Create a Draft
When people are too hot in the office, a regular action is to open the window, but the effect can be increased greatly by encouraging a draft. I mean if you think about it, you open a window, trying to let air in, well where is the hot air already in the office going to go (simple fluid dynamics???)? You need a flow of air so the hot air has somewhere to go, and the cold air from outside is sucked into the office space.
So here is the theory bit. Hot air rises. That’s the key point. If you were to take a good thermometer, and take measures on the floor, and near the ceiling of your office, you’d probably see about a 2C difference.
So, if you have an open window, and assuming there isn’t a breeze outside, the first thing that will happen will be hot air will try to escape. What you need, is a second window open, at a lower level to allow cooler air to come in. The ideal office space looks like this.
If not, even having them at the same level is good.