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?
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.
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.
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.
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).
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