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Bath exhaust fan pulling air from basement?

whitenack | Posted in Green Building Techniques on

Hey guys,

We have been in our “Pretty Good House” for about 9 months now and still dialing in all the systems. Our blower door test was a .08 ach50, and we still have some holes to fill in the basement where the hvac line sets were cut through the walls. We have panasonic whispergreen bath fans that I have been fiddling with to get the right amount of continuous fresh air.

We are currently in the extreme humidity portion of the year here in central KY, with humidity in the 80’s and 90’s for many days. The new construction is still drying out (hopefully) , and our indoor humidity is in the high 50’s to low 60’s with help from our AC. To combat the humidity, I have only one bath fan running continuously at just 30cfm to limit the amount of humid air entering the home but to still provide fresh air.

The past few days, I have noticed an area in our house by our door to the basement that seems a little warmer and humid than the rest of the house. It also has a smell that is consistent with the current basement smell…not moldy/mildew, just the dusty, post-construction, unfinished basement smell. It is obvious to me that there is basement air passing under the door and into our living space. I have to believe our bath fans are contributing to this.

I have a few options, and would like some thoughts.

1.) Put weather stripping under the door to limit the amount of air being pulled into the living space. This is the easiest.

2.) Turn the bath fans off completely during this extreme time.

3.) Provide a source for more makeup air in the living space. Don’t like the idea of creating an opening in the living space

4.) Switch to a supply or balanced ventilation strategy instead of my current exhaust only choice. Don’t like the idea of this from a design and expense standpoint.

5.) Put an exhaust fan in the basement to draw conditioned air into the basement instead of pulling basement air into the conditioned space. I already have a pipe leading from the basement to the outside for a passive radon system, so I could incorporate a fan into this and exhaust the basement as well.

I kinda like the idea of option #5, because it would also help with moving some fresh, dry air into the basement and help with the air quality down there. It would also help with getting more aggressive with radon removal. I’m not sure we have a radon problem in this house but radon in a problem in our region.

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  1. whitenack | | #1


    6.) Get a couple of dehumidifiers and run them in the basement. This would help with the humidity difference as the air is pulled into the living space, but wouldn't help the smell.

  2. Expert Member
    Dana Dorsett | | #2

    Exhaust-only ventilation in house that tests at 0.08 ACH/50 (are you sure the decimal point is in the right place?) is a bit silly. Even at 0.8 ACH/50 it's problematic, since there is no defined entry point for the ventilation air, and can be coming from less-than-clean areas, and doesn't direct the ventilation air where it's needed most.

    Exhaust-only ventilation depressurizes the house, not the slab, and tend to draw MORE radon and other soil gases into the house. How "fresh" is that "fresh air", really?

    Switching to supply only would give you the same amount humid air into the conditioned space, but you'd know that it's at least as clean as the outdoor air, and it won't aggravate a radon problem.

    Option #5 would potentially back draft your kitchen & bath exhaust, which isn't always an ideal path.

    Option #1 is worth doing, but you still don't know where the make up air is coming from, only that less of it is coming via the basement.

    If you don't smoke and don't fry a lot of food, don't use aerosal sprays indoors, a combination of #1 & #2 during periods when the outdoor dew points are in the 70s F, turning on the appropriate fans only when the baths or kitchen is in use.

    The second half of #4 (or at least a partial implementation), BALANCED ventilation is the "right" solution in a tight house. A pair of ductless Lunos e2 HRV on the first floor (locating each half where the ventilation is most useful) would allow balanced ventilation for up to 22 cfm, and when you need more ventilation than they are providing you can run the bath fans. By their mere presence become a large fraction of the make-up air for the bath fans when they're running, providing a known path. The additional unbalanced cfm induced by the bath fans will cut into the heat recovery efficiency, but it will still be doing some heat recovery.

  3. brp_nh | | #3

    I'm assuming your blower door test was 0.8 ACH50, but either way, the advice might be the same. We built a house in NH (zone 6) with PGH like specs. Our ventilation plan was bath fan exhaust (Panasonic WhisperGreen) with passive air inlets (Panasonic). I can probably offer some good advice based on our experience.

    Our house is about 1300 sq ft conditioned space and we have a Panasonic WhisperGreen in each bathroom, a model that can run as low as 30cfm. Final blower door tested at 0.38 ACH50. Our initial plan was to run one fan at 30cfm continuous and install two Panasonic passive air inlets (rated 12-18cfm each). Our final Energy Star inspection (includes a test to make sure the fan can do at least 50cfm) showed that this arrangement doesn't work for such a tight house. In fact, the passive air inlets can't really do 18cfm, they are more like 10cfm each. Even running one fan at 30cfm was slightly de-pressurizing the house. We eventually installed a 3rd and then 4th passive air inlet (one in each bedroom and two downstairs). This arrangement now works ok because the continuous 30cfm fan gets enough air. Boosting one fan to 80cfm is ok when windows are open, but de-pressurizes when the house is closed up. In our cold climate, the winter indoor humidity can get a bit low, down towards 20%.

