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I'm trying to keep the moisture content fairly low in my house.

I have a humidity sensor in the house. Right now it is showing a temperature of 17C and a humidity (RH) of 62%.

I also have a humidity sensor outside. That shows a temperature of 11C and a humidity of 90%.

At what point would it be best to start closing the windows to keep moisture out? Is there a graph or a calculation that I can do?

4 Answers 4

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The calculations are not very straightforward. You want to look for a Hygrometric chart, that shows the relationship between absolute humidity (amount of moisture in the air), relative humidity, and temperature.

Here's one I found from https://manual.museum.wa.gov.au/book/export/html/89. I added the two data points you gave to the chart, the red dots. This tells me that your indoor air has more moisture than the outside air, and so the moisture would tend to move from the interior to the exterior, under this specific condition.

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  • So, if I've understood it right, it is the Absolute Humidity that is key. I'd seen one of those charts after googling but, tbh, couldn't understand it! Thank you for your explanation and the two points on the graph. It is interesting that if I want to keep my RH level under 60% and keep my indoor temperature at 17C then it is almost always beneficial to open my windows if the temperature 10C or less outside...regardless of the what the RH of outside is.
    – atreeon
    Dec 28, 2022 at 13:25
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    Nice answer to the question, but @atreeon : If the objective is comfort, temperature is important too. Why are you asking about moisture alone? You obviously won't open windows in freezing weather to remove a small amount of humidity indoors. Unless you just took a hot shower or boiled pasta ... and then you don't need a formula.
    – jay613
    Dec 28, 2022 at 20:26
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It would really be best to work and think in absolute humidity, or the grams of water per cubic meter of air. You want to ventilate the house when the outside air has less absolute humidity than interior air.

However, nobody ever works in absolute humidity. All anyone ever talks about is this rather inconvenient unit called "relative humidity".

See, at any particular temperature, air has a limited capacity to hold water. For instance, a 10°C (50°F) temperature, air can hold 9 grams of water per cubic meter (assuming sea level, good grief). Likewise, 20°C (68°F) air can hold 18 grams of water per cubic meter (note: double, and we've only gone up 10 degrees). Any more will condense.

So if they give you relative humidity (which will be a percentage), you need to find out the absolute capacity of air at that temperature and now you will have grams per cubic meter. You get that from a place such as the chart that SteveSh posted in that answer above, except that chart is truncated off so it is useless above 25°C (77°F).

Then you multiply the air's capacity at that temperature by the percentage, and that gives you the "grams per cubic meter".

E.G. 50% relative humidity at 20°C means 18g x 50%. = 9 grams/m3.
33% relative humidity at 10°C means 9g x 33% = 3g.

Isn't that just perfectly awkward?

When outside air comes into your house, it will be the exact same grams per cubic meter that it was outside. However, as you warm it, the relative humidity will lower, and as it passively cools, the relative humidity will raise. If you mechanically cool it (air conditioning), the A/C evaporator is quite cold (maybe 2-5°C) and will condense a lot of the water out of the air.

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  • Maybe most people don't work with absolute humidity. But almost anyone should be able to understand the concept of dew point, which is a good surrogate for absolute humidity.
    – SteveSh
    Jul 14, 2023 at 17:52
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Statistically, the best time for an air exchange is before sunrise in the morning, since the absolute moisture levels are the lowest at that daytime.

This is contra-intuitively, since most tenants open the windows f.e. when the sun is shining.

Preferably for airing, windows should be opened when it is dark outside. With windows/doors opened at 2 walls at 2 different directions (west-east is best), 1 to 3 minutes is enough in most cases.

With measuring devices, a calculation from relative to absolute values is possible, as mentioned before.

There are even fan units which can be mounted at a wall. They do the calculations based on inside and outside sensors and will start accordingly, inclusive counter stream heat recovery.

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    before sunrise in the morning, I imagine, would also be the coldest time of the day when the air is less able to hold as much moisture. Very intersting. Many thanks!
    – atreeon
    Dec 28, 2022 at 13:26
  • I don't follow your line of reasoning. The absolute moisture level in the air - the humidity - does not change from night time to day time. It changes when the air mass changes. What does change is the relative humidity, which generally increases in the early morning because the air is cooler.
    – SteveSh
    Dec 28, 2022 at 13:59
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    @SteveSh Also the absolute moisture level is statistically at a minimum in the morning, since the air is cooled down during the night releasing water to all surfaces like plants and soil = morning dew. When the same air (assuming no wind) is heated up again by sunshine, the air can retake the moisture from all surfaces and plants. Thus the daily night/day temperature change works like a natural dehumidifier. At least in Europe, it is well known, that airing/venting should be done in the morning, if any moisture problems are present.
    – xeeka
    Dec 28, 2022 at 14:43
  • What you described only happens if the surfaces you mentioned are at a lower temperature than the dew point pf the air. If the dew point is 40F, and the air/surface temp drops to 50F in the early morning, then no dew. I know that surfaces like vehicles can be cooler than the air because of radiative cooling over night. But I don't think that changes the basic argument.
    – SteveSh
    Dec 28, 2022 at 15:32
  • @SteveSh Yes, the dew point is not reached every morning. But still the air has often its minimum of moisture before sun rise. Simply because with the rising sun f.e. the plants do evaporate more water near the ground/buildings (photosynthesis) and the air gets warmer = increased capacity to solve water, thus there is a high probability that the absolute water in the air will be increased during day time, even without morning dew. If in doubt, just try it out with a moisture meter: Preferably airing before sun rise vs. airing at afternoon (13:00- 15:00).
    – xeeka
    Dec 28, 2022 at 18:24
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Dew points.

Keep it simple. Instead of worrying about absolute or relative humidity, just reference the atmospheric dew point, and if it’s higher then what you prefer close the house up and let your HVAC lower the humidity in your home. If the dew point is pleasant outside, open the windows.

The experts, too, consider dew point to be an important value. Addendum ae to ANSI/ASHRAE Standard 62.1-2016, "Ventilation for Acceptable Indoor Air Quality", considers dew point when deciding whether or not air is sufficiently comfortable:

Buildings or spaces equipped with or served by mechanical cooling equipment shall be provided with dehumidification components and controls that limit the indoor humidity to a maximum dew point of 60°F (15°C) during both occupied and unoccupied hours whenever the outdoor-air dew point is above 60°F (15°C).

(PDF source.)

Please see also:

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