulation, any comment of what R-value the duct wrap needs to be?
2.Since the duct run is going up to the roof, does the duct need to run perfectly vertical or on a slight angle to the roof. Does it make a differnce or does using two 45 to make a 90 degree bend take care of that?

Thanks again.
June 5, 2013 at 6:59AM     
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PRO
Harding Construction & Sustainable Solutions
You can use any UL151 approved duct tape tor an approved duct sealing mastic to seal all the holes and keep the connections tight. Mastic is always better. Its important you use the right materials here otherwise a hot attic with corrode the tape adhesive and fail, so don't use MacGyver duct tape.

The best way to insulate the duct would be to purchase adhesive and a roll of insulation wrap made for ducts. Spray the duct with the adhesive and wrap it with insulation twice, should be a minimum of R-5, but the requirements for your climate may ask for a higher R value.. Most HVAC suppliers will carry this and you may be able to find it at a big bog home store, ask the sales rep if they know the minimum R value on an exhaust duct.

It doesn't matter which direction the duct travels as long as there is a minimum of 1/4" rise per foot. Just don't want the air traveling down, should always have some rise or be straight up.

You can use 2" wide HVAC strapping to support the duct work. Wrap the strap fully around and then both ends to a joist.
June 5, 2013 at 7:29AM   
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kesler_f
Great!
You mention adhesive to attach the insulation. Is there an advantage to doing this vs. wraping duct tap over the insulation to secure it?? I'll be using 4" round. Otherwise, is there a specific adhesive i should look?

thanks again!
June 5, 2013 at 7:51AM   
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PRO
A Kitchen That Works LLC
I would concur with Harding Construction. One additional item you asked about, supporting the ducting, you will not likely need to support it if you are installing straight runs with rigid duct, but if you find you do, run metal straps around the duct and adhere to the rafters. Insulate first, then strap if possible.
June 5, 2013 at 7:59AM   
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Harding Construction & Sustainable Solutions
A strong spray adhesive that you can find in the paint department should do the trick. You can wrap it with tape instead but I would wrap it at least every 12" or gravity may pull the insulation off the duct. If the duct is vertical it won't need strapping but if it is horizontal I would strap it every 4' or gravity may again take over.
June 5, 2013 at 8:08AM   
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kesler_f
Hcss. Last question. Im attaching photos of the vent in the space. You mention using two 45 to make a 90 bend to go to the roof. The unit is 2 feet from exteroir wall and 2 1/2 feet vertical till it meets the slope roof.
My question is once the 45 are in place, and the vetical duct......to the roof cap.
Should i be concern the the roof cap location will be to close to the edge......about 1 1/2 feet.

Can i connect two 45 together to the unit and a vertical ( 2feet) to the roof cap?
or do i have to have a duct between the 45?
June 8, 2013 at 8:10AM   
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Harding Construction & Sustainable Solutions
About 6" of duct between would be best but two 45s together will work. You want a long sweep instead of a quick 90. This helps prevent static pressure.
June 8, 2013 at 3:19PM   
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kesler_f
And if put a 6 in. in between would put the roof cap about 1 feet from edge of wall.... is this ok or I'm I going to run into a problem. Do I need to pull it back more?
Thanks again, you have been very helpful.
June 8, 2013 at 4:31PM   
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Harding Construction & Sustainable Solutions
Happy to help. It doesn't matter if it is foot from the edge of the wall or right on the top of the wall as long as you can make all the connections tight.
June 9, 2013 at 6:00PM   
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kesler_f
Sorry, have to ask one more. regarding the duct tape, I'm having trouble finding a duct tape as you suggested.....ul151 and being mastic........some many come up when I Google. Not sure which is the right one. Would you mind posting a product link or product name? And for the duct insulation? If it something I can find at home depot....only place I can go would be best too. Otherwise product link please.

Thank you thank you thank you again!
June 9, 2013 at 7:49PM   
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Harding Construction & Sustainable Solutions
Any UL151 tape will work, the two major differences is that one is aluminum and the other is a plastic tape. You can use the plastic tape for exhaust vents. the other option is to skip the tape and paint mastic on the joints. You will be able to find this really easy at Home Depot. The sales folks at HD will be able to help. Once your in the store this will come together for you.
June 9, 2013 at 8:12PM   
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kesler_f
Hey, any tips on appling the tape or mastic?
Tape........... connect ducts together and secure with #8 screws, tape over seam?
mastic........apply on inside end of one duct and apply on outside end, connect ducts together and apply more mastic over seam and done? Or wrap seam with fiberglass tape and then apply mastic over that to cover? Or apply mastic over seam and cover with foil tape?

I ask bc I came across variations application for the mastic.......I'm leaning towards the mastic over the tape. But I'm wondering if these variations are overkill for 2feet run of duct for bathroom vent?

Would you mind informing me how you would apply the mastic?
It will be the unit, connect 45, connect 6 inch ridge, connect 45, connect 1 1/2 feet ridge, connect to roof cap. What would be you procees?

After this I should be ready to complete the installation and not bother you with any more questions :)

Thanks again for all the advise and for helping understand this process.
K
June 12, 2013 at 5:20AM   
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PRO
Harding Construction & Sustainable Solutions
You can do a variety of the things you mentioned but this is how I would move forward.

Ok install: tape only.
Better install: tape and screws.
Best install: screws, tape and mastic over the top of the tape.

The tape is really just to hold things together while the mastic dries. You can skip the tape if the seams are tight with the screws and go right to the mastic.

I would apply the mastic with an old or disposable paint brush on the exterior only. No need to mastic the interior on an exhaust vent. Most of the information you see out there on how to apply mastic to ducts relates to heating and cooling and sometimes connecting flex duct to rigid duct.
June 12, 2013 at 7:43AM   
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kesler_f
HCSS, i finally got around to start installing the duct work for the bathroom vent. I have all my suppies and an going to use mastic to seal. Heres my last question.
Does the duct route need to stay at a 90 degree or can it run on an angle. I bring up the question bc of the slope of the roof and the angle the roof cap will be in.
Do i keep the duct in line with the roof cap to connect to vent with one bend.Image #1
or
Do i try to add bents to create a 90 degree? Image #2

I say its image #1bc there is less bends, but not sure if there is a requirement to keep at 90 degrees.

Thanks!
August 4, 2013 at 8:28AM   
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PRO
Harding Construction & Sustainable Solutions
It can run at an angle. Shorter the distance to the roof the better.
August 4, 2013 at 8:31AM   
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