Helpful? 1

Does bathroom exhaust fan piping need to be insulated?

I am building a two story home (mixed-humid climate), and will be venting the bathrooms to the outside. The upstairs vent piping will run in the attic.

Is there any particular type of piping to be used through the attic (metal ductwork, PVC, etc.)? Any typed of insulation to go on or around the vent piping? And is it ok to have the vents discharge in the soffits?

Thanks for any advice.

Asked by Chris Johnston
Posted Thu, 08/26/2010 - 13:57
Edited Fri, 08/27/2010 - 10:51

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9 Answers

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1.
Helpful? 1

Chris,
Q. Does bathroom vent piping need to be insulated?

A. Yes. Otherwise the exhaust duct gets too cold when it runs through the attic. A cold duct allows condensation of moist indoor air, leading to drips from the exhaust fan.

Q. Is there any particular type of piping to be used through the attic (metal ductwork, PVC, etc.)?

A. You can use round galvanized ductwork or PVC sewer pipe.

Q. Any typed of insulation to go on or around the vent piping?

A. Duct insulation.

Q. Is it OK to have the vents discharge in the soffits?

A. That's frowned on. Best choices are (a) gable wall, or (b) roof termination. A soffit termination is possible in a pinch -- there's even a commercial flapper designed just for that purpose -- but most experts note the disadvantages (humid air is easily drawn back into the attic, and they tend to form ugly icicles.)

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Answered by Martin Holladay, GBA Advisor
Posted Thu, 08/26/2010 - 15:43
Edited Wed, 11/19/2014 - 12:15.

2.
Helpful? 0

Chris,

Does this question pertain to bath fans or plumbing vents?

Answered by Doug McEvers
Posted Thu, 08/26/2010 - 17:04

3.
Helpful? 0

bath fans, sorry I should have been more clear

Answered by Chris Johnston
Posted Thu, 08/26/2010 - 17:29

4.
Helpful? 2

I don't like roof terminations for bath exhaust fans - both because it's an extra hole in the roof and because condensation will drip right back into the fan.

The optimum termination is through a wall, with a good quality wall cap with spring damper. If the horizontal run to the wall cap is pitched downward at ¼" per foot, then condensate will drain outward. If installed in this manner, and with rigid plastic duct (with glued joints), then insulation is not necessary. Plastic is somewhat insulative and it won't rust if condensate pools in the duct.

Answered by Riversong
Posted Thu, 08/26/2010 - 19:37

5.
Helpful? 0

Followup question. Is it ok to combine the vent pipes from two bathroom fans vent them to the outside, or should they individually be vented? What about three vents?

Answered by Chris Johnston
Posted Fri, 08/27/2010 - 19:27

6.
Helpful? 0

You can combine exhaust ducts only if the joint duct and termination cap has the same cross-sectional area of all tributary ducts combined. And then, you'll have to incorporate backdraft preventers in each tributary duct so that an operating fan won't return air into a non-operating duct run. And then you'll have to increase fan outputs to compensate for the additional static pressure of the backdraft preventers and potentially increased turbulence due to mixed flows and additional elbows, wyes and tees.

In other words, don't.

Answered by Riversong
Posted Fri, 08/27/2010 - 19:40

7.
Helpful? 0

We have installed them where there is a single remote fan in the attic serving several bathrooms - single penetration to the exterior - we have had good luck with this system.

Answered by Danny Kelly
Posted Fri, 08/27/2010 - 20:18

8.
Helpful? 0

We have installed them where there is a single remote fan in the attic serving several bathrooms

But then they're no longer source removal or point-of-use exhaust fans - they effectively become a whole house exhaust system since every port will exhaust when there is demand for any one, also either making the demand port less effective since the pressure differential is shared with multiple ports or exhausting far more air than is necessary for source removal because the fan is oversized.

Answered by Riversong
Posted Fri, 08/27/2010 - 21:05

9.
Helpful? 0

Danny, you could probably wye two fan outlets together... fans in the attic next to each other a few feet from the outlet wall, adapt them from 4" to 5" and into a 5" wye that sticks out the wall. That way each room gets its own fan and there's only one hole in the wall. I have yet to install one fan serving multiple rooms, because I'm not sure customers will want a fan drawing in a room where they didn't turn it on, even an ultra-quiet fan.

Answered by David Meiland
Posted Sat, 08/28/2010 - 06:31

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