Retrofitting bathroom exhaust fan

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Old 09-12-09, 05:57 AM
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Retrofitting bathroom exhaust fan

I'm about to retrofit exhaust fans into two adjacent 2nd floor bathrooms. Please assist with any do's and don'ts, and any gotchas that I should look out for. Especially regarding the roof vent and cutting the roof. The forum search function was giving a database error.

My plan:

- Two fans, separate 4" hard ducting, real aluminum duct tape, insulated, venting out to roof

Q's:

1. Is hard ducting really required over flexible? The big boxes sell a retrofit kit that comes with flexible ducting (obviously for versatility of installation). I will have to piece everything together by going hard ducting.
2. I'm assuming one roof vent and using a Y adaptor is a no-no?
3. The roof vent will be new territory for me. How far apart must the roof vents be, considering the two bathrooms are adjacent?
4. Do I need to caulk the roof vent to the plywood? Between the vent and the shingles?
5. Anything else, ESPECIALLY related to the roof vent?

Thanks!
 
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Old 09-13-09, 11:38 PM
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1. Is hard ducting really required over flexible? The big boxes sell a retrofit kit that comes with flexible ducting (obviously for versatility of installation). I will have to piece everything together by going hard ducting. Rigid ducting is preferable although a short section of flexible is okay for vibration isolation and to make fitting up easier. The ductwork should be insulated to reduce the chance for condensation.
2. I'm assuming one roof vent and using a Y adaptor is a no-no? That is correct; two fans need two separate roof jacks (the proper term) for exhaust.
3. The roof vent will be new territory for me. How far apart must the roof vents be, considering the two bathrooms are adjacent? They can be as close as you desire although putting each one in its own "bay" (between a pair of rafters or trusses) is probably preferred for roof strength.
4. Do I need to caulk the roof vent to the plywood? Between the vent and the shingles? No, you follow the instructions that should be included with the roof jacks. Generally, the upper edge of the roof jack slips under the row of shingles above the jack and the lower edge is nailed through the sheet metal and shingles. Use some roof cement over the nail heads. If you are not sure you can run a bead of roof cement under the the outer edges of the roof jack on the sides only and also run some cement over the outside on the sides. Do not seal the bottom of the jack.
5. Anything else, ESPECIALLY related to the roof vent? The above information applies to asphalt shingles. If you have wooden shingles or any other type of roof you may want to get a roofer to install the roof jacks. A good way to find a roofer for this tiny job is to see if anyone in the neighborhood is having a roof replaced and then ask the foreman if he or one of his workers wants to make a few bucks on the side. If you have a steep roof or anything but asphalt shingles this is the best fifty or hundred bucks you can spend.
 
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Old 09-14-09, 07:10 PM
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Yes, furd covered it. My two cents from my files: The roof mounted exhaust is 5-10% more efficient at removing warm, moist air (hot air rises) than a soffit mounted one. Roof terminal should have a pipe neck attached for a positive connection. Use straight pipe (metal or PVC) whenever possible, wrapped with insulation and a vapor barrier. Corrugated, flex pipe creates turbulence and has almost double the surface area for water exhausted from baths to collect on. Use as few bends as possible with 45*s instead of 90*. Tape metal joints with silver tape, not duct tape (4 year life). Tape the individual joints of each elbow. Use 3 screws on each metal joint. Use HVAC black tape for plastic covered insulation. Insulate the whole pipe and the fan box. Use caulk to seal the small gaps at the fan to wallboard joint. Let no heated moist air escape into the attic which causes mold and mildew, wets insulation (defeating its purpose) or possibly making frost in your attic and ice dams on the roof. Notice the increase for accordian duct: Bathroom Fan Sizing
Be safe, Gary
 
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