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Tiny house ventilation/low CFM fan/ flashing

tannerc | Posted in General Questions on

Hey folks, after reading the ventilation articles here I have a few questions.

Background: 30′ tiny home, new construction, about 225 square feet and 2400 cubic feet volume. All sheathing seams meticulously taped, tyvek completely taped. Located in coastal northern California. Heat source will be a small wood stove with a dedicated air inlet. I will soon install cedar tongue-and-groove siding over a 3/8″ rainscreen.

1. ASHRAE standard (just a reference point) suggests about 10 CFM for my place. Does anyone have a “gut feeling” for whether an exhaust only system would function properly without a dedicated make-up air source here? No blower door test results sadly. The lunos HRV fans look like a great balanced option but may not be cost effective in my relatively mild climate.

2. Does anyone have any recommendations for an exhaust fan that could do low-volume continuous (10 or less CFM) as well as boost enough for the TINY (25 sq ft) bathroom? So far I have only identified the Lunos RA 15-60 as an option.

3. How important is it that I get everything planned and flashed before the siding goes up? Planning to Flexwrap all duct penetrations to the tyvek. If I need to add a 6in duct later, could I cut a larger (say 10 in) hole in the siding, tape the smaller duct penetration, then find a hood that will fit snugly in the larger hole? 

Thanks again!

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  1. Yupster | | #1

    Maybe look at the products produced by Vents US. I've never used them but they look like they might have something in the range of what you are looking for. They also have some HRV's like the Lunos for a lower cost.

    If you're trying to figure out how many CFM you need for boost mode, just multiply your area (25 ft²) by your height (say 8') to get the volume of the room. 200 ft² in this case. Divide that by the CFM and the result is how many minutes it takes for one full air change. In this case, 200 ft²/10 CFM = 20 min.

    You'll kick yourself for not planning ahead with the penetrations. Finish your design, pinpoint where they are going to go, then build it. It will be so much easier.

  2. rockies63 | | #2

    I was reading articles on a site by builder Matt Rasinger about common rookie mistakes he's seen and one of the articles was on how builders flash wall penetrations incorrectly. He suggested in his blog post on plumbing mistakes (there are also blog posts on electrical mistakes and HVAC mistakes) to use products from Quickflash. This might be better than Flexwrap.

  3. GBA Editor
    Martin Holladay | | #3

    If you want balanced ventilation, you could install a Panasonic WhisperComfort ERV, which has a variety of settings (10 cfm, 20 cfm, and 40 cfm).

    Remember, too, that any ventilation appliance can be put on a timer. For example, if you want 10 cfm, you can control a 40 cfm appliance with a timer, so that it is on for 5 minutes, then off for 15 minutes, and then on again for another 5 minutes, ad infinitum.

  4. tannerc | | #4

    Thanks all!
    Martin, my understanding is that an HRV would be preferable to an ERV in my Northwest climate as I struggle to keep the humidity down in my interior in our wet winter. This understanding comes primarily from Dana's comments in this article:
    Also do you have a feeling for whether an exhaust only system would struggle for air, or is that just one of those delightful 'wait and see" aspects of home construction?

    1. GBA Editor
      Martin Holladay | | #7

      A tiny house doesn't need much air exchange to overcome stuffiness and humidity problems. While an HRV would be preferable to an ERV, either would work to achieve your goals.

      Whether or not an exhaust-only system would struggle for air depends on the meticulousness of your air sealing (and the tightness of the weatherstripping on your door and windows). If you decide to install an exhaust-only system, and you aren't getting the air flow you want, you could install a passive air inlet. (That's a remedy I rarely recommend, but it might be appropriate in your case.)

  5. rockies63 | | #5

    Martin, what timer do you recommend?

    1. GBA Editor
      Martin Holladay | | #6

      Here are two examples -- I haven't used one, but this is the type of timer I'm talking about:

      24 Hour Tork Timer

      Grasslin KM2 ST 24 Hour Timer

  6. Expert Member
    Peter Yost | | #8

    Hi Scott -

    While it does not take much ventilation for your space volume, you also have less air to deal with so ventilation is critical for an airtight small space.

    The other variable, particularly related to moisture, is the hygric capacity of your space. Many tiny homes have a higher proportion of "hard" surfaces or interior finishes that can safely hold that much moisture, so you need to be more thoughtful about moisture management from "household" sources of moisture.

    We have a Tiny House Festival here in Brattleboro VT each year and for the 1st one, I gave a presentation on moisture in Tiny Homes, driven by this example:

    If one person sleeping for 8 hours in a tiny home had no ventilation, how much would the interior relative humidity (at say 68 F temperature) rise just from the respiration of that one adult? The answer is roughly from a starting relative humidity of 30% to 80%!

    I like the idea of a variable capacity exhaust fan, coupled with either a "cracked" window or passive inlet. I have had really good luck with variable speed low capacity fans fro FanTech:


  7. rockies63 | | #9

    Hi Peter, thanks for the information. Here in BC it is a code requirement to have a 24/7 air ventilation system installed in all new construction. Would a Fantech exhaust fan coupled with a passive inlet qualify (instead of using an ERV or HRV)?

    For a small (under 900 sq') off grid home which Fantech model would you recommend?

    Martin, as to the 24/7 requirement, if a timer is used on a HRV or ERV to limit the units operation should the timer be installed after the inspection just in case it violates the code (after all, it will decrease the amount of air supplied to the building).

    1. GBA Editor
      Martin Holladay | | #10

      When it comes to code interpretation, my opinion doesn't matter. The only opinion that matters is the opinion of your local code official -- so that's the person to ask.

      That said, many homeowners use one type of system to pass inspection, and then change out their equipment (or modify the control systems) after obtaining a certificate of occupancy. It's one option to consider.

  8. tannerc | | #11

    Thanks again folks, it's much appreciated.

    Peter, that is a wild figure regarding humidity! Thank you very much for the link, I was also thinking a continuously variable speed fan would be best. My aunt and grandfather Wells live in Brattleboro, I'll have to try and make it to that festival some time!

    Martin, how would I go about properly flashing in a passive air inlet (or HRV) after the siding has gone up? Is there a good method for new penetrations in a rainscreen wall? I've been cringing watching lots of videos of remodelers just caulking a vent hood to the siding and calling it good.

    1. GBA Editor
      Martin Holladay | | #12

      The flashing details will depend on the type of WRB you have, and how much access you have. Unfortunately, there is no simple description of a single method. Ideally, you would be able to slip some type of flashing under the WRB above, and lap the flashing over the WRB below -- but often, that's not possible. Of course, if you use a liquid-applied WRB, you can't lap anything.

      Manufacturers offer a number of rubbery flashing systems for penetrations. What you use depends on the details, siding type, and access.

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