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When to Open Windows Based on Humidity Levels

W Ramsay | Posted in General Questions on

The general rule is that indoor RH should be kept between about 40% and 60%. I’m guessing that this is at perhaps 68°f in winter and 72°f in summer?

So, on a cool summer morning… Inside is 70°f @ 52% RH. Outside is a very comfortable 58°f @ 72% RH. Is it OK to open windows and get a bunch of fresh air through the house for an hour or two (or until outdoor temp rises above perhaps 68°f)?

The moisture content of the air (absolute humidity) outside is actually lower than inside but the RH is much higher and higher than recommended.

??

Thanks,

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Replies

  1. Trevor Lambert | | #1

    The absolute moisture content is what matters in this case. The outside air will get warmed as soon as it comes in the house, and the RH will drop, the overall effect being a lowering of the whole house RH. Eventually, if the house temperature equilibriated to the ambient, that might be a concern. Given the relatively slow rate of air exchange and the heat energy contained within the interior house structure and contents, not to mention the ongoing solar gain, I don't think would ever be a concern within the time frame of a few hours or a day.

  2. Jon R | | #2

    If the interior came down to 58°f @ 72% RH, then there would be a little moisture accumulation in interior materials - RH is what matters for this. But this won't happen, so say the open windows bring the interior to 66°f @ 52% RH - which causes no accumulation for later release when the house is 70°f @ 52% RH. Even more so when using a more common Summer interior of say 76°f and 60%.

    Summary: for this example, you accumulate some sensible cooling in thermal mass (a plus), lose some moisture in materials (a plus) and lose AH moisture in the air (a plus).

    For maximum comfort - set the HVAC to optimal and never open the windows.

  3. W Ramsay | | #3

    Thank you guys. This is one of those things where I'm wondering what I'm missing. :-)

  4. Walter Ahlgrim | | #4

    Teach me how to decide when it is a good idea to shut down the AC and open the windows.

    Lets assume the AC has the house a 78° and 60% and that just before sun rise when the temp is lowest the air is full of fog and at 100% humidity.

    My gut says no when the low is 75 and yes 60 but more often than not the low is in my gray area.

    Walta

    1. Charlie Sullivan | | #7

      Here's what I recommend: Convert both do dew point, either using a chart, a phone app, or a website calculator. http://www.dpcalc.org/ is a really simple one. It says 78 F 60% humidity is a dew point of 63. 60 F and 100% humidity is lower dew point and thus lower absolute moisture content and so I'd open the windows.

      Even better, get a temperature/humidity monitor that can display the dew point directly. I have one of these:

      https://ambientweather.com/amws3000x3.html

      You can configure the display to provide your choice of stats, and plot one. I have it displaying indoor and outdoor temperatures and dew points, indoor humidity, and graphing dew point for all probes.

  5. W Ramsay | | #5

    Here's my not-really-qualified-to-speak-on-this take... I lean towards opening windows as much as possible as often as possible. I think that there are two things to think about; 1) Human comfort / health and 2) House/structure health.

    I think RH is a better measure for human comfort than AH though what RH is comfortable varies by individual. I'm generally more tolerant of temp and humidity than my Swedish wife so there are many times in both summer and winter when she'll want windows closed when I'd otherwise have them open. Her upper comfort limit seems to be about 55% RH.

    I think for the house itself it's important to keep the interior RH between 40 & 60% most of the year though we'll keep it much lower during parts of the winter to reduce frost on windows.

    I think generally as long as the AH outside will result in your desired RH inside (< 55% RH or 0.010 AH for my wife) then good to go on opening windows. Lacking a device to do it for you... I think you have to plug in the outside temp, BP and RH in to a calculator to see if the AH (total moisture content) is acceptable. I aim for 0.010 kg/m3 AH. So for 90% RH the outside temp would need to be 55°f. For 99% it'd need to be 53°f.

    HOWEVER, that air coming in will FEEL damp until it warms up. As it warms up its total capacity for moisture increases and so RH decreases and it is better able to pull moisture away from your body which feels more comfortable. SO, I'll be cautious if the outside RH is too much greater than inside RH.

    It's also important to check outside air quality, particularly PMx.x. But that's another discussion.

    1. Tyler Keniston | | #6

      Don't forget that your body regulates to a rather constant temp, so while RH will ebb and flow with ambient temp variation, your body core is not ebbing and flowing in step. Your skin and the air film around your body is a heated and therefore the RH at this boundary is different than at whatever meter you're using. For this reason, absolute humidity (dewpoint) is quite relevant to human comfort, especially temperature regulation. Heat index anyone?

