Our bathroom extractor fans are vented into the attic of our ~70yr old house in Philadelphia. According to our home energy auditor this is pretty common here. We want to vent these to the outside before we proceed with insulation which might involve spray foam spray foam bringing the attic into the envelope of the house.

The best solutions seem to be infeasible:

  • Go straight up through the roof. We have a slate roof and I have concerns about the aesthetics, cost, and impact on an older roof.
  • Go out through the soffit. We have minimal soffit space to install any vent, so we'd have to go through the stone wall. Doesn't sound like fun.

We have a whole house fan on one gable of the house which is not stone. This seems like the only easy place to get to the outside.

Unfortunately this is a pretty long run (25 ft plus). Assuming we use rigid and insulated pipe, how far can we run a vent pipe?

  • Any idea what the make and model of the fan is? Any elbows in the ducting?
    – Tester101
    Commented Apr 30, 2015 at 17:00
  • 1
    This answer might be helpful.
    – Tester101
    Commented Apr 30, 2015 at 17:19
  • I wish I could provide a link to validate, but am having google-fu problems. However, I read recently (fan instructions -- who'd have thunk it!) that 90 degree elbows stuck together are particularly bad. They suggested 2 feet between. Only other advice is that if your run turns out to be too long, you could use an inline booster fan. Commented Apr 30, 2015 at 23:23
  • I'm going to take a closer look at the fans. One of them seems pretty standard Home Depot or Lowes fan. Commented May 1, 2015 at 1:03
  • Whatever you do, don't vent into a soffit. Much will come back it and not fully exit the house.
    – user20127
    Commented Jul 5, 2015 at 21:17

1 Answer 1


In rooms with no shower or bath, 25' should be no problem. Just run the smooth wall pipe and go with 4" diameter.

For the rooms with showers and baths it should not be a problem if you can get it go vertical into the attic, then slope down towards the exit. Even at 25' it should not be a problem. You could even possibly use ABS pipe for the sloped run because moisture will condense in the pipe and create water. This water, however, should drain and evaporate and not cause a problem.

  • Thanks. How much of a slope is required? I can take it up 3-4ft, into a bend, and then run with a constant slope to the exit point. It's going to be special in its own way but it doesn't affect the space very much. Going to make sure that the pipe can be replaced without breaking the connector to the exhaust fans. Commented May 1, 2015 at 20:20
  • General rule for drainage in smaller pipes is about 1/4" per foot.
    – Damon
    Commented May 2, 2015 at 5:47
  • I think building codes require ducts to slope back towards the fan, not towards the outside. Though I think the reasoning behind this, is to prevent ice from building up and blocking the duct. So it may not be a major concern if you live somewhere warm.
    – Tester101
    Commented May 6, 2015 at 16:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.