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Community and Q&A

Ventilation options

JeremyArch | Posted in Mechanicals on


  I have completed some energy upgrades to my 1983 2 story slab on grade house in climate zone 6. My final blower door test revealed an ACH rate of 1.9 for 1900 sq feet. There is no ductwork (heating with minisplits) and no bathroom fans and it was recommended by my energy advisor to add an hrv/erv. I guess i am wondering if you in the community think its worth it to retrofit ductwork and install an HRV. The house isnt super tight and its a big expense/renovation to start installing the equipment.
 I guess at a minimum I should install some bath fans, but would this be enough? Also, I dont want to install the bathroom in the ceiling on the upper floor as I would prefer not to punch a hole into my attic, can most fans be installed on a wall?

 Thanks in advance for any thoughts/comments


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  1. Expert Member
    Dana Dorsett | | #1

    Bath & kitchen fans could be enough, or not depending on your sources of indoor air pollution. While not a perfect measure of indoor air pollution, monitoring the indoor air's humidity during cold weather could indicate whether active ventilation is required to be protective of the house. Ideally the house would run 30-35% RH @ 68F during the winter. If it's bumping on 40% or higher more ventilation would be a good idea.

    Ductless ventilation (Lunos , etc.) would also be an option, and probably cheaper than retrofitting ducts everywhere.

  2. JeremyArch | | #2

    Thanks for the feedback. The main floor usually hovered around 35%RH last winter the upstairs was around 40% (I did run a a dehumidifier upstairs at times) . The upstairs is where the 2 showers are for the house. I was hoping by adding some ventilation in the 2 bathrooms I would be able to drop that down a bit. In your opinion is using a pair of lunos e2 in the bathrooms a possibility instead of a standard bath exhaust fan. I see lunos has an ego but purchasing 2 of those would almost double the price of a pair of e2.
    I could start by putting a pair in the bathrooms to see how these manage everything this winter, if I am still unsatisfied i could add another pair on the main floor.

  3. Expert Member
    Dana Dorsett | | #3

    ASHRAE calls out 0.03 cfm x floor area plus 7.5cfm per occupant (defined as the number of bedrooms +1) but has another formula for discounting based on the anticipated infiltration deduced from the blower door information.

    Building Science Corporation has proposed 0.01 cfm x Area +7.5 cfm per occupant. For a 1900' house that's 19cfm plus 7.5cfm per occupant. With 2 occupants (ignoring the number of bedrooms, just counting actual residents) that would be 35 cfm.

    A single Lunos e2 is good for 22 cfm, and that would probably keep your house from hitting 40%, but would come in shy of the the BSC recommendation, and WELL shy of ASHRAE 62.2.

    Current codes (based on ASHRAE 62.2) calls out 20 cfm continuous ventilation for bathrooms that don't have exhaust fans controlled by a wall switch.

  4. JeremyArch | | #4

    Thanks again Dana, so it seems the lunos e2 would meet the continuous ventilation based on Ashrae 62.2 with 22cfm. Any recommendations for a bath fan that can be installed in a wall and then duct to the outside? I see a lot of ceiling only installs here in canada, and some direct through venting fans.

    1. Expert Member
      Dana Dorsett | | #8

      No, every PAIR of Lunos e2 would meet ASHRAE requirements for just one bathroom, but would fall well short of the whole-house requirement. But since each half of the pair is only 11cfm, it doesn't meet the ASHRAE bathroom venting recommendation.

      A bath fan on a switch capable of 5 air changes per hour for the bathroom volume would be a good way to limit the moisture buildup from showering. Putting it on a timer or humidity sensor would be even better, to purge post-bathing moisture from the house for at least 15 minutes after the event. If you buy a pair of Lunos, put them where people will be spending more time, not the bathrooms.

      1. JeremyArch | | #12

        I see, so a lunos e2 would not work in each bathroom and would need a dedicated bath fan. By the time I pay for a pair of lunos and a couple decent bath fans im getting closer to the cost of a balanced system that would service more rooms.
        Looks like i have a couple bulkheads and some drywalling in my future!

  5. Jon_R | | #5

    Ventilation is about not breathing polluted air and humidity is a horribly inaccurate way to gauge this. Your house is tight enough to need mechanical ventilation.

    A centralized ventilation system and open interior doors will save money and be better than what you have.

    The TwinFresh Comfo may be useful where you want to close doors.

    I haven't done the calculation, but I'd guess around 70 CFM running whenever the house is occupied.

    1. Expert Member
      Dana Dorsett | | #10

      To be clear, the humidity measurement is only to gauge the relative risk to the house, not the occupants. If as reported it has been hitting 40% & higher in winter it needs some ventilation just to keep the springtime mold spore counts down in the house.

      For ASHRAE 62.2 whole house ventilation on a 2 bedroom (presumptively 3 occupant) house would be (0.03 cfm x 1900' ) + (3 x 7.5 cfm)= 80 cfm.

      But that usually results in overly dry indoor conditions in colder climates, which is one of the reasons BSC uses only a 0.01 cfm per square foot multiplier.

  6. JeremyArch | | #6

    Thanks for the comment Jon. I am just looking for options and a centralized system may be the way I go. My goal would be to minimize disruption to the interior house but if the other options arent as good or prove to be to expensive, i will. I guess i wasnt sure if 1.9ACh is really "tight" enough to require it. I read alot on here of 1ACH or less where it seems it is mandatory. Im happy i was able to tighten my house to where it is, but its still a good way from being a super tight high performing house

    1. Expert Member
      Dana Dorsett | | #11

      The 3ACH/30 requirement in the IRC is also the threshold of where active ventilation is also required by the IRC, so yes, at 1.9ACH/50 you're in the range, as is evident from your wintertime indoor relative humidity.

      Even leaky houses need ventilation- there's no telling just how clean the incoming air is from unknown paths, or whether the ventilation is being delivered where its needed the most. There's no particular magic to 5ACH/50.

  7. Jon_R | | #7

    You are well under the 5ACH@50 where you might consider not using mechanical ventilation. 2018 IMC, 401.2.

    Once you fix this, I doubt you will have problems with Winter humidity being too high.

    1. JeremyArch | | #9

      Thanks Jon, I didn't realize 5ACH@50 was when to start considering ventilation.

  8. JeremyArch | | #13

    I appreciate both for your feedback. I plan on being in this house for awhile so i just want to make the best decisions i can. It will be great to have some justification when informing my wife of the upcoming expense and mess.

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