      Apparently mucous membranes react more to RH.
      And wood and other fibrous materials certainly react to RH.

      My 2 cents on 'when to open' would be that you would need to run calcs on the latent/sensible balance. If the sensible is lowered by x, but the latent is increased by 1.5x, then no go on window opening. If the sensible is lowered by x and latent is increased by only 0.5x, then it's a go. Of course I'm not sure how exactly you'd run the calcs but that would seem to be the analytical way.

  6. DCContrarian | | #8

    Air at 72F and 50% relative humidity has the same moisture content as air at 60F and 76% RH or 53F and 100% RH. I would argue that if you are air conditioning you never want to introduce air with higher moisture content than the air inside your house, even if it is cooler.

    Air conditioning cools the air and removes humidity. The control for the air conditioner runs off of temperature -- the thermostat. If you are introducing cool air the air conditioning doesn't run so much and won't be able to remove the humidity.

    Note also that when the outside air is cool the air conditioning runs more efficiently and uses less energy.

    1. Tyler Keniston | | #9

      "if you are air conditioning you never want to introduce air with higher moisture content than the air inside your house, even if it is cooler."

      The answer to this question all depends on the objective, which as of now is ill-defined. If the objective is pure comfort, and we fairly assume that more humidity is less comfortable, then I definitively agree with this. Just covert everything to dewpoint and voila. (assuming adequately sized equipment).

      If, on the other hand, the goal is efficiency within some given parameters, I think a sort of latent/sensible tradeoff calc as I mention above would be in order.

      And if I'm not mistaken that 'calc' is really just the enthalpy of the air. Which conveniently is available on most psychrometric charts. So one could consult a chart and determine if air at a given dry-bulb temp and dewpoint has more or less total energy than interior air.

      Stumbled on this which seems applicable: https://www.computrols.com/enthalpy-hvac-free-cooling/

      And of course if the goal is 100% pure 'efficiency with no comfort parameters,' the answer is obviously to never run an AC and just open/close the windows with the weather.

      1. Tyler Keniston | | #10

        As an example, using DC's numbers where dewpoints are about equal:

        72F @ 50%RH ≈ 43.1 kJ/kg
        60F @ 76%RH ≈ 37.8 kJ/kg
        53F @ 100%RH ≈ 33.2 kJ/kg

        If inside air is 72F @ 50%RH, even air at 60F and 90%RH has lower enthalpy (40.6 kJ/kg).

        The catch may be, as said by DC, that the set-point is solely temperature based. So if you have to heat the incoming air to the correct temperature/humidity relationship, the gains may be lost in the heating (or dehumidifying/heating) process. My feeling at this point is that in humid environments, just going by dewpoint alone is the simpler and better way. If the climate is not unduly sticky, perhaps an enthalpy based approach could occasionally work out.

        1. Charlie Sullivan | | #11

          I agree. If you do the enthalpy approach, you end up with a low temperature and high humidity in the house, and you can't get back to a nice combination of temperature and humidity by running an air conditioner, because that will just make it colder, so you need to run a dehumidifier, but if you had just avoided opening the windows in the first place, you could have run the air conditioner at a similar energy cost to running the dehumidifier.

          1. Jon R | | #13

            Or, per the OP's plan, just wait for the temperature to rise, which will convert the low temp/high RH to a higher temp/lower RH.

  7. W Ramsay | | #12

    "The answer to this question all depends on the objective, which as of now is ill-defined."

    In our case:
    1) The primary objective is air exchange. Fresh air in, stale air out.
    2) Second is to reduce energy use

    Human comfort/health figure in as does structure health. These are going to establish upper and lower bounds for both temp and moisture content.

  8. flyingdutch | | #14

    In our house, we have a Davis Instruments Vantage Pro2 out in the backyard. If the dewpoint is 13C (56F) or lower, windows are open. Above that, they are closed, and we'll run the A/C. If it's really hot upstairs, and a coldfront has passed, and the dewpoints are dropping from the high teens with a downtrend, we'll open the windows to cool the upstairs down (as we know the air is getting drier outside)....

    1. W Ramsay | | #15

      And likewise, and to some earlier points made about the house being cool and so AC not being able to remove moisture...

      If it's cool in the morning and the moisture content of the outside air is at or below the moisture content of the indoor air BUT you know that it will be getting warmer and AC will be needed then excessive cooling of the indoor air in the morning shouldn't be a problem as by mid to late morning or early afternoon the AC will need to run.

      Of course that could also result in a period of higher and less comfortable humidity until the AC removes the moisture so care must be taken.

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