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Ventilation through windows, any way to know approximate CFM?

Markiz_von_Schnitzel | Posted in General Questions on

Even though there is still dispute over how many CFM is actually needed, this is not what I am interested in.

What I am interested in, is there a way to estimate how much actual ventilation is happening though windows?

I know this is a VERY broad assumption. Could depend on wind, temperature, temperature difference, orientation, size of the window, how much is the window opened, type of window..

But is there some sort of a “average” case, if something like that exists?

Say, two 4×4 foot widows, wide open, outside temp and inside temp the same, no wind, how much air would be exchanged?

Been wondering this because I currently use this ventilation method, since I don’t see much point in mechanicals if they are not at the same time HRV/ERV, and those are prohibitively expensive.

If I open a window in my bedroom, it gets subjectively fresh after ~2-3 minutes already.
The room is ~15×15 feet, 8 feet tall, which comes in at 1800 cubic feet.
Window is 5×4 feet, so 20 square feet. In this room, there is no “cross ventilation” mostly, it’s usually just this window.

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  1. Jon_R | | #1

    > wide open, outside temp and inside temp the same, no wind, how much air would be exchanged?

    Let's add "humidity and pressure the same" too. At that point, I see no driving force and therefor no air exchange (in theory, not practice).

  2. Expert Member
    BILL WICHERS | | #2

    To really calculate this, you’d need at a minimum the pressure differential across the window opening. Airflow depends on lots of things (pressure differential, shape and size of the opening, resistance to flow, etc). It’s really too complicated to have a rule of thumb that will be “right” enough to be useful.

    There is more info here:
    And here:

    If you have no pressure or temperature differential, in theory you’ll get no flow, except for diffusion which would serve to equalize the chemical makeup of the air on both sides over time. Wind would make gusty air flow, thermal differentials would make convective airflow. I’m hesitant to even make a SWAG here, but I’d assume a very low rate of flow barring outside factors (large temperature differential, stack effect between first and second floor windows, crossflow from wind, etc).


  3. Markiz_von_Schnitzel | | #3

    Ok, I see your point, and it obviously makes sense :)

    I only assumed no temp diff, for.. I don't know why.
    Actually, i assumed it for spring, when we don't heat or cool our house, so the temp diff is minimal. Maybe it's 20 inside, and 15 outside during night, average.

    But, how do people making energy modeling then take this into account, if there is no ROT?

    1. Expert Member
      BILL WICHERS | | #5

      I don’t think people commonly model houses with the windows open. All the modeling I’ve ever seen basically assumes a sealed structure and attempts to optimize the HVAC system by accounting for things like insulation in the walls, U values and solar gain numbers for windows, weather patterns, stuff like that. Open windows just have too many variables that can’t be controlled or predicted.

      The most effective passive ventilation uses convection and stack effect to move air. The best way to do this is to open second floor windows on one side of a house and first floor windows on the other side. This results in passive crossflow through the entire house with no energy use. The only problem is you can’t control how much wind you get, so you won’t know exactly how much air movement you’ll get.

      You can use a whole-house fan to move a known amount of air, but that’s more than just opening windows. I suspect using tilt’n’turn windows may also help to maximize convective airflow due to the way they can be cracked open in differing orientations.


  4. MattJF | | #4

    From a design perspective, all modeling is done at the "extremes" design conditions, so moderate temps and windows open is not a common scenario.

    The advantage of mechanical ventilation is that it runs at a more applicable timescale of hourly than opening the windows once a day.

  5. Expert Member


    The reality is when you rely on windows, as I have always done, You end up fluctuating between under and over ventilating, since you open and shut the windows using comfort as the metric. With the window cracked, a shift in wind direction or velocity will dramatically change the temperature in our bedroom at night.

    Most of us around here who use windows for ventilation also heat with wood. It's good combination because we can remain unaware of how much of an energy penalty our ventilation strategy causes.

  6. Markiz_von_Schnitzel | | #7

    Thanks all..

    But then.. How can ERV/HRV manufacturers make any claims for "savings" by using their devices?

    I'm not saying ERV/HRV is in any way cost effective (it most definitely is not), but they do put out some numbers for promotion materials? Do they just take the "recommended" ventilation rate, and assume that even people using manual ventilation will ventilate that much?

    1. MattJF | | #8

      ERV/HRV claims are all made vs supply or exhaust only ventilation. Or they make a comfort/health argument vs only natural ventilation. No one says adding mechanical ventilation will save you money over no ventilation at all. ERV/HRV will save energy over exhaust/supply only. They are not always cost effective though.

      Opening windows is not a great comfort option for many of us in parts of the country with more extreme weather. Ventilation is certainly more critical the tighter your house is. My wife is not going to be happy when I get up to open the window at 2am because I forgot to before bed and it is 5 degees F outside.

    2. Expert Member
      MALCOLM TAYLOR | | #9


      The claim ERV/HRV manufacturers make about energy savings are in comparison with manual or mechanical ventilation - which is demonstrably true. Whether it makes economic sense to install them is another issue:

      Ventilation rates are (at least in theory) set to ensure indoor air quality. If users of manual ventilation don't meet them, they potentially risk both health concerns, and the longevity of the building through excess humidity. So I think it's fair to use ventilation rates as a metric for all types of ventilation strategies whether manual users choose to meet them or not.

  7. Peter Yost | | #10

    It would be interesting to use a CO2 monitor to gauge the impact of window ventilation, assuming that windows are being opened and closed based on occupancy. In our own home in the master bedroom (and no forced-air space conditioning or whole house mechanical ventilation running), CO2 levels rise quickly and substantially as soon as the bedroom door is closed and just as quickly and substantially drop with the door open.

    Sure seems as though the primary or only driver in this case is diffusion.


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