Live in Central Texas where the temp is in the upper 90's, low 100's. My AC went so I had to put a single room AC unit for my Master bedroom and bath. Square footage for coverage is 600 sq. feet. Well, it's humid in the rest of the house and my Mother keeps reminding me not to open the windows because it will develop mold. Well, I have a dehumidifier floating from living room to kitchen. Is it safe to open the windows at night to release the heat in the house?
I have no idea about the climate in Texas. Is it tropic/ rainforest-like? I live in Germany and we have such temperature days in summer, too. We do not have an air condition at all (not because of money, its not typical). Mode of operation is: during the day, lower the blind, to keep the sun out. At night, open windows (as soon as felt temperature outside gets lower than it is inside). We don’t have any problems with mold by this. If we have get mold, then it is typically in winter because walls are cool and humid air condenses, especially behind furniture positioned on walls to the outside.
The answer is: it's complicated. You want to manage both humidity and heat, and sometimes the needs of the two conflict. (I'll assume that less of each is better, i.e. it isn't winter outside.)
Heat is actually pretty easy: if it's cooler out than in, then you want to increase ventilation to get the heat out of your house. So, open up in the evening, close up in the morning.
Humidity is more complicated, because there's moisture in your home's air, but perhaps more moisture in its furnishings (unfinished wood, plaster and "dry"wall, rugs, etc). The "relative humidity" measurement most commonly cited (e.g. 80%, 20%) depends on temperature; if you warm some air up, its relative humidity level will go down, even though the amount of moisture in the air hasn't changed. To measure the absolute amount of humidity in air you measure its "dew point", or the temperature at which, if you cooled the air, the relative humidity would hit 100% and you'd get fog or dew.
Why does this matter? Well, as an extreme example, say it's hot and dry in your house, and cool and foggy outside; you may not want to ventilate, as you'll pull all that moisture into your house; yes, it'll be cooler, but it may end up feeling warmer and muggier.
So, if it's cool and dry outside, ventilate; if it's hot and humid outside, don't. If it's mixed outside, you'll need to think about that humidity level, and whether ventilating will dry out the house, or just add moisture.
In all of this, having fans helps, and most specifically a whole house fan. Such a fan takes heat from high in the living spaces (where it'll be warmer) and pulls it out, to be replaced by (hopefully cooler and/or drier) air from outside. These work best in areas where it gets cool and (relatively) dry at night, but they can be useful elsewhere.
In non air conditioned buildings in Texas in the 1950s and 60s we kept the screened windows open day and night. When we got a/c window units in the 1950s we still kept the windows open in the parts of the house that were not served by an a/c.
Human bodies give off a lot of moisture from sweating and in exhaled air. Cooking, bathing and other domestic activities produce heat and moisture. Even if the humidity is moderately high outside it will be even higher indoors with the windows shut.
Keep the windows open in the un-air-conditioned part of the house.
If you keep the windows open to allow air exchange, then you can do away with the dehumidifier. The dehumidifier is removing moisture from the air, but it is generating a lot of heat which is raising the temperature of the rooms.
One activity that produces both heat and moisture is boiling water in an open pot, like a lot of people do to cook pasta. If you cook pasta try using a minimum of water, reduced burner output to the minimum to achieve boiling, and cover the pot.
Take showers with the minimum water temperature that is comfortable and make the showers as brief as practical.
Get more window units to air condition the remainder of the house.
This is a little off the beaten track, but within the confines of the question. Besides having a AC system, one of the very best things you can do is to invest in a whole house fan. Having one of these will solve a whole lot of heat problems..
Depending upon the design of your home, a whole house fan mostly ( but not always ) during the day and especially at night, keep your entire house cool.
If your home has an attic, one of the reasons it's hot is because of heat buildup.
https://youtu.be/CQZmWIYsyDU. Check this out. I am not advocating any brand over another, but rather showing you an idea for more choices of cooling.
Never run AC and whole house fans simultaneously.
Opening at night isn't a good choice, because at night/early morning humidity rate are peacking, best time to change air, in my opinion, is early evening (19-21): outer temperature start falling and humidity isn't as high as in the morning.
If you have a skylight in your attic, remember open it: hot air in attic will start a quite stong draft giving a good breeze across all house.
Just a door at ground floor and skylight will make a good breeze in all house, at least in my house (100 m^2)