    Based on our experience, exhaust ventilation with no official makeup air in a house at or below 0.8 ACH50 just doesn't work. You should continue your basement air sealing of the HVAC holes and run a long term radon test (try this, My take on your options:

    A) Open windows for makeup air or turn off bath fans. Neither are good options.

    B) Install enough passive air inlets (living area, bedrooms) to at least allow enough air for a continuous 30cfm or whatever your base continuous cfm should be. The older continuous ventilation standard of (7.5 cfm/person) + (1 cfm/100 sf) seems more reasonable than the new one. Start with one or two passive air inlets, evaluate, possible do more. The install is a bit of a pain once siding is already there, but not too bad. This option would be the least costly (especially if you install) and I think would work ok in a KY climate.

    C) Install something like the Lunos e2 for continuous ventilation. Use bath fans as needed, but you still might get some de-pressurizing without windows open. Expensive, some wiring work, and the Lunos are big 6" diameter tubes.

    D) Install HRV, possible a big expensive retrofit.

    I think option B would be worth exploring the most. Let me know if you have any questions.

  4. whitenack | | #4

    Thanks for the replies!

    OOPS! I guess .08 ACH50 would be like living in a plastic sack? Haha! So I got back to my notes and see that it was .08 ACHn, not ACH50. I also found my cfm50 rating, so I did some math to find my ach50. Looks like it is right at 1.0 ACH50.

  5. lance_p | | #5

    Agree with others - 1.0 ACH50 is pretty tight to rely on an exhaust-only ventilation strategy.

    If it's hot outside with high humidity, ventilation air you draw in is FULL of moisture. This will elevate the indoor RH and put added strain on your AC. Air at 95F and 65% RH has a dewpoint of 82F, meaning any incoming ventilation air will be condensing its moisture as it enters your house. This can lead to problems and could have something to do with that "basement" smell you're getting. With an exhaust-only ventilation strategy there's a chance some of this condensation is happening inside your walls, as air will be drawn in through tiny gaps everywhere. Here's an easy to use Dew Point Calculator:

    An ERV can cool and dehumidify incoming air by passing heat and moisture to the cooler/drier outgoing air, which has the added benefit of reducing the load on your AC and making it run more efficiently. The energy savings will not be huge, but should be kept in mind when considering the cost of balanced ventilation. If your basement is unfinished a basic ERV install shouldn't be overly complicated or expensive.

    I just happen to have the spec sheet handy for Panasonic's Intelli-Balance 100 ERV, and they claim 73% Total Recovery Efficiency with incoming air at 95F (humidity not specified). It's about $800 and a competent DIYer could install it themselves. It also has individual intake/exhaust fan settings that allow you to pressurize (summer) or depressurize (winter) your house. You'll get added humidity control if you're able to get the exhausts located high on the walls in damp areas like near showers and your stove.

    Dedicated dehumidification should also be considered. At night when your ventilation is working but your AC is not, indoor RH will rise. Ideal indoor RH is between 40-50%, which should be a reasonable goal given the tightness of your envelope. We've had a real hot spell here recently (for us, anyway) hitting 95F and +60% RH regularly over the last week and a half. Our very average house (read: leaky) was easily kept below 50% RH with a portable 60 pint dehumidifier running in the basement along with our AC.

    Lance Peters

  6. GBA Editor
    Martin Holladay | | #6

    I agree with the comments that advise you to install a balanced ventilation system (an HRV, an ERV, or Lunos fans) instead of an exhaust-only system for a house as tight as yours.

    Here are links to some relevant articles:

    "Designing a Good Ventilation System"

    "Revisiting Ventilation"

    "Ensuring Fresh Air in Bedrooms"

    Finally, I disagree with Dana's conclusion that exhaust-only ventilation systems are associated with increased radon levels. For more information on this question, see "Exhaust-Only Ventilation Systems and Radon."

  7. tech1234 | | #7

    Great info guys! I don't have much to add as I am still building my PGH in NH.

    Brian P- I would love to chat with you about your build and what you would and would not recommend in our exact climate. If you're interested post up and I'll shoot you my email

    James Walters

  8. GBA Editor
    Martin Holladay | | #8

    I assume that "Brian P" is Brian Post, the author of a series of GBA blogs. To learn more about Brian's project, read his blogs. The first one is "Building a Small House in the White Mountains."

    Subsequent blogs in the series can be found by clicking the links in the "Related Articles" sidebar on that page.

  9. brp_nh | | #9

    James, no problem. I think Martin has offered to connect people who don't want to share their personal email online. I think you can send him an email at [email protected], he should have my email on file and can connect us.

  10. whitenack | | #10

    Hey all. Thanks for all the replies. Looks like I need to do some more study/research.

    I think the decision is going to come down to whether I want to just add some passive air inlets and use my existing whispergreen bath fans as the power source, or whether it is worth it to spend the money on another unit to supply the fresh air.

    Brian P., thanks for the link to Airthings. I didn't realize there were IAQ monitors that measure radon.

    Did you guys see that the latest option (Wave Plus) claims to monitor Air Pressure? I'm not sure if that means it monitors the outdoor barometric pressure (like what you see on the weather reports) or weather it measures the indoor pressure (like whether you have more exhaust vs. supply). I reached out to Airthings customer support and they claim it is the latter. Does that sound possible?

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