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Bath fan in an airtight home

Erik Olofsson | Posted in General Questions on

How do you reconcile leaky bath fan ducting within an airtight new home?

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  1. Aj Builder, Upstate NY Zone 6a | | #1

    Put in new non-leaky ducting?

  2. Dick Russell | | #2

    For any exhaust fan to work, air pushed out must be replaced by air brought in by either leaks in the building shell or ventilation air designed in. In the case of a very tight house, the latter is needed, unless you crack open a window somewhere when the fan runs. While a single bathroom fan may be able to depend on leakage through the shell (even a "tight" house leaks at some low rate), filtering makeup air through the walls often is undesirable.

    You may have heard the saying: "Build tight, ventilate right." In a heating dominated climate, the controlled ventilation air perhaps ought to come in through a heat recovery ventilator (HRV), to recover some of the heat from the air being pushed out. Houses with an HRV often draw continuously from the bathrooms and supply fresh air either to bedrooms or to the ductwork supplying warmed air to the whole house. Do a search on this site for info on HRVs and ventilation in general.

    Edit: Erik, what did you mean by your question, anyway?

  3. Erik Olofsson | | #3

    dick, what i was getting at was how to make the actual ducting and damper system to be as airtight as possible when not in use. i am considering switching the range hood fan from exterior venting to recirculating but a similar solution does not seem available for the bath fan...

  4. David Meiland | | #4

    Erik, you can install an inline damper in the ductwork leaving the bath fan, and a good quality wall cap, in addition of course to taping all the joints in the pipe. Not sure it gets any better than that if you're not going to have balanced ventilation. The damper in the fan itself it not likely to be that effective.

  5. Erik Olofsson | | #5

    david, by balanced ventilation are you referring to mechanically made-up air? if so, i plan on using some sort of hrv but this does not seem to address the intrinsic leaky-ness of conventional ducting/damper systems.
    i really do not have much experience with anything other than conventional bath fans but i would think there are some alternatives once you start looking at hrv's?

  6. Aj Builder, Upstate NY Zone 6a | | #6

    Erik, u r talking about a tiny amount of air. Humans need air. U r approaching and surpassing overkill my man. U built a tight home, pretty good already. Perfection lies at a point before zip lock tight.

  7. Erik Olofsson | | #7

    aj, that's what i wanted to hear!

  8. David Meiland | | #8

    Erik, by balanced ventilation I mean HRV or ERV, where there is ducted incoming air to replace outgoing air. Your bath fan is exhaust-only ventilation, which is what most people are currently doing. And, it works fine, at least in terms of finding air to replace what it exhausts. You can easily test exhaust fan flows and my experience is that a bath fan installed with a short, straight duct will still flow near its rating even in a tight house (something in the 2-2.5 ACH50 range, which is the best I have tested so far).

    There is no need for your exhaust fan duct to be leaky. Get aluminum foil HVAC tape and tape all the seams and joints. Foam the exterior wall penetration carefully. Install a good wall cap, like a Seiho with a damper. If you're really picky, install an inline butterfly damper in the ductwork just downstream of the fan itself. During a blower door test you will get very little air backdrafting through your fan.

    I don't know what you mean by "alternatives once you start looking at HRVs". Personally I would install a good bath fan like a Panasonic WhisperGreen even if I was going to install a HRV, because I want high volume exhaust during/after showers. A HRV is not really supposed to provide that.

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