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Community and Q&A

Questioning the Need for Mechanical Ventilation

JohnsonDesign | Posted in Mechanicals on

After reviewing the informative articles on the site, I’m still struggling with the necessity of mechanical ventilation in our circumstance.

We are building a 1500 ft2 home and 500 ft2 guest house in the arid high desert of southern Utah. In the coldest part of the winter, daytime temperatures are in the low 40s and in the hottest part of the summer, daytime temperatures are in the 90s. Average annual rainfall is just shy of 11” and snowfall is around 30”. Relative humidity caps out at 60% in the coldest part of the winter and bottoms out in the summer at 20%. The buildings are well-insulated: 2” Zip R sheathing on the walls with 2×6 framing and 14” TGIs (no exterior rigid foam). We plan on 1-2” of closed cell foam in the walls plus blown in fiberglass. In the un-vented roof, 2-3” of closed cell foam plus blown-in fiberglass. Heating and cooling will be mini-split, plus a wood stove in the home. The buildings will be occupied more or less continuously from mid-March through September and intermittently the rest of the year. In the summer, it gets cool enough at night that windows are typically left open.

Our HVAC contractor doesn’t think mechanical ventilation is necessary. What do you think? If you recommend it, HRV or ERV, and how big?

As an aside, given the tight seal provided by the Zip R, would you skip or reduce the closed cell foam in favor of more blown-in insulation?

Brett

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Replies

  1. Walter Ahlgrim | | #1

    More than a few people are likely to disagree with me but I think HRV/ERV is an over hyped product.

    There is some time and places where they are a necessary but most houses in most locations will be fine without one.

    If you build a 700 square foot .4 HCH passive house on the Arctic Circle and have 5 people occupying it all day long. The humidity in that house will get high enough that the very best windows on earth will be covered in condensation all winter long without an HRV.

    Other people are convinced they will suffer brain damage from low oxygen without an HRV installed in 100 year old Florida beach shack.

    When I look heat exchangers it seems to me the larger the temperature differential the more efficient and effective the unit is and at 15° of less they don’t seem likely to do much. The ventilators claim to recover 90% of the energy and it is likely true when it is -40 outside and 80 inside giving them 120° of differential to work with. My guess is when it is 100° outside and 85° inside they don’t get a 90% recovery.

    I say if the air in your house feels stuffy crack open a window. I doubt excess humidity is a big problem in Utah.

    Walta

  2. GBA Editor
    Kiley Jacques | | #2

    Read this short piece from building scientist John Straube:
    When is a House Tight Enough to Need Mechanical Ventilation? Number of occupants, blower-door test results (the airtightness factor), and indications of condensation can help inform your decision. (A good rule of thumb: If the blower-door test value falls much below 5, add mechanical ventilation.)

  3. Expert Member
    Akos | | #3

    It doesn't take much care to build a sub [email protected], sometimes even without trying the builder will hit that target. This means the house will most likely need ventilation, in your climate you can open windows as humidity is low. There is an energy penalty and extra heating cost for this, depending on your local energy costs, energy recovery (ERV/HRV) might or might not be worth it.

    From my point, something like a Panasonic WhisperComfort ERV costs only a couple of hundered dollars more than a good quality bathroom fan. Install is about the same as a bath fan, so it is a no brainer for most new builds.

    The nice part with an ERV/HRV, is you can leave it running on low speed when the building is unoccupied to keep the inside air fresh.

  4. JohnsonDesign | | #4

    Thanks for the input, folks. Unfortunately, pretty evenly divided, which is what I feared! Any thoughts on the Zip R plus closed cell foam question?

    Brett

    1. Expert Member
      Akos | | #8

      Spray foam on the inside of ZipR doesn't make much sense. Because of the thermal bridging of the studs, it would barely budge the assembly R value plus with an already sealed sheathing like ZIP, it won't do much for air leaks.

      You do need the SPF for the unvented roof for condensation control. Make sure to follow the correct ratio of cc SPF to fluffy here:

      https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies

      P.S. As someone that has lived in houses with proper mechanical ventilation, I would not want to live in one without it. Coming home, opening the door and not smelling a stale house as you walk in is worth the cost.

      1. JohnsonDesign | | #9

        Thanks for your advice. I think I'll skip the spray foam in the walls and go with 3 - 4" in the ceilings. For 14" TGIs, the that should clear the 30% foam requirement for Zone 5B.

  5. Trevor Lambert | | #5

    Every new build should have mechanical ventilation. The science is settled on the fact that we've been living in houses with poor air quality for a long time, and that includes "leaky" houses. Leaky houses don't leak enough when the air is still outside. Will you suffocate? No. Is that really the bar we should set? No. We know that brain function is reduced at CO2 levels below 1000ppm, and we know that naturally ventilated houses exceed that by a lot, a lot of the time. Other contaminants are also going to be elevated without consistent ventilation.

    Mechanical ventilation doesn't necessarily mean an HRV or ERV, though I would say it should be the default unless it's demonstrated it's not worth it. Just cracking a window when it feels stuffy fails not only the scientific test but even the common sense test. When you live in a stuffy house, you get acclimated to it; just cracking a window isn't going to even do much in a lot of circumstances. Opening windows in the window is not something most people are going to want to do for comfort reasons, and if you actually do that enough to improve the air quality in a heating climate it's certainly going to cost more than the HRV would have.

    1. JohnsonDesign | | #10

      This is a great explanation. Thanks Trevor.

  6. Jon R | | #6

    I'd build to the 2021 IRC (code). Even if you don't have to.

  7. Steve Knapp CZ 3A Georgia | | #7

    You might want to run the ducts (supply and returns for a ducted ERV) while you can. Then you could buy an Awair unit and monitor your indoor conditions. If the CO2 is staying high, you could then retrofit an ERV.

    That said, the Panasonic FV-04 is pretty cheap and would probably provide enough fresh air for most circumstances. (Note that you shouldn't mount this one in a bath or count on it as an substitute for a kitchen exhaust fan.)

    To provide feedback on your insulation plan, you need to tell us your climate zone. You can identify that by going to https://www.energy.gov/sites/prod/files/2015/10/f27/ba_climate_region_guide_7.3.pdf.

    1. JohnsonDesign | | #11

      Thanks Steve. I'll look at the Panasonic. I'm Zone 5B, so for 14" TGIs with blown-in fiberglass, it looks like I need 4" of spray foam to hit the standard.

  8. plumb_bob | | #12

    Another factor is a tight house with a wood stove. You could easily get into a negative pressure situation that can be dangerous. Also, wood stoves impact air quality with the inherent ash and smoke.

    Mechanical ventilation will balance pressures and can filter air.

  9. Brad | | #13

    Somewhere on this site there is an article by Martin which suggests that an ERV/HRV is not always necessary. I'm not an expert, but I'm not convinced it's needed either. As mentioned by plumb_bob, the wood stove complicates things, I would think you'd need a fresh air intake for that.
    Another big factor is how many occupants? I know it's usually done according to the number of bedrooms, but if the code doesn't require it, you can build it for yourself.
    I have a crawl space exhaust fan in my conditioned CS which I run for a few hours a day, no ERV or HRV, and the house is at 720 PPM CO2 right now. 2200SF, 1.6 ACH50.